Hey all, the ol' lady drives a 2007 HHR, LT I believe. It's a temporary car but will likely need to have it at least another year or two. The check engine light came on and it was of course the P0420. The light will sometimes come on and turn itself off after a few days, last a day or two then come back on, ad nauseam. I decided to hook it up to a reader and check the O2 sensors and see what was going on in that department. The front is reading how it should, sporadic between .1 and .8 V. The rear sensor stays pretty much straight as it should, moves around between .73 and .75. I'm not sure if that is high or not to be honest. I have done thread searches here and all over the internet. I'm not exactly sure what to do at this point. I would hate to throw a few hundred dollars at a new cat and that not be the issue.


I guess long story short, what other tests can I run to narrow the issue down? I use Torque for my OBD scanner if that helps. Also, just to itch my curiosity, I reset the code on it the other day to see what would happen. She ended up driving it over 100 miles and no light, with 3 on/off cycles of the vehicle. Upon the 4th crank, she said the light came on right at startup. Not sure if that helps, or if it just took that long to present the stored code.


Thanks ahead of time for any input

 

Welcome to the site, this worked for me


https://www.chevyhhr.net/forums/2-2l...20-code-58413/

It's the rear O2 sensor

 

Well I understand that moving it out of the stream simply negates that reading to the ECM. But that just sort of hides the issue doesn't it? If the rear sensor is throwing the code, it must be reading an issue what's coming through. This may mean a rich or lean condition, which could be an entirely separate issue, right? I just want to make sure I'm narrowing the issue and not just hiding a problem that will come back to bite me.

 

Ok, I tested this theory, it worked for me, my readings are normal now, not hidden

The alternative is to replace the catalytic converter.
Which as you'll see is what I had to do.

 

Maybe I was misinformed about this method. I was told it takes it out of the stream so that it no longer reads it. If the readings are normal after pulling it back some, what is abnormal about my readings? Is the voltage too high on my sensor?

 

The intermittent part bothers me. There is a chance there is a wire laying on something hot or a loose bolt/burned gasket.

You need to compare the freeze frames from several different occurrences. Maybe that will show something.

I never figured out mine. Everything I did temporarily fixed it. .73 is about what mine read, it really doesn't matter if it stays steady, the OBD test is a very fast change that you cannot detect.

This is what the book says about it:

 

Could be the catalytic converter failing, clogging up, giving false information