Hello, im new to this website, Im driving 2005 Honda Civic LX special Edition with 95k miles on it, recently the check engine light turns on and goes off very often, finally i checked the light and the code P0420 popped, now some people say its cat and a lot of them say its prob O2 sensor, i dont know what to do, if any one can help i would appreciate

 

P0420 usually means the catalytic converter is bad, but it's also possible that the o2 sensor(s) needs to be replaced, or another possibility is that the timing belt slipped.


I'd start with checking the timing. If it slipped, fix it. If it's fine, check/replace the o2 sensor(s).

If the problem still persists, replace the cat.

 

wouldnt the car make a sound if the timing belt slipped? and the light goes off and on sometimes, i dont know which o2 sensor is it like rear or front

 

at 95K mi, it ain't likely that the cat has gone bad already, so it's possible that it's the sensor(s).

usually the code reader would tell you which sensor, either sensor 1 (upstream before cat) or sensor 2 (downstream after cat). make sure u reset the ecu before installing the new sensor. take it for a long drive to readjust the fuel trim.

 

I'm having this same issue, except my car has 140k miles... Should I replace the o2 sensors first or is it most likely the cat?

 

I get this every now and then too. If your gas mileage isn't suffering, just keep resetting the code. What I've been told is the secondary O2 sensor fouls more often than the first because it's at a much lower temperature than the primary, and deposits are less likely to burn off. The second O2 sensor serves no purpose other than to tell you that your cat is dead, closed loop fueling is metered off the primary.

 

Is it possible to be Primary O2?