P0420 Fault Code Quick Tips!

catalytic converter P0420 fault code indicates a failure in the catalytic converter (CAT) as specified by the post oxygen sensor. As in numerous events like this, it is a worthy idea to evaluate the voltage fluctuation of the oxygen sensors located at the front and back of the CAT. At the front (usually located at the exhaust manifold), it should oscillate about 0.1 to 0.9 volt at
the signal wire whereas the back ones (located after the CAT) it should oscillate less say around 0.3 volt to 0.6 volt. Depending on the vehicle models, if the oscillation voltage value is greater than this, it indicates a CAT failure.


1. For P0420 fault code, most cat is guaranteed up to 8 years or 80,000 km so check with your dealer and service bulletins if your vehicle is covered. If your vehicle is not covered, you can buy an after market cat but beware it follows the manufacturer’s efficiency guidelines. To confirm this, get an exhaust analysis reading before and after the catalytic converter is installed.

2. On some imported models P0420 fault code test, there could be no voltage oscillations at the post oxygen sensors. To be safe, contact ATS for the right voltage readings. Make sure the engine computer (ecm) is fully cleared of the code after installing the new cat. If not, the P0420 code can return and give you a false error code.

3. If you are not equipped to check this P0420 fault code yourself, you can go to a small garage and have them check your CAT this way. Put the vehicle on a hoist and with the engine warmed up, ask them to check the temperature of the CAT on the inlet and outlet openings. There should be at least 100 degrees difference. If none or small difference, the CAT is bad and needs replacement. Make sure you use only OEM CAT parts (you replace it only after verifying your 80k emission guaranty from the dealer).


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