# Top Ten Reasons for Check Engine Light To Come On



## maz60 (Jan 14, 2005)

Since I joined the Check Engine Light club last week, I reviewed some of the postings on this subject and came up with the following list of phrases used to describe the problem areas that have been associated with this warning light:
1. "intermittent misfires in various cylinders due to differences in octane ratings, plus additives"

2. "an air pipe in the intake,"
3. "burnt vacuum line in the secondary air system"

4. "vacuum"-related. Replaced some "vacuum" parts under warranty and now all is well."
5. "solenoid for an air injection pump"

6. Engine Thermostat
7. "car's water temp reached 220°F"
Some seem like realtively straight foreward problems in the air intake, fuel or temperature control systems.
However, we also have:
8. a transmission sensor, 
9. a transmission control module
10. need a new torque converter

The transmission problems are very worrisome.

Last week, the service folks kept my car for two days. On the first day, they checked the emission system and vacuum lines and could not find a problem. They turned the light off and drove it for a while on the second day and pronounced it fixed.
The light came back on in two days and the car will be back in the shop tomorrow. 
I have 2004 Phaeton with a V8 engine and have about 26,000 miles on it.
Recently Michael wrote about a "problem inside the torque converter. Funny thing is that the driveability of the car is not compromised - the car still works OK - but the torque converter needs to be replaced in order to turn the light out."
Michael, any more info from ZF Transmission? What is the answer to your question: Is the fault code that supposedly mandates replacement of the torque converter a) spurious, and b) related to cold temperatures?
When these transmissions have been replaced, do they find something wrong with them? Or is there a faulty sensor in the transmission responsible for the "false positive"? 
Does the fact that both the 5 and 6 speed transmissions are having problems suggest a faulty sensor?
Have the new transmissions that are used to replace the originals been redesigned to prevent this "false positive" or to correct the real problems?
If not, I fear a number of owners may be stuck with replacing these $13,000 transmissions every 30,000 miles or putting some black tape over the CEL.
Maz


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (maz60)*

Hi Maz:
The Phaeton tech at my VW dealership and I determined through experimentation that the transmission only generates that particular fault code (the one that relates to the torque converter) if the car has been parked outside at night (cold-soaked) at temperatures below -10°C. Furthermore, whatever condition it is that causes the light to come on is momentary - the light comes on when the car is started, but if the fault code is then cleared right away, the fault code will not return.
Based on those findings, we kind of looked at each other and decided to take no action at all for the moment until we can gather further information. This sort of reminds me of the discovery process we followed two years ago when we were hunting down the cause of all the low voltage problems on Phaetons, and eventually discovered it was the B version battery controller. In other words, neither one of us suspects that there is a serious problem with the torque converter, and for that reason, the last thing we want to do is replace it.
I'm in Europe now and hope to get to both Dresden and Friedrichshafen (ZF Transmissions home town) within the next month. I want to gather more information before making any further posts on the matter. For the time being, my advice to any other owners who encounter that torque converter fault code is to just have it noted and documented on a work order (in case further action is required later on), clear the fault code, and keep driving. BTW, the thread that discusses this particular fault code in more detail is here: Does my W12 need a new Torque Converter? There is a different thread for discussion of 6 speed transmissions here: 6 Speed Automatic Transmission Concerns (V8 Phaetons). I am going to beg everyone's cooperation in keeping transmission discussions on those threads, not here on your new topic about triggers for the CEL.
-------------------------------------
As for the CEL itself, first and foremost we have to keep in mind that *the CEL is an emissions control warning light*, not an engine health warning light. There is a different warning light called EPC (Electronic Power Control) that tells you if you have an engine control problem, and if you have other serious but non-emission related engine problems such as overheating, low oil pressure, low oil quantity, etc., you will get a plain-language warning in the display between the speedometer and tachometer. See this post for a list of possible plain-language warnings, and also for a list of illustrations of all the warning lights: Instrument Cluster - what do the symbols mean? (list with a photo of every possible symbol that can be displayed).
Keeping in mind that the CEL is an emissions control warning light (mandated by the US Federal Government) and not an engine health light, here is my personal guess at what the primary causes are for CEL illumination. These observations are based on my experience moderating the forum.
*1) Loose gas cap* - this could allow fuel vapour to escape. The fuel tank goes through its own pressure-test routine on each driving cycle, if a pressure leak is detected, the CEL comes on.
*2) Temperature changes in winter* - if you park in a heated garage, drive outside to a very cold day, open the gas cap, partially fill the tank, then put the gas cap back on again, you can sometimes get a spurious CEL because the warm air in the tank is shrinking at the same time the pressure test is being carried out.
*3) Ethanol* - Look in the owner manual - I think the Phaeton engines were designed to run on a maximum of 5% ethanol. Legislators all over the place are mandating higher ethanol levels all the time, either to polish their environmental image and look green, or to keep the campaign contributions from Archer Daniels Midland coming in.
*4) Poor quality gasoline* - this can be caused by any number of reasons, ranging from long storage time (stale fuel), poor QC at the refinery or during transportation, or flaky regional legislation that governs fuel formulation (e.g. Chicago area in the winter, and W12s stalling). Poor quality fuel causes misfires, misfires illuminate the CEL.
*5) Vacuum related things* - many of the emission control systems on the car use vacuum. If there is a leak in any of the vacuum systems (a loose connection, damaged hose, etc.) then the light will illuminate.
*6) Transmission* - I'm referring to the torque converter slip issue for the W12 cars here - but, the only reason that the CEL illuminates is because if the torque converter is found to be slipping (not locking up), this will increase fuel consumption, and increased fuel consumption creates more emissions, therefore the CEL must, by law, illuminate. The fact that the fault condition (the slip) only exists for a few seconds on start-up, before any attempt is made to move the car (take it out of Park) is beside the point as far as the legislators are concerned.
----------------------------------
In short:
Some of the factors that cause the CEL to illuminate indicate problems in the motive system of the vehicle, and these problems should be further investigated to determine the cause (the fault code). *You cannot make a decision about the significance of the CEL warning until you see what the fault code is.* Most of the times the CEL _on Phaetons that have been well maintained _(serviced in accordance with the service schedule) illuminates, it is a nuisance warning. Some specification is not being met, but it does not signify an engine health concern (e.g. gas caps, bad fuel, etc.)
If you encounter a serious engine problem, you will see the EPC light or get a plain language warning in the instrument cluster.  *The CEL is an emissions control light!*
Michael


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## Motorista (Oct 28, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

Here's my situation:
- Check fuel cap light turned on several weeks ago. Condition 1 applied, and tank was half-full. It also occurs to me that I had last filled it at a different gas station, which I had never visited before.
- gas cap light disappeared when I restarted the car after leaving parked approx. 2 hrs outside.
- The following day the check engine light went on.
- As of now the light is still on after at least 5 full driving cycles and about 500 miles. The tank has been refilled once, at my regular gas station.
- The car is now parked in my home garage as I'm travelling. I just received my vag com and will read a code when I get back home, in about two weeks. I will report here instead of my original thread.
- There is no discernible difference in operation, except for a barely discernible faint surge at around 2,300 rpm in sixth. I noticed it a couple of times on my drive back from NY, but I'm not sure if it was there before: it's so faint that I might not have noticed it before.
Stefano


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## Gobuster (May 19, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (Motorista)*

I can add one to the list! On Monday my car was in for it's 20k service, no fault codes were found and I got it back running perfectly, just as it had been. Wednesday night I went out and noticed the CEL came on and stayed on, so I took it in this morning and was told the fault code related to a bad catalytic converter! A new one is on order and in the meantime I'm driving a Highlander


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## car_guy (Aug 19, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (Gobuster)*

I'm not a mechanic and I know I'm sticking my neck out suggesting this, but I'd be very surprised if a catalytic converter with only 20K miles on it is the _root _ cause of the CEL coming on. The converter is a non-electronic device that is very unlikely to fail that soon without an outside cause. Typically a fault code that includes the converter as a possible cause can be the result of another component failure as well. Even if the converter actually caused the fault code, it is likely that something else caused the converter to fail.
If there are no driveability issues, the first thing I would have done would have been to simply erase the code and see if it returns. Occasionally you just get a spurious error.
Is there any way to find out what the actual code was?
Perhaps Chris, who has more experience in this area than I, can now stick his neck out and speculate. He's probably smart enough to wait for the code first!


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## nogood911 (Feb 7, 2004)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

Hi Michael,
Do you have the code number? Audi has a TSB for some TCC concerns on the A8 A6


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## Paldi (Sep 4, 2004)

*Re: Top Ten Reasons for Check Engine Light To Come On (maz60)*

My CEL was definately vacuum related to the secondary air system. It took eight tries to find it and three dealer service departments. The mechanic found a slightly compromised vacuum hose routed too close to an exhaust manifold.


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (nogood911)*


_Quote, originally posted by *nogood911* »_Do you have the code number? Audi has a TSB for some TCC concerns on the A8 A6

Hi, and welcome to the forum.
Here's the fault code my car kept generating:
Address 01: Engine
Part No SW: 07C 906 018 C HW: 
Component: D1-6.0L-AG-LEV G ª5211
Coding: 0000173
Shop #: WSC 01065
1 Fault1 Found:
18032 - MIL Request Signal Active (Check TCM for errors too!)
P1624 - 008 - Implausible Signal
Readiness: 0000 0000
Address 02: Auto Trans
Part No: 3D0 927 156 N
Component: AG5 01L 6.0 W12 USA 1114
Coding: 0000102
1 Fault Found:
17125 - Torque Converter Clutch: Stuck OFF / No Power being transferred
P0741 - 003 - Mechanical Failure - MIL ON
There's a lot more discussion about this particular 5 speed transmission fault code at this post: Does my W12 need a new Torque Converter?. VW of America have a TB out that addresses that particular fault code - it is referenced over at that post - but the technician at my VW dealership and I don't want to start ripping the car apart and replacing the torque converter, which is basically what the TB suggests we do. We're taking it nice and slow, I want to gather more information from the staff at the Phaeton assembly plant in Dresden and the staff at ZF Transmissions before we do anything. The kicker is that the fault code only shows up on cold starts when the car has been cold-soaked overnight. For me, that's not sufficient reason to replace a torque converter.
I would be curious to see the Audi TB, though. Click my username at the left of this post to reveal my email address.
Michael


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## maz60 (Jan 14, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

I got the Phaeton back yesterday. The problem was traced to debris in the combi valve. The valve was replaced and so far so good.
So this was emission problem and not a transmission problem.
My original impression that other V8 owners were having transmission problems signaled by the CEL seems to be wrong. In reviewing the forum links about the six speed transmission problems, I do not find the Check Engine Light has been the signal for the transmission problem.
I sure hope a workaround can be found for the five speed transmission that does’t involve transmission replacement.


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (maz60)*


_Quote, originally posted by *maz60* »_I sure hope a workaround can be found for the five speed transmission that doesnt involve transmission replacement.

I'm pretty optimistic about that, at least, at this point in time anyway. It appears to both the technician and I that we don't have a transmission problem, we have a spurious CEL illumination and fault code generation problem. 








Michael


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## nogood911 (Feb 7, 2004)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

I just looked up the TSB 2010879/9 it appies to 2002 A6 V8- with happens to be an 01L transmission-may be something to think about.


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (nogood911)*

May I trouble you to email that TB to me? Click on my name to the left to reveal my email address. I am in Europe now, therefore no access to dealers.
Thanks, Michael


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## 1pt8t (Mar 14, 2007)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

I'm shocked to see so many problems with the flagship model of VW. I'm even more shocked that you guys are so calm about it







Doesn't seem like honoring warranty service on the Phaeton is much better than a base model VW.


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## VWGlf00GL (Feb 14, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (1pt8t)*


_Quote, originally posted by *1pt8t* »_I'm shocked to see so many problems with the flagship model of VW. I'm even more shocked that you guys are so calm about it







Doesn't seem like honoring warranty service on the Phaeton is much better than a base model VW.


There are some problems out there, sure. You get that with any car flagship or not. We all have our horror stories, some owners have no problems at all. I've been through a lot in the past 5 months with my Phaeton.. Better now, then later I say.. Patience = Perfection.
She's at 52,214 miles (maybe a little more now, she's been road tested) this week. I've had a lot, I mean a lot of CPO Warranty work completed at two VW Dealers in the Bay Area. This latest warranty repair is between $11,000 - $15,000 (just in parts). I am glad VWoA does not want to buy the car back, I won't sell it too them!
Latest saga, but it's coming to an end in a few days...
http://forums.vwvortex.com/zerothread?id=3974132
The car is overall great, with such rarity and not many on the road. These cars are a dime a dozen in some states. I really enjoy cruising in my Phaeton, this once in a lifetime car is a wonderful auto!! 
http://****************.com/smile/emthup.gif


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## jlindy (Mar 21, 2006)

After seeing people discussing the possibility of a CEL for poor gasoline quality, I was wondering a couple of things.
Does anybody know if using regular unleaded or mid-grade really causes engine problems or is it jus a manufacturer's suggestion? I know it is recommended that that gas rating is 91 or higher, but the reason that I ask is that I have a buddy who uses regular (87 I think) in all his cars with no problems and has never had a check engine light. He has driven several Infiniti's that all say premium fuel only so I was just wondering how much is truly hype and how much of a risk is it. I am too scared to used the regular grade, but with gas prices, I have been known to use the mid grade from time to time. 

_Modified by jlindy at 6:38 AM 10-15-2008_


_Modified by jlindy at 6:39 AM 10-15-2008_


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## 1pt8t (Mar 14, 2007)

*Re: (jlindy)*

car talk has some great info on this. i think all the experts basically say 'no'. i only use premium if i'm driving up mountains or in strenuous conditions... basically to prevent pre-detonation from the engine adding its own heat to the cylinder. and while it's a small engine it does get hot b/c of the turbo. i know nothing about your engine, however, but that's my 2 cents.


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## Gobuster (May 19, 2005)

*Re: (1pt8t)*

I have used regular in several cars that call for premium only, BMW, Phaeton and Audi. At no time did this trigger a CEL or make any noticeable difference to how the car ran.

Manufacturers specify premium to a) ensure maximum power is developed b) cover all operating conditions. The engine is fitted with a knock sensor that prevents destructive pre-ignition when lower octane fuels are used. However in normal gentle driving conditions, you may not ever engage it. 
From what I've heard, it is best not to operate in conditions where the knock sensor is constantly in use so, where you want and need premium fuel is when: a) frequent use of maximum acceleration, b) hilly conditions that require heavy throttle, c) towing, d) hot, dry conditions combined with heavy throttle use. 
Bear in mind that some premium (top tier) fuels have a better additive package than regular or plus grades and help maintain fuel system cleanliness.
I've noticed no difference in fuel economy between regular and premium, but usually use 1 tank in 3 of premium (Shell V-Power) for fuel system maintenance!


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## Itzmann (Jan 16, 2006)

*Re: Top Ten Reasons for Check Engine Light To Come On (1pt8t)*

My CEL light likes to comes on the day after I have left the dealership. It has a mania for staying on for a couple of weeks and then it may go away for months... or days. I used to think the three engine cycles from cold starts would kill it, but it rarely does.
In the last quarter, it has been ON mostly, by far, since my last service which was in late May or early June (it was off for maybe two weeks in July, but that's it). It only went away after leaving the car parked for three full weeks while on vacation... and it has yet to come back on this week.
Keep fingers crossed.


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## Gobuster (May 19, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (Itzmann)*

Francisco, make sure your gas cap is tight! A loose or faulty cap can trigger a CEL frequently - not to mention about a thousand other possibilities!


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## car_guy (Aug 19, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (Gobuster)*


_Quote, originally posted by *Gobuster* »_Francisco, make sure your gas cap is tight! A loose or faulty cap can trigger a CEL frequently - not to mention about a thousand other possibilities!

Actually, the Phaeton has a specific message identifying a loose gas cap that comes up on the small display between the tach and speedo. Guess how I know... I was pretty impressed that I got that message rather than the CEL lighting up. Is that a hidden feature?
Many car parts stores will read the code(s) for you free of charge. However, there may be several possible causes for any single code.
Steven


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (maz60)*


_Quote, originally posted by *maz60* »_
...Recently Michael wrote about a "problem inside the torque converter. Funny thing is that the driveability of the car is not compromised - the car still works OK - but the torque converter needs to be replaced in order to turn the light out."
Michael, any more info from ZF Transmission? What is the answer to your question: Is the fault code that supposedly mandates replacement of the torque converter a) spurious, and b) related to cold temperatures?

Hi Maz:
That particular problem is unique to the W12 engine Phaetons (those with the 5 speed transmission) and does not affect the V8 vehicles.
W12 Phaetons sold outside of North America do not have a 'Check Engine Light". They also have identical transmissions to the W12s sold in North America. I have never heard of a W12 owner outside of North America needing to replace their transmission.
The 'check engine light' does not mean that the engine is sick. It means that one of the parameters that are mandated by American EPA regulations is not being met. If you don't mind just ignoring the light, you can drive the car. But, the presence of the light might make it difficult to pass an emissions test in jurisdictions that require such a test. If this is the case, then I expect that the emissions warranty (which is longer than the basic new car warranty) would cover the work required to stop the light from illuminating - this being replacement of the torque converter.
Having said that, and having personally replaced the torque converter in my W12 to stop the light from illuminating, I would advise against replacing the torque converter unless you absolutely have to. Driveability is not affected (as all the Europeans have proved), and it is an absolute SOB to replace that torque converter. The amount of disassembly required is staggering, and if the work is not done correctly more problems will show up later on.
Michael


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## murphybaileysam (Dec 29, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*


_Quote, originally posted by *PanEuropean* »_
I would advise against replacing the torque converter unless you absolutely have to. Driveability is not affected (as all the Europeans have proved), and it is an absolute SOB to replace that torque converter. The amount of disassembly required is staggering, and if the work is not done correctly more problems will show up later on.
Michael

I have also had my torque converter replaced recently and am curious as to what problems you feel may show up at a later date (as well as when they might start to show up).
My car continues to drive perfectly, as it did before the torque converter problem, so I'm sort of apprehensive about what problems, and why they would occur, may be forthcoming.


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## Itzmann (Jan 16, 2006)

*Re: Top Ten Reasons for Check Engine Light To Come On (murphybaileysam)*

My CEL light just came back on this Sunday... And since my return from vacation I am getting a "TPMS failure" warning on the MFI display... so I may schedule a shop visit...
... Which is not a problem, since the dealer is only a couple of miles away, except they simply reset the idiot lights and usually question whether I close my gas cap correctly and whether I attempted to set new tire pressures on the infotainment. I don't like it when the basic assumption is that I am the problem. But I guess it is what it is!


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## PanEuropean (Nov 3, 2001)

*Re: Top Ten Reasons for Check Engine Light To Come On (murphybaileysam)*


_Quote, originally posted by *murphybaileysam* »_I have also had my torque converter replaced recently and am curious as to what problems you feel may show up at a later date (as well as when they might start to show up).
My car continues to drive perfectly, as it did before the torque converter problem, so I'm sort of apprehensive about what problems, and why they would occur, may be forthcoming.

If the torque converter is replaced by a competent and detail-oriented technician, then there are no future problems to worry about.
If the person who does the torque converter replacement is anything less than highly competent and seriously detail-oriented, there may be problems down the road arising from all of the disassembly and reassembly work required.
Michael


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## VWGlf00GL (Feb 14, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (PanEuropean)*

I had my CEL light up the past two days.. I have a failing O2 sensor, not sure which one, there are four of them. 
I took the car to the VW Dealer yesterday morning, and my VW Tech turned the light off with the VW Diagnostic Tool. He said it's nothing to be "too" concerned. When the light illuminates, gas mileage is affected, I assume because the car is running in "safe mode".
During my 210 mile road trip last night, I plugged in my cheap-o Auto X-Ray OBD II reader at a rest stop to disable the light (slow response from the sensor). It's been off for today, we'll see what happens later. 
So my BIG question is.. Should I replace all four O2 sensors or the malfunctioning one? This would be a normal "wear and tear" item I assume. The car now has 53,810 miles on the clock. 
Thanks!


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## 7even5ins (Nov 11, 2008)

*Re: Top Ten Reasons for Check Engine Light To Come On (VWGlf00GL)*

Somone explain to me this,
My check eand AND traction control light came on the very second in turned on my newly installed deck.















The traction control works, its just that the light is on.
When my friend installed it, he hacked thi sh*t outta the factory harnes







and now theres also 2 grounds? Plus, the secruity wires arent connected to anything, so im thinking the lights are coming on dude to the secruity wires not being connected to anything, ya know, like to warn the "theif" and that theyll take it to a shop, shop knows that that certain kind of car has been stolen lately?
Idk. Someone help?








(Wow, i just realized this is the Paheton thread...i didnt even click on the Paheton thread, i clicked diagnostic thread and then searched check engine light and it brought me here, my bad.)


_Modified by 7even5ins at 2:26 PM 11-24-2008_


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## car_guy (Aug 19, 2005)

*Re: Top Ten Reasons for Check Engine Light To Come On (VWGlf00GL)*


_Quote, originally posted by *VWGlf00GL* »_I had my CEL light up the past two days.. I have a failing O2 sensor, not sure which one, there are four of them. 

What's the number of the code you are getting?


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