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Fault codes po420 and po430

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I have a 2000 996 C4 that I purchased about two years ago. It has run perfectly with regular oil changes. Yesterday when I started the car in the mid afternoon, the check engine light came on. My frined used his durmetric and it shows two codes. They are po420 and po430. Dpes anyone know where I can find out what these codes are?? Assistance or advice appreciated.

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Online here you can look up fault codes...


P0420 TWC Conversion, Bank 1 – Above Limit

P0430 TWC Conversion, Bank 2 – Above Limit

Unless you have O2 sensor faults also this would say that both cats are bad and need to be replaced.

Very odd to have both go out at the same time (unless someone used leaded gas in the car).

I would double check the faults and certainly clear them and see if they come back.

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P 0420 and 0430 are codes for low three-way catalytic converter conversion on both banks. The system is looking at the O2 sensor readings before and after the TWC and not liking what it sees. Problem is that it would be very unusual for both TWC’s to crap out simultaneously. Five separate test sequences should be run in order to see if the cat is bad or the problem lies elsewhere. As the TWC’s are expensive, it would be worth getting the tests run by a competent tech.

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Thanks to all that replied. much appreciated. I have spoken with my tech, and we suspect that I may have over filled my oil after a recent oil change. Does anyone else agree with this possiblity :unsure:

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