So I've done a bunch of research on the forum to figure out possible causes and solution to my P420 check engine code, with no real luck. When I am on my Tephra 7 ROM, I have zero issues with the code, but as soon as I switch back to my Stock ROM so I can get an inspection, after 20 miles of driving the code comes on. Maybe the stock ROM is more sensitive to picking up the code? Rear 02 periphery bits in both the ROMs are turned on. Either way gotta get this sorted out.

• Replaced the sensor, so don't believe it could be that.
• I have full 3 inch buschur exhaust with slip connections, test pipe and clamps, so I am pretty sure there is no exhaust leak from there.
• Rear 02 has the usual anti-fouler extenders and have ran it like that for 60k+ miles with no issues what so ever (can these get clogged after many miles?)

The only thing I can think of is: I know the exhaust manifold gasket has leaked for a while (maybe getting bigger) but from all the info I gathered the P420 code is a reading difference between the front 02 and rear 02. The manifold gasket being before the front 02, I don't see how this would cause a discrepancy between the two 02's.

Any info or insight to my problem would be greatly appreciated, as inspection is up by the end of February. Thanks in advance.

 

Why do you need to swap ROMs for inspection?

 

This. Unless the OP is also switching parts on the car to go back to stock if that area has a visible inspection.

 

2 years ago I went to get an inspection with my tephra ROM running. Everything went fine but apparently the car got flagged because the ROM Id said Europe as country of origin instead of US. When I went to go get an inspection last year the car automatically failed when the inspection was run and I was instructed to go to the main inspection station. They provided detailed information that the car ROM had switch origin of country lol. Hence why I have had to switch back to the stock ROM...

 

That's really odd. I just got mine inspected last week on Tephra V7 running speed density and had zero issues. I have full visual and emission testing.

 

I have a close friend the exact same thing happened to him with getting flagged and failing the following year because of the ROM. Maybe its a Massachusetts thing...

 

Anyways, ROM or not, does anyone have any suggestions to solve my P420 issue? As stated, I have the usual causes of this code ruled out (02 extender, bad cat). Any help would be greatly appreciated.

Can a "starting to go" bad ignition coil cause this? Possibly causing the car to run too rich and throwing the sensor reading off.

 

I'm no ECU Guru but can't you just turn the periphery bits off for that test?

 

When u turn off the rear 02 in periphery bits it doesn't allow it to "get ready," thus causing it to not register during inspection and fail. Tried that last year over a few week period and the rear 02 was never ready. As soon as I turned it back on it checked out fine. I wish the solution was that simple haha

 

check out TSB-12-13-001REV

Engine Controls - MIL ON/DTC P0420 Due To Abusive Driving

 

https://www.evolutionm.net/forums/ec...us-mrfred.html

 

Okay I think I know where ur going with this. Disable the rear 02 to get rid of the SES light and use the patch to pass because it wont matter if it reads the rear 02 or not because it will think its working either way...

 

Well after downloading the patch from the above link and reading the attached thread, I have determined this will not solve my problem. It is noted towards the end of the thread that cars in Massachusetts are also getting flagged for this patch because it is force passing tests that are not performed on an Evo during a normal inspection. So I'm still up the creak with no paddle...

Instead of trying to find a work around, can anybody shed some additional light on what the root cause of this problem could be, taking into consideration the above I have listed?

Again: there is no notable exhaust leak on the car, there is no cat present, there are anti fouler extenders added on the rear o2....

 

Stlevoix, I appreciate the info, but this service bulletin is pertaining to a car with a catalytic converter, which I do not have.