Happy to be a new member, Looks like a great forum for Monte Carlo fans. I have a 2003 Monte Carlo SS with the 3800 V6. Bought new. The car has 133000 miles at this time. At 105000 miles it needed a catalytic converter. Replaced with a good quality converter $260 wholesale but didn't buy the GM replacement. Check Engine light is on and reading the code 0740 or 0470 I'd have to find the paper work. Cleared the light twice and then replaced the sensor behind the converter and the light came back on the next day.

Any help would be great.

Thanks
DJ58

 

it could be both the front or back sensor o2. good rule of thumb is to replace them both at the same time due to how the old one went could have taken them both out. but it could be a limp mode due to when the old one went it might need to have the battery disconected for a while when you clear the codes for a little to see if the sensor codes will clear finnally

 

Um, P0740 is a torque convertor clutch solonoid malfunction in most cases.

 

Also if the code is P0420 then it's probably due to the fact that you are using an aftermarket Catalytic converter.

Nothing would be wrong with the cat or the sensor, but since it probably flows better than the crappy stock cat, it will throw that code. Many people who mod their cars have a problem with this when they do aftermarket downpipes.

Only options are to fake the sensor or to get the code programmed out of the PCM

 

Thank you for the info, I think we may replace the sensor's in front of the converter and behind and see if that takes care of the problem. My mechanic cannot cancel the codes from the PCM and doesn't know how to fake the sensors. Any info on faking the sensors might be helpful even in other problems they may run across.

Thanks
DJ58

 

It's by putting a resistor in place of the O2 sensor

 

Thanks, will pass on.


DJ58

 

My Dad's LS has an aftermarket converter and does the same thing. Since we know its ok, I just clear it occasionally and we don't worry about it. It would be nice to get rid of it permanently, though.

 

The part I don't understand is this didn't start happening untill about 14 months after the new converter.

DJ58

 

That IS odd.