Which sensor did you change. If it was the rear first you shoudl change the front one. That is the one that sends the signal to the computer whether to richen or lean out.

 

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.

 

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:

 

I'd urge the OP to check the rear O2 sensor including the connection and wiring.

Also see:
http://www.clubwrx.net/forums/general-maintenance-troubleshooting-accidents/134327359-o2-sensor.html
if error is Bank 1 sensor 1 vs. Bank 1 sensor 2 etc.

EDIT: Somewhere there is a picture thread but I am sorry I cannot find it. If I do I will update.

 

Bank 1 is the front O2 sensor bro, a common problem with subarus. I recently changed mine as well. Although the rear can go bad highly unlikely as the front one is what tells the car if to richen or lean out.

 

Bank 1 is the front O2 sensor bro, a common problem with subarus.

There is only one bank. It therefore does not specify physical location. The code number itself is used for that.

 

Known to be an issue with the rear O2 sensor. have you done anything different with your car lately? Fuel additives, downpipe?

 

po420 means that the car got too high... lol:rotfl: