Hey guys this is my first post here. I just bought a 98 4cyl accord coupe with 140,000 miles. It runs great and drives just like a new car. I have had one problem with it though, the check engine light is on. The error is catalyst below threshhold. I had a local muffler shop replace the Cat. and the light still comes on. I took it back down to the shop and they put their computer on it and everything looks good. Will this light eventually go off or could my O2 sensors be bad. Any advice on what to do or if I should just ignore the light. The car still runs well I just don't want to damage anything else. Thanks

 

Welcome to the forum!

Did they by chance give you the actuall error number? If they re-tested it and there is no code present then they may just need to clear the code (autozone will pull and I believe clear codes for free). I'd have someone try and pull a code and if nothing is present then clear the error and see if it comes back. If it comes back then based on what you've said I'd assume it fouled your O2 sensor from having a bad cat... hope this helps! Post the code if you know it.

 

No there was not a code number. I was watching him while he did it. I did have him clear the error and the check engine light came back on the next day. We were also able to look at both O2 sensors and they appeared to be functioning correctly. However the second one did jump around a bit, but would eventually catch up with the first one. When they replaced the converter they used a universial cat with out the sensor hole in it. Now the O2 sensor is located right before the cat. They assured me this would be ok. I guess I can swing by Autozone and have them check it out.

 

The first O2 sensor and second O2 sensor should NOT look the same. When i say first and second, i mean on the same bank. So sensor 2 would be the sensor behind the cat. Sensor 1 should have a reading that osscilates b/w 0 and 1v. Sensor 2 should have a reading that is near 0v (lean mixture).

 

Wow.. If that is right then my sensors were all out of wack. I was watching the handheld computer the mechanic was using and the voltage on the sensors were jumping around all over the place. Not sure if I trust this mechanic and what he is telling me. Is there any software out there I can install on my laptop and get this dignostic info myself?

 

If your willing to shell out about $200, then yes. AutoEnginuity There is a whole thread about this somewhere. Remember that the first O2 sensor is suppose to oscillate.

 

The 2ndary o2 voltage should be bouncing around too. Not quite as much as the primary though, and a little out of phase.

 

I'm kind of curious about that. The reason you have excess air in the exhaust is for the catalyst to work. After the cat, that excess is mostly used up.

So... since the O2 sensor measures oxygen, shouldn't it read low levels of oxygen? (Which would be interpreted as rich.)

 

You guys both raise a good point and i looked up the O2 sensors in my reference book at it seems that the second O2 sensor is suppose to bouce around as well, but it does not explictly say that. The doubt i have is that the caption says something about a bad cat and i think its demonstrating a bad cat and not a good one. I always thought that the O2 sensor would read lean after the cat b/c there shouldn't be any fuel left in the exhaust after this point. The reason that i don't think it should be bouncing around is b/c the cat should burn up all the fuel, so in the spots where the O2 was lean the cat wouldn't do anything but when it was rich the cat would burn up the fuel and make it lean. I've looked at O2 sensor reading in auto shop and i have a gut feeling that the 2nd O2 sensor was showing as lean all the time but i can't say that 100% for sure.

 

Never ever ignore the check engine light. It will hurt gas milage and performance.

Sounds like the o2 sensor may be bad. You can actually read the voltages from the sensors yourself with a multi meter from radio shack. I'm pretty sure the green wire is the signal for the sensor, but someone better double check me.