First time poster here on the forum, but I have been viewing threads for quite some time.

I own a 2006 Hummer H3. The truck now has just under 85,000 miles on it. For about a year now, I have had a code on the truck. It is as follows...

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

After much research, I am still stumped, and I am hoping someone has experienced this before and can lend me a hand.

I have read all of the things that can cause this, and they are as follows...

1. An oxygen sensor is not reading (functioning) properly
2. The engine coolant temperature sensor is not working properly
3. Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
4. Retarded spark timing
5. The oxygen sensors in front and behind the converter are reporting too similar of readings

I think I have ruled out the cat converter being faulty by performing the following...

As per a suggestion from another thread I read, I attempted to trick the downstream O2 sensor to detect a different reading from the upstream one. I did this by using sparkplug nonfouler to back the senser out a bit to "sniff" less exhaust. This worked for about 50 miles or so, then the code came back on. Also, I felt the converter after the truck was running, and it is hot as hell, so I am assuming that it isn't blown out, also, when tapped, does not sound hollow. And finally, if it were clogged, I would notice a loss of power, which I do not decect any such loss.

I don't detect any leaks anywhere in the exhaust system.

So, it seems to me that the next logical step would be to replace the O2 Sensor. I wanted to pick your brains before I did so, because I for one hate throwing money at a problem before having a pretty good notion that is the problem.

Anyone elso have this code thrown? What did you do to resolve it? Does anyone know how to test a O2 sensor? Possibly with a volt meter or something?

Thanks in advance for any help. I tried to search the forum for an answer, but did not find anything helpful. Hopefully we can figure this out, and have this issue resolved for future H3 Owners.

 

Oh, and it might be helpful to note that my gas mileage is a whopping 11 mpg.

This is approx. 80% city, 20% highway.

I know these vehicles arent exactly green, but that seems excessive.

Could this be due to a faulty o2 sensor?

 

I don't know about that code, but does seem logical to start with the O2. We are averaging 15-16mpg and we have 33" tires. So 11 is really low I would say. With your mileage also have you ever cleaned the throttle body? That should be done. Not too hard to do. There are threads on here to do that also step by step. Our mileage improved after cleaning it. I think scheduled cleaning for the TB is at 70,000 miles.

 

Yeah, I just cleaned it. Thanks though!

 

Trouble Code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions

Some suggested steps for Troubleshooting a P0420 error code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter


Chris
(PS borrowed this from another site/dude...)

 

Most likely it is the O2 sensors. Most sophisticated scanners can give you the readings of the sensors while the engine is running. Plus people don't realize that O2 sensors are a routine maintenance item like spark plugs are. I suggest replacing O2 sensors every 50-60 thousand miles.