The P0420 errors are more common with Elise.

Did you advise your tuner that you have headers and install the GRP O2 spacers on the back O2 sensor? Below is from GRP site on shorty headers.

• O2 extension harnesses included for header orders. Please leave a note at checkout so we know which car they will be fitted to!
• Rear O2 spacers included. These are to be used on the rear o2 sensors only. These are not always a 100% way to ensure not getting a cel for catalyst efficiency but they normally work pretty well. The true 100% way to ensure no codes is with a tune.

Below is how to check O2 function

marcinr
1,502 posts · Joined 2008

#4 · Feb 17, 2016
Don't change the O2 sensor until you know it's busted. Log the voltages from the pre-cat and post-cat O2 sensors while you drive around for a few minutes after the car is warmed up. This will tell you exactly what is wrong.

I'm attaching two graphs from my car when I got P0420. The pre-cat O2 sensor in blue, the post-cat O2 sensor is in red.

The first graph shows a good catalyst. Notice the pre-cat O2 sensor is constantly oscillating between high and low (blue). This is normal, it's how it hunts for the right A/F ratio. The catalyst should then burn off the excess HC's using excess O2 and have a much smoother O2 level. This is exactly what's happening, the red line is a very coarse low-pass filter (average) of the blue line. This is what a good catalyst looks like in data logs.

The second graph shows a busted cat. The red and blue lines are very similar, meaning the cat doesn't do anything.

A bad O2 sensor will either flat-line, or read high, or show complete nonsense.



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You are always by my side. I feel safe with you man. Thank for the detailed instructions. Actually I have not the headers. I have only grp 3rd kat delete on the exhaust,rest is stock. (Sooner i ll study something...cannot accept too many kats). I heard that CAI can create errors too

 

Yes CAI can and that's why with Elise BOE warns about needing tune. I could have sworn you posted you were purchasing GRP shorty headers. I wonder if using the extenders on O2 would help like with Elise. This is why I have new tune on one ECU and previous known good tune on my spare ecu to help with troubleshooting. Sorry couldn't be of more help other than maybe check O2 operation per the post above.