I have a 2000 honda accord sedan, 2.3L (f23a) with a manual. I have a cold air intake... I justinstalled a new header, test pipe, and cat-back 2 1/4" exhaust. The header has a 02 bung on the top half,(whichonly reads one cylinder) where the stock manifold02 reads where all four cylinders merge together.So I decided to lengthen my 02 wires so I could use the bung in the downpipe portion of the header(where it would read all four cylinders). I also removed the cat, and replaced it with a test pipe/resonator. I put my rear 02 sensor in the bung on the test pipe. So literally my two 02 sensors are really close together(within a foot or so). I am now pulling a 0420 code, which means "catalyst below threshold". I understand that the car is telling me it knows that the cat is gone... If I move my front 02 from the downpipe back to the bung on the header, Does anyone think this will solve my problem? Or should I just invest in an 02 simulator for the rear 02 sensor? Any ideas? I just don't like my engine light on... The car is still really young(57,000 miles). Thanks everyone!!!

 

That code shows up when the two o2 sensors are reading nearly the same. The CEL on my 1999 has been on for 7 years with same code. Apparently the dealer said not to worry about it to the previous owner. I would move the sensor to the header bung and reset your ECU and see if the code goes away.