1999 Chevrolet Prizm MIL light on all the time

Peter’s question:

” Hi , I drive a 1999 Chevrolet Prizm. It has MIL on all the time. There is a code: PO420. Where is the problem? I am seeking your advice. Thank you. Regards, Peter”

Answer:

OK, PO420 is the code for catalytic converter low efficiency. There are three possible causes: a bad catalytic converter, bad wiring, or bad oxygen sensors. THE most common problem is a weak catalytic converter, but pinpointing if this is your problem is rather hard. The oxygen sensors give feedback to the main computer about how the cat itself is working, and PO420 sets if the computer decides the cat is bad using this information. This is a very complex algorism in the computer software, and checking can be done by a professional mechanic indirectly using an oscilliscope. Since this costs a ton, and isn’t always completely accurate, you might just try replacing the rear oxygen sensor (the one after the cat.) It is the cheapest part in the system and a bad rear sensor can trip that code. Now if that doesn’t do it, a new aftermarket cat could be welded on your Prism, which would save you hundreds over the cost of an OEM cat. But it’s still going to cost you two to three hundred dollars for that guess. So I’d try the rear oxygen sensor first if you want to take a stab at it. BUT, on the plus side, an inefficient cat code really isn’t going to make your Prism run any different. It will just make your car fail the yearly state inspection if your area does emissions testing.


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