I have a 03 civic DX with a 02 EX engine and ecu in it.

Recently the p0138 code popped up with a pending p0138. the code means the the downstream o2 sensor is giving off a high voltage signal (post cat rich condition) meaning the cat is bad or the sensor is bad. I replaced the sensor with one from Honda since i had a cheap aftermarket sensor in there. CEL came back on during second start up after highway driving. freeze frame showed this happening at 50mph, so i thought it would have to be the cat. To make sure that cat was in fact bad i made a mechanical o2 sensor simulator out of two spark plug non-foulers but that did not keep the code from coming back.

Anyone have any ideas on what could be the problem and how to diagnose it? Thank you in advance.

 

Does anyone have a diagnostic chart on how to check the sensor as well as the sensors wiring?

 

Have you found any solutions for this problem I'm having a very similar one anything will help. Thanks