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2003 Buick Century

I have 2003 Buick Century. I started having problems with the car after coming to a stop, accelerating - the car starts to go hesitates, then jerks a bit to go. This does not happen every time after a stop but when it does the service engine soon light comes on. Initially I got the P0401 code (I think) and took it in to a mechanic. They could not find anything wrong with the car but did clean out the EGR valve, that didn’t work, they worked on my transmission and replaced other parts ($800) still no success. I took the car to another mechanic who replaced the EGR valve and several other small parts ($500). I still had the problem but this time the error code was P0420. I took it back again and replaced the spark plugs, O/2 sensor, catalytic converter ($800). That hasn’t fixed the problem. The mechanic said that they have taken my car for several test drives and the car has not demonstrated the problem. The error message still reads P0420 even after all of the parts have been replaced. The 2nd mechanic has washed his hands of my problem. Can anyone help?

P0420 is “catalytic converter efficiency low bank 1” (or words to that effect). Since your catalytic converter is new, it’s probably a failed oxygen sensor. Since the car is stumbling on acceleration, it’s probably the upstream sensor.

You need another mechanic. One that can put the oxygen sensor traces on an oscilloscope and see if the output from the upstream sensor is within spec and correct.

Note that the actions taken for the P401 code were appropriate. And, if your upstream sensor was bad in a way that was causing rich operation under load, the carbon deposits could be what messsed up the EGR valve operation.

The descriptions of the other work described are insufficient to make any guesses. But I’m not gettting a warm fuzzy feeling here.

Thank you for your advice. The 2nd mechanic still has my car. He is going to drive it around again today to see if he can get my car to demonstrate the problem. If he can’t, the company this mechanic works for is done trying to help me. I will take it to another mechanic and ask them to put oxygen sensor traces on an oscilloscope. I can’t tell you how much I appreciate your help, I am out of money and patience!