We recently installed a 4-2 header on my 98 honda civic ex.
when we drove it for about 15 minutes, the CEL came on. got a free "diagnosis" and it was the upstream o2 sensor needing to be replaced. that was ridiculous because that particular OE o2 sensor was less than a year old!

we however went ahead and bought a new one for $80 bux and replaced it and after 10-15 mins later, the CEL cuts back on. Another "diagnosis" reveals it to be the downstream sensor going out..

we got a price for that one at advanced auto parts for $200..i said no way and drove for about 6 days...

then today, we did a little research and found the exact same OE downstream sensor at NAPA for $130! i bought it and its sittn it my car

so the question is:
why would the sensors go out together?
If i do replace the downstream sensor, will the CEL light maliciously cut back on to warn me about the CAT perhaps??<< someone told me that could happen...

i cant get a straight answer for any "expert".. and i have tried! think anyone can help me?

 

strange. there's no reason for a new header to cause the o2 sensors to go bad.
i'd reset the ecu and see if the CEL comes back on eventually

remove the "backup" fuse from the under-the-hood fuse box for like 5 minutes, and put it back in. then start the car and idle for like 5 minutes. then you're good to go. then just return to driving your car as normal and wait and see if the CEL comes back on.

 

I had somewhat of the same problem, just in reverse order. I didnt replace my header but had a CEL light on and it said my rear 02 sensor, replaced it, got the code cleared and 20 mins later my lights back on saying my front 02 sensor is bad...im right there with you on this confusion.

 

If you damaged the sensors when you did the install then they will go out. 9 times out of 10 that is what happened.

 

hmm that must be why mine went out. would over tightening them make that happen? sorry to thread jack but the same thing happened to me. i got an o2 simulator for the downstream(secondary) o2 sensor but now it say both are out.