# P0420 Cat Effieciency Code



## nmbrinkman (Jul 17, 2009)

Well, the engine light has been lit on my wife's '03 Jetta GL 2.0 (97k miles) for a while now. The code is P0420: Catalyst efficience below threshold bank 1. If I remember right, it came up shortly after the temp gauge on the dash would give a reading and then suddenly shut off, then come back on later. It's completely random when the gauge works and I'm not sure what the problem is. Sending unit? The sensor? But that _may_ be another issue all by itself. 
Anyhow, I took the car to my mechanic since I need to get it smogged in a few months. His immediate diagnosis is a bad cat. converter. Since in CA we cant use aftermarket cats anymore, this looks to be a spendy fix. I kinda want to get the minor stuff fixed first incase they might be indirectly springing the cat code.
Any idea how I can fix the fluctuating temp reading... Also, could that have been the demise of my cat (engine running too rich if the car "thinks" it's cold)? I do plan to have the O2 sensors checked before anything else but I'd like some advice.


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## MkIII_Paul (Sep 7, 2008)

*Re: P0420 Cat Effieciency Code (nmbrinkman)*

not sure if running super rich will damage the cat quickly... misfires will do it, since fuel is burning in the exhaust instead of in the cylinders... now that I think about it, even if it is running rich, since the burning mixture expands and does work on the cylinder, the temp of the exhaust gasses should not be hot enough to damage the cat.
the gauge is most likely a loose wire... trace them and wiggle them to see if the gauge shuts on/off. 
and you are dead on about the o2 sensors... get those replaced before you shell out for a cat.
good luck.


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## germancarnut51 (Mar 23, 2008)

*Re: P0420 Cat Effieciency Code (nmbrinkman)*

From experience, I can tell you that Pre-Cat O2 sensors fail much more often than Post-Cat O2 sensors, and cause more problems. It could be because the Pre-Cat is exposed to higher exhaust gas temperatures, and contaminates.
When a O2 sensor fails (both pre and post cat O2 sensors) they usually send a signal indicating they are reading a lean mixture.
In the case of a pre-cat sensor, when the ECU receives the too lean signal, it will increase the fuel being injected in the cylinders to richen the mixture. When the defective pre-cat O2 sensor does not read the change to a richer mixture, the ECU will continue to increase the fuel, up to the maximum possible. What this does is lower Fuel MPG, and overwhelm the cat, (causes overheating, and could damage/destroy the cat internals). Then, because the cat cannot handle the HCs from the excessive fuel (and because the pre-cat O2 sensor is reading so lean), the ECU sets a cat efficiency CEL when the post-cat O2 sensor reads the gases coming from the cat as having the more HCs than the pre-cat O2 sensor.
My suggestion would be to use a OBDII code reader to look at the fuel injector trim levels. High positive fuel ADJUSTMENTS would be an indicator of a failed pre-cat O2 sensor. Hold off on the cat replacement and post-cat O2 sensor replacement, until after you replace the pre-cat O2 sensor (don't forget to reset the ECU memory). You could very well find that replacing just the pre-cat O2 sensor cures all your problems.


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## darin 1 (Jul 17, 2009)

*Re: P0420 Cat Effieciency Code (nmbrinkman)*

germancarnut is correct in my experiance also after the b1s1 ox replacment give it a few days of driving for the cat to light off good and clean up before proceding further.


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## aa2p (May 25, 2009)

*Re: P0420 Cat Effieciency Code (nmbrinkman)*

Dude I had the same code! I cleaned my MAF and it was gone almost within seconds. Try it.


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## Fitzgerald (May 16, 2008)

*Re: P0420 Cat Effieciency Code (nmbrinkman)*

Not sure how much progress you have made since your post, but I am in a similar position: 
P0420 and P2181 Codes. The P2181 is for Engine Coolant Perfomance - The fix for this is ECT (Engine Coolant Temp. Sensor = $10-$15) and thermostat (about $25?). These are both fairly easy fixes, but changing the thermostat can get messy, so get a wide bucket to catch your drippings. 
As for the P0420 code - I have been given a little bit of mixed instructions: some say its a cat because the sensors are "doing their job" in stating that the cat is below its threshold. Some say its just sensors, but I really don't know what's the cure there. I am going to purchase the O2 Sensors from partsgeek (.com) and see if I find remedy there, but with California's new emissions law, I am not sure if I am ready to drop $1,000 on a new cat. 
My car is at 102K, I have a rough idle and slowly trying to chip away at this ice block.
Have any of you guys experienced a "whistling" sound coming out of the exhaust?


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## M3 E46 (Jun 30, 2009)

*Extended Cat Warranty 2.0 AEG Only*

To the OP: Not sure if this has been covered or what the warranty on the cat is through VW, but most car manufacturers have longer warranties on cat converters. Some states have different restrictions too where manufacturers have to have an extended warranty on cats. Worth a try. 
http://www.epa.gov/OMS/cert/recall/vw2.htm
The above link doesn't include your year model, but it might help you finding something. I have the identical issue as described in the above link, but I have more than 120K miles.

_Modified by M3 E46 at 9:45 AM 7-21-2009_


_Modified by M3 E46 at 9:47 AM 7-21-2009_


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## Fitzgerald (May 16, 2008)

*Re: Extended Cat Warranty 2.0 AEG Only (M3 E46)*

I wish I had the luxury of 120K warranty, but unfortunately, it does not cover my 04 Golf.


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