Got Code P0420(Catalyst System Efficiency Below Threshold Bank 1) on a 2002 Honda S2000 after emission test, got 37000 miles on the car, Manual. It was in the shop since 2017. It was barely driven since just a couple of months ago. I have driven
You might have a bad 02 sensor. You might have a component in the exhaust system that has deteriorated. If you have a Blue Drive scanner, or some unit with Enhanced Diagnostics and/or Mode 6 Data, check to see if the 02 sensor is functioning correctly.
See here: https://exhaustsystemsguide.com/oxygen-o2-sensors/
The point is that if the sensor is bad, or there's a problem with the exhaust system, the engine might run rich, which could trip the P0420 code.
If it's not that, Scotty has videos on catalytic converter diagnosis and treatment.
https://www.youtube.com/watch?v=qJTmddbOxMk
https://www.youtube.com/watch?v=5icTmYItwiE
https://www.youtube.com/watch?v=Qdl6m4GjkMk
1) Try an Italian Tune Up. No joke. Reset the codes, and get on the freeway, and drive as fast as legally possible for an hour or so. Helps to clean out the exhaust, and you will see if the codes come back or not.
2) Very rarely the codes will be due to bad O2 sensors. Try cleaning them. Or try switching their locations. In either case reset the codes, and see if the codes pop back up after trying these interventions.
3) If worse comes to worse, I recommend MagnaFlow cats. I replaced my Prius cats with direct fit MagnaFlow.
Check your own states requirements, because some allow you to just use universal fit and others require special cats that are usually only OEM. (Sometimes the direct fit can fit like an OEM, but there is no way universal fit will ever look like OEM)
