Code P0420

[Rick34]

My 2000 Miata has 27000 miles. I got the code P0420 Federal System below threshold. Does this mean I will need a new cat or could it be the O2 sensor?

[NoorAutoExpert]

I wouldn’t assume you need a new cat just from P0420. On a 2000 Miata, an aging O2 sensor, exhaust leak, or another engine issue can also trigger it. The best approach is to check for exhaust leaks and look at the front/rear O2 sensor readings before replacing anything.

With only 27k miles, I’d definitely diagnose it first rather than automatically buying a catalytic converter.