02 bonneville keeps throwing service engine soon error

Hello group,

I have searched high and low for the answer to my question with no avail. I currently own a 2002 Pontiac Bonneville. the vehicle had been in an accident right before I bought it. but has since been fixed up and road worthy.

the car started throwing a Service Engine soon code, ( I cant remember what the original code was). I had it tested by Orilies and by the garage I bought the vehicle from. Each code indicated the 02 sensor needed to be replaced on Bank #1. I replaced the 02 sensor and drove the car for another 3 days when the service engine soon light returned. I took it back to the dealership I bought it from and the error code this time was

"PO420 Catalyst Efficiency Low Bank 1"

I was instructed to take my car to an Exhaust shop to have the Cat converter tested. The Exhaust shop drilled a hole in my muffler between the Cat and the Engine and tested for blow-back to see if the Cat was clogged. This test was negative and it was determined that my Catalytic converter was not clogged. I was then instructed to swap out the second 02 sensor down past the Catalytic Converter. I replaced the 2nd 02 sensor on Sunday and the "service engine soon" light appeared again today.

so just to recap I did the following.
1) swapped the Bank1 02 sensor with a brand new one.
2) had the Catalytic converter tested.
3) replaced the bank2 02 Sensor with a new one.


any help as to what this problem might be linked up to would greatly needed.

A cat inefficiency code is caused by a cat not effectively doing its job. It has nothing to do with the cat being plugged so the back pressure test was an ineffective test. The O2 should have been monitored with either a scan tool or a voltmeter to see if it is holding steady or switching. This would have given you a much better idea of the state of the cat.

The basic answer to you question is this.

The post cat O2 sensor monitors the cat efficiency. If the PCM sees the rear O2 sensor switching rich to lean simular to the upstream 2 sensor than it knows that the cat is no longer functioning as intended. There are some more technical things going on with how the PCM determines the health of the Cat but that is the basic answer.

A new Cat will likely solve your issue.

Here's the tricky part. Not all cats are created equal. An O.E. cat will work the best in your vehicle. A direct fit aftermarket cat from a reputable supplier should also perform well. Stay away from universal type cats as they usually cause more issues than they solve.

If he wants an OE design he will need a Heavy Metals Cat... Even some of the Direct fit High flow Cats can set the SES off for the rear O2.... If it says Not for use in Arizona and or California, then that will not be an OE Cat...lol

Good Luck