I just bought a H2 a few days ago and the previous owner said he replaced an O2 sensor because of a check engine light and that it came on and off after it was replaced too.

So today, the CEL lit up and I put my scanner on it. Turns out it is a P0420 code, which is a catalytic converter efficiency code. (meaning the cat isn't doing it's job) There are a few things that can cause this, but I find that some people just decide to throw parts at it because they don't know how to diagnose the code.

It took me 2 minutes to figure out what was wrong and that includes jacking it up to crawl under it. I get under the truck, look and see the new sensor is behind the converter on the drivers side. I see the two converters under the truck, and put my hand on each one. (truck was running a little bit before that). The drivers side is cool, pass side is smoking hot. Converters are a restriction in the exhaust, so if the element is all there, it will retain a ton of heat. So, the drivers cat is completely gone inside, and knocking on the case of it also confirms my suspicion with a hollow sound.

As a temporary work around, I took the rear o2 out and installed a spark plug non-fouler between the o2 and the exhaust bung. This simulates the cat being in tact by lessening the exhaust gas signal to the sensor.

The computer uses the rear O2 for one thing, and one thing only.. to monitor the efficiency of the catalytic converters. (yay hippies..) The PCM will compare the signal from the front O2 and the rear O2. Basically, if the two signals are too similar, the PCM will know that the cat isn't doing it's job and throw the CEL on.

Anyhow, that is enough rambling for me.

Later,

 

The Cat Converters are supposed to be covered under warranty for 80,000 miles so you can probably get yours replaced free
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Creampie Vids

 

Interesting.. I'm about 1000km away from 80,000 miles, so I think I'll check it out. It is a US truck that was imported into Canada, so I hope that it doesn't complicate things..

Thanks for the heads up!

 

Your diagnostic abilities made this H2 owner a bit envious.