Q: P0420 code comes back repeatedly, emissions readiness issue. Takes about a week for Check Engine Light to come back on.

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Repeat code P0420 for Catalyric inefficiency bank 1. However Catalytic and 02 systems show readiness on scanner, Secondary Air system shows not ready.

I have checked for vaccum leaks and fixed a couple. Replace coolant temp sensor and flange becuause of a minor leak. Replaced all spark plugs because of a bad coil. Changed the o2 sensors with OEM Bosch parts.

Also previous owner thought the air pump was bad and replaced it earlier this year.

When I changed the o2s I inspected the Cat. Looks good externally and doesn't make the rattle noise when you shake/hit it.

I have experience working on VW/Audi but this code just keeps on coming back.

I don't want to guess and just change the catalytic if it is not actually bad, but this would be my next diagnostic.


My car has 130000 miles.
My car has a manual transmission.

If this is the only code that keeps coming up, and the data stream on the oxygen sensors are reacting correctly, the only real other thing left would be the convertor. If there was a wiring issue, or a sensor not responding correctly, you should be able to pick that up with the numbers on the scanner.

According to AllDataPro, the suspect would be the convertor due to the fact that P0420 is coming up by itself. If there were additional codes for the oxygen sensors, then there would be other places to look. If you still have doubts about the function of the convertor, you could always perform an exhaust gas diagnosis.

To avoid any unnecessary repairs, have one of our mobile technicians come to your home to perform these checks and properly diagnose the trouble code firsthand.

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