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Pesky P0420 Code on an '04 WRX

20K views 12 replies 5 participants last post by  speedracer866 
#1 ·
Alright, I got the intermittent CEL for the pesky P0420 Catalyst System Efficiency Below Threshold (Bank 1). :screwy: Generally, I can reset the CEL code and continue on; however, I think that I have a bad oxygen sensor. The sensor is operating at 100.2% efficiency...enough to throw me a code.
There are no performance issues when driving; however, when started I can smell gasoline vapors when the engine's cold and only exhaust fumes when warm.

I've been reading through SEVERAL postings on this and cannot figure out if my culprit is the front or rear mounted o2 sensor. The car only has 62K miles, so my thought is a front sensor. Does anyone have any suggestions before I crawl under to checkout the situation??
 
#5 ·
P0420 is "Catalyst system efficiency below threshold". Bank 1 isn't specified. The computer is simply noticing that there is no difference between air into and out of the cat, which indicates either the cat has gone bad, which is why I asked about fuel additives or down pipe (high flow cat, or no cat), or the rear O2 sensor has gone bad (unlikely as there should be other codes as well). The front O2 sensor would throw a different code (of which there are many).

If he recently replaced his down pipe, this error would be the first thing I would expect.
 
#10 · (Edited)
#11 ·
Just replaced the rear 02 sensor and the CEL came on again...the code reads as a P0420 (same as last time) in Bank 1. I failed to verify whether it was Sensor 1 or 2, but with a car that only has 62K on the odometer how often do oxygen sensors really go out? Went and bought 2 anti-foulers for the rear 02, prolly install within a few days and update progress.

On a side note...I've also got a DP that deletes the cat (not installed yet) that includes the rear bung for the 02 sensor. Has anyone had any problems with CEL codes if they are not using the Cobb AP or revised maps?

Unfortunately, I have to deal with Maryland emissions restrictions and I really hate jumping through these damn hurdles. :sadwave:
 
#12 ·
If the cat in the downpipe has been deleted, this code will be thrown, no matter what. You can get the mechanical fix for the O2 sensor which basically pulls it further out of the exhaust stream, or you can get some form of tuning, as any stage 2 map should be ignoring this code anyways.

On an 04 WRX, you shouldn't be running the DP without engine management of some kind or another anyways.
 
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