Q: Catalytic converter is working, but will not kick in to pass smog.

asked by on November 07, 2016

Replaced sensors in 2000 Grand Prix, because it would not pass smog. Now all the sensors work, but the Cat will not kick in, to pass smog. The mechanics all say there is nothing wrong with the Cat. They said to drive it, and it would finally kick in. Well after 1000 miles, it has not kicked in. Three different repair stations can not figure out why it won't kick in. All the sensors are fine, and so is the Cat. They said it does not need to be replaced. Can you help? Thank you.

My car has 173000 miles.
My car has an automatic transmission.

The O2 sensors monitors catalytic converter efficiency, it does not control the catalytic converter. Catalytic converters either work well enough or they don’t. If it won’t kick in, it needs a new one. Cat’s work independent of all other systems. They simply catalyze the remaining HC’s (hydrocarbons) in the exhaust stream. If it isn’t doing this, it is bad. If the cat is original on your car with 173,000 miles, it is definitely due for one. This would be a normal maintenance cycle.

I have to point out, if the cat is not working, how can it be good? If it is good, your car would pass emissions. There are not any electrical components to turn it on. It is turned on when it reaches operating temperature as supplied by the heat that is created by the motor and transferred through the exhaust. There is only one possible system that can affect the cat operation. That would be the air injection system that is used to warm the cat up as quickly as possible when it is cold. If this system is not operating, then the cat will warm to slowly and will not operate soon enough in a drive cycle. Other the air system, a cat either works or it does not.

I recommend contacting customer service to see if there is a technician in your area with the tools and experience to diagnose this issue and service your catalytic converter as necessary. I find it disconcerting that three shops could not figure this out.

Good luck!

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