Have a civic type r 2002 135k on the clock. Fitted an aftermarket cat, and 2 new sensors cleared the code. Now after 3 days the code has unfortunately returned which has gave me a real headache.

When the cat was installed plugged in a laptop with diag software which again showed mirrored fluctuating signals in both o2 sensors in closed loop mode.

Any Ideas out there

 

I'd be willing to guess the aftermarket cat is what's causing your ECU to freak out and show a deficiency in your O2 sensor. You might need a defouler to move it away from the stream a little bit.

 

Yes I used an o2 sensor spacer right angled, code no longer appears and car appears to run a lot better.

Cheers