# Tutorial-How to change coolant temperature sensor (4.2 engine) and improve fuel economy (gas mileage)



## Phaeton2 (May 18, 2013)

Is your fuel economy worse than normal? If your fuel economy is very poor (5-7 mpg city with my experience) you might have a bad coolant temperature sensor. *So how does the coolant temperature sensor affect fuel economy?* I asked this question to one of our members who has been of great help to me, Jason Young. As he explains:

_"Colder engines require richer fuel/air mixtures. Warmer engines can run on leaner (more economical) fuel/air mixtures, but will run fine on a rich mixture as well. It gets more complicated on turbo engines but we don't have to worry about that here.

If the coolant temperature sensor degrades or fails, it typically does that toward the cooler/cold direction. Based on that, the ECU may end up adding more fuel than necessary. If the CTS fails entirely (or is declared to be failed by the ECU) the ECU may be forced to use "safe" conservative fueling maps that are not as economical."_



*So how do I know if I have a bad temperature sensor? *

This is simple, simply use VCDS (vag-com) scan address 01. If you have a bad sensor you will receive a fault code similar to this:

Address 01: Engine
Controller: 4D0 907 560 CS
Component: 4.2L V8/5V G 0030
Coding: 0007873
Shop #: WSC 01065

1 Fault Found:
16502 - Engine Coolant Temp. Sensor (G62): Signal too High
P0118 - 002 - Lower Limit Exceeded
Readiness: 0000 0000

The Phaeton is unique in that it has two coolant temperature sensors on the 4.2 engine. Most engines only have one sensor in my limited experience. We call that German engineering! The lower (front) sensor is located on the coolant hose which attaches at the radiator on the right (passenger) side. Access is easy for the lower (front) sensor. The upper (rear) sensor is located behind the engine. Access is difficult for the upper (rear) sensor and requires multiple items to be removed. See attached diagram for both sensor locations. Both sensors are identical, part #5, which is VW part 06A919501A. If you're changing the sensor I recommend buying a new rubber o-ring, part #4, and the plastic clip, part #3. Both parts are cheap. I bought all 3 items together, genuine vw parts on ebay for about $15 or you can buy them at your dealer.













The diagram shows the radiator hose connected to the top of the radiator. This may be incorrect as the radiator hose on my Phaeton is connected to the bottom of the radiator on the left (passenger) side.



*So how do you know which sensor is defective?*

Your fault code tells you. G62 is the upper (rear) sensor. G83 is for the lower (front) sensor.

1 Fault Found:
16502 - Engine Coolant Temp. Sensor (G62): Signal too High
P0118 - 002 - Lower Limit Exceeded
Readiness: 0000 0000



Once you determine which sensor is defective you can begin the work. *I will outline the steps for changing the upper (rear) sensor. * I have not changed the lower (front) sensor so I cannot give directions for this. I understand changing the lower (front) sensor requires a complete drain of your antifreeze, replacing the sensor, and then refilling the antifreeze. Unfortunately, the Bentley repair manual does not cover replacement of the upper (rear) sensor. Please understand that the following procedure maybe incorrect and repair following this procedure is at your own risk. I am not a mechanic.

The procedure for changing the upper (rear) sensor took me about 5 hours. With instructions, I would guess you could do this work in 2-4 hours if you're a skilled wrencher. Before you begin, I recommend that you watch this video. It's has nothing to do with changing the coolant temp sensor on a Phaeton, but it is hilarious and you'll get an idea of the journey you are about to embark on.










*Step 1* Bring a radio. This is going to take some time and is frustrating. Music makes it a little more enjoyable.


*Step 2* I recommend that you protect the vehicle with fender protectors. I small step stool makes access easier as well.

*Step 3* Understand what we are trying to accomplish. The upper (rear) coolant temp sensor is located below the main fresh air intake duct between the engine and firewall. This main fresh air intake duck is attached to the left and right air intake ducts connected to the air filter boxes. To get to the temp sensor you must tilt the main fresh air intake duct up on the left side. You cannot and should not attempt to remove the main fresh air intake duct. You are simply removing items out of the way so you can tilt the main fresh air intake duct to gain access to the temp sensor (as seen in the third photo).




















*Tilting of main fresh air intake hose after steps 1 to 14 is completed*











*Step 4* Remove rear engine cover by pulling up on cover. It is attached with 3 snap connectors. It comes off easy and really no fear of breaking it as long as you pull straight up.

*Step 5* Remove left and right air intake duct between main fresh air intake duct and air filter boxes.










*Step 6* Remove two bolts holding main fresh air intake duct (left bolt removal shown in step 4)











*Step 7* Loosen hose clamp holding main fresh air intake duct to intake manifold. Do not remove hose clamp. Simply loosen it. Do not attempt to remove main fresh air intake duct.

*Step 8* remove hose preventing ability to rotate main fresh air intake housing. Hose should come off with by pulling and twisting to loosen.











*Step 9* Now, we need to remove the relay wiring housing mounted on the firewall. It is in the way and prevents rotation of the main air intake duct. Steps 9-14 covers removal of the relay wiring housing.

Unclip hoses in front of electrical relay housing. Do not disconnect hoses just relocate them out of the way so you can remove the relay housing cover. Remove relay housing cover.











*Step 10* Cut plastic tie holding wiring harness to relay housing so relay housing can be removed.

*Step 11* pull wiring harness away from relay housing












*Step 12* remove wiring and relays off of relaying housing. Pay close attention to wiring routing so it can be reinstalled. Do not cut wiring or disconnect wiring.










*Step 13* remove two nuts holding relay housing to firewall.

*Step 14* remove relay housing. This will permit access to rotate the main fresh air intake duct to gain access to coolant temp sensor










*Step 15* Success! now you can rotate the main fresh air intake housing to gain access to the coolant temp sensor. Lift the left side of the air intake up while pushing the right side down. Take a good look at the newly uncovered coolant temp sensor.

*See end of thread for remaining photos uploaded on 1/14/21


Step 16* Don't pull out the temp sensor! You must first drain the coolant reservoir fully otherwise you are going to leak antifreeze all over your motor. Antifreeze is corrosive and will eat paint. If you don't care, then I guess you can pull the sensor but I wouldn't do it. Below is an image of what the coolant temperature sensor, rubber o-ring, and clip look like to aid you in locating the sensor.











*Step 17* Place the car in "jack mode". With the ignition switch on, hold both the shock button and suspension lift button on the center console for 5 seconds. Sometimes the jack mode confirmation will not appear in the instrument display. To confirm you are in jack mode try and adjust either the shocks or suspension lift. If you are in jack mode you will not be permitted to make adjustments.


*Step 18* Check to see if engine has cooled down. If engine is cooled, remove the coolant tank lid on the reservoir. Do not remove the lid if the engine is warm as you can burn yourself.










*Step19* Use siphon pump (tube with suction bulb, available at most auto stores for about $10) to suck antifreeze completely out of reservoir. I didn't have one handy, so I sucked on a tube to start the suction. I don't recommend this as if your are not careful you can swallow the anti-freeze which is deadly.











*Step 19* The temp sensor is roughly at the same height at the bottom of the reservoir. To lower the anti-freeze in the system more, I lifted the left rear of the car with a jack to drain more coolant into the reservoir. Make sure you placed the car in jack mode. Suck the anti-freeze out of the reservoir using a siphon pump.










*Step 21* unclip the wiring harness connector from coolant temp sensor. I recommend using a pick with a hook on the end to release the connector.










*Step 22* Pull the clip securing the temp sensor to the hose. You can insert a flat blade screwdriver between the clip and hose to pry loose. This is the black one seen here. I recommend having a spare, I lost the original. Part # 032 121 142




















*Step 23* Pull the temp sensor out by lifting and twisting. It may be stuck so you will need to use some force to get it out. You may be able to use a pair of pliers to aid in the twisting. I didn't try this as it wasn't necessary. Remove the old o-ring. Before removing the sensor, I reinstalled the coolant reservoir cap on to create a vacuum to minimize any anti-freeze spillage. I didn't loose a drop of coolant so apparently it was sufficiently drained or the vacuum helped.










*Step 24* Congrats you're half way done!! Everything should go together much faster. Install the new coolant temp sensor with a new o-ring. Reconnect everything removed and refill the anti-freeze removed back into the reservoir. I wouldn't worry about burping the coolant system as the car should do that over a few days.


*Step 25* Clear the error code on VCDS. Open controller 1, click fault codes and clear.










*Step 26* finished! Have a beer!


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## Jxander (Aug 5, 2008)

Who cares if Bentley doesn't cover the replacement of the rear coolant sensor, this DIY post trumps anything Benley might do. Thanks for all your work on behalf of the rest of us that might have to do this later.

Jim X


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## Phaetonlvr (Feb 26, 2013)

Great tutorial that will come in handy if the coolant sensor on my P ever goes out. Thanks for writing this up!


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## Phaeton2 (May 18, 2013)

Thanks, it's important to me to give back. Without the forum, ownership of a Phaeton just wouldn't be feasible for me once you start factoring in the maintenance. I'm glad you found this helpful.


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## coolsig (Jan 4, 2011)

Awesome write up, very detailed! Thanks for sharing.. This should be added to TOC.


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## Phat One (Jul 10, 2009)

I just replaced mine and thanks to these instructions I was able to do it without taking everything off. I loosened the air duct as described and was able to pivot it up just enough to get my hand in there. I used a bent piece of wire to hook the clip from underneath so it didn't fall down into the engine compartment and used the wire and my fingers to pull it out. These are breakable so having a spare is advisable but I was able to reuse mine. I then jiggled the sensor and connector together out of the opening. The connector was then closer and easier to access. I used a long flat blade screwdriver to gently twist in the connector lever until I heard that little click when it unlocks. Be gentle with this so you don't break it, practice on the coil packs if you like, they are the same type. Once it clicks, it is easy to get off. Replacement is the reverse and care was needed not to drop the clip on installation. So it only took about an hour or so. I did this while I was doing my timing belt etc so the coolant was already drained. While I was at it I replaced the lower one which is very easy.


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## Phaetonlvr (Feb 26, 2013)

For future reference: there is an easier way to drain most of the coolant out of the V8 by using the drain plug on the LH side of the engine oil pan. Access that drain plug by removing the forward belly pan. Here's a photo (lower arrow shows the drain plug):


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## invisiblewave (Sep 22, 2008)

I've just done this job on my V8 (G62). Apparently, VCDS does NOT read this sensor when outputting coolant temperatures. I had all 10 measuring blocks for coolant under Engine showing the same temperature after I changed the sensor.

The job itself is pretty easy. The "relay box" is, in fact, the cover over the lambda sensor connectors, this had me confused for a while since the air intake is nowhere near the actual relay box. Other things of note: The sensor is on the driver's side (LHD), with the intake connector pipe off you can just about see it, but it's not the first connector you can see, it's behind and a little below that, you can't really see it properly until you've moved the air intake pipe a bit. Pull the sensor out before removing the connector, once the clip is off it just wiggles out, then the connector is much easier to removed. I bought a sensor with a new clip and o-ring included (I wouldn't re-use a plastic clip in that area if you can help it, particularly since the sensor comes out easily without the clip), but the o-ring didn't come out with the old sensor, from what I could feel with my fingertip I believe it's in a groove and would be extremely difficult to remove, so I left it in and didn't use the new one. You don't need to remove much coolant from the reservoir, although I did have my car pointing downhill on the drive so the sensor was slightly elevated. Don't be afraid of the lambda sensor connectors, unclip them from their mountings and then undo them.

I now have to run around every post I've made in the last 5 years about my thermostat being stuck open and correct them. The thermostat is fine, the problem is with VCDS.


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## Phaetonlvr (Feb 26, 2013)

invisiblewave said:


> I've just done this job on my V8 (G62). Apparently, VCDS does NOT read this sensor when outputting coolant temperatures. I had all 10 measuring blocks for coolant under Engine showing the same temperature after I changed the sensor.


That's how it looked like to me when I tried helping a fellow Phaeton driver. Never figured out why I couldn't see that sensor in VCDS. I did have a suspision that the label file wasn't fully done and maybe preventing from showing it but I never got around to contacting Ross Tech to get their take on this.


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## invisiblewave (Sep 22, 2008)

I don't think it's a simple label file problem, unless there's a measuring block that's completely mis-labeled. I looked at every block that had "coolant" in the description, there are 10 under Engine, and I found a few under another controller (Instruments maybe?). All of them appear to be pointing at the other sensor, unless my thermostat IS stuck open but somehow the car isn't detecting any problem from that. Judging by the dramatically-improved MPG, though, I'm pretty confident there's no issue now. Given that this engine is in every Audi 4.2 on the road, Ross-Tech should have a solution to this! I'll drop a question on their forum.


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## Phaetonlvr (Feb 26, 2013)

The only reason I suspected the label file is because the typically used MVB for the coolant data in ECU 1 has none of the 4 data field defined for the V8, I believe it's MVB 130. On the W12 that's were you find them (plus in a few other MVBs).


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## invisiblewave (Sep 22, 2008)

If I can see the MVB numbers, I'll take a look at 130 shortly. I haven't ever had reason to really dig in to MVBs and understand exactly what's going on. The picture in my head is of a numbered table with a label against each. Assuming that's the case, I'm thinking if I select the right one it should show the reading from the G62 regardless of what the description says, the problem would be finding the right one.


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## Phaetonlvr (Feb 26, 2013)

When I had a failed thermostat on my W12 recently I looked up the fault code on Ross Tech's website and it's a generic fault code, not just for Phaetons. 18613/P2181/008577 - Ross-Tech Wiki

It makes it sound like that on VW or VAG Group cars, MVB 130 is were you will find the temps readings from both sensors and that I can confirm for the W12. Which is why I was surprised that the label file for the V8 has no description for the four data field of MVB 130 nor do they show any data if you select it. Keep us posted what Ross Tech says about that.


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## invisiblewave (Sep 22, 2008)

I took a look last night and the numbers for the measuring values on Engine 1 didn't include 130, it jumps from 127 to 132 or something.


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## Phaetonlvr (Feb 26, 2013)

That's what I remembered running into.


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## Phaeton2 (May 18, 2013)

*Uploaded photos. I was limited to 10 per reply so here are the rest. Can't believe I still have the car after all these years. Enjoy. *

Step 15 Success! now you can rotate the main fresh air intake housing to gain access to the coolant temp sensor. Lift the left side of the air intake up while pushing the right side down. Take a good look at the newly uncovered coolant temp sensor.












Step 16 Don't pull out the temp sensor! You must first drain the coolant reservoir fully otherwise you are going to leak antifreeze all over your motor. Antifreeze is corrosive and will eat paint. If you don't care, then I guess you can pull the sensor but I wouldn't do it. Below is an image of what the coolant temperature sensor, rubber o-ring, and clip look like to aid you in locating the sensor.











Step 17 Place the car in "jack mode". With the ignition switch on, hold both the shock button and suspension lift button on the center console for 5 seconds. Sometimes the jack mode confirmation will not appear in the instrument display. To confirm you are in jack mode try and adjust either the shocks or suspension lift. If you are in jack mode you will not be permitted to make adjustments.


Step 18 Check to see if engine has cooled down. If engine is cooled, remove the coolant tank lid on the reservoir. Do not remove the lid if the engine is warm as you can burn yourself. 











Step19 Use siphon pump (tube with suction bulb, available at most auto stores for about $10) to suck antifreeze completely out of reservoir. I didn't have one handy, so I sucked on a tube to start the suction. I don't recommend this as if your are not careful you can swallow the anti-freeze which is deadly. 











Step 19 The temp sensor is roughly at the same height at the bottom of the reservoir. To lower the anti-freeze in the system more, I lifted the left rear of the car with a jack to drain more coolant into the reservoir. Make sure you placed the car in jack mode. Suck the anti-freeze out of the reservoir using a siphon pump.










Step 21 unclip the wiring harness connector from coolant temp sensor. I recommend using a pick with a hook on the end to release the connector.




Step 22 Pull the clip securing the temp sensor to the hose. You can insert a flat blade screwdriver between the clip and hose to pry loose. This is the black one seen here. I recommend having a spare, I lost the original. Part # 032 121 142












Step 23 Pull the temp sensor out by lifting and twisting. It may be stuck so you will need to use some force to get it out. You may be able to use a pair of pliers to aid in the twisting. I didn't try this as it wasn't necessary. Remove the old o-ring. Before removing the sensor, I reinstalled the coolant reservoir cap on to create a vacuum to minimize any anti-freeze spillage. I didn't loose a drop of coolant so apparently it was sufficiently drained or the vacuum helped. 


Step 24 Congrats you're half way done!! Everything should go together much faster. Install the new coolant temp sensor with a new o-ring. Reconnect everything removed and refill the anti-freeze removed back into the reservoir. I wouldn't worry about burping the coolant system as the car should do that over a few days. 


Step 25 Clear the error code on VCDS. Open controller 1, click fault codes and clear. 











Step 26 finished! Have a beer!


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## Amish Phaeton (Feb 16, 2021)

Thank you so much for updating the photos!!!! They are extremely helpful!


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