What does P0420 mean?
What the code actually says
P0420 is a generic powertrain code that translates to 'Catalyst System Efficiency Below Threshold (Bank 1).' Bank 1 is the side of the engine that contains cylinder #1. On a V6 or V8, there may also be a P0430 for bank 2. The code doesn't say the converter is definitely bad—it says the computer's math doesn't add up.
The engine computer compares the oxygen sensor before the catalytic converter (upstream) with the one after it (downstream). A healthy converter stores and releases oxygen, so the downstream sensor should show a fairly steady voltage. If the downstream sensor starts switching like the upstream one, the computer sets P0420.
Common causes and fixes
The most common cause is a worn-out catalytic converter, especially on high-mileage cars. But other issues can trigger the same code: a faulty downstream oxygen sensor, an exhaust leak before or after the converter, an engine misfire or rich condition that damaged the converter, or even oil burning from worn valve seals. Diagnose before replacing the converter, because it's expensive.
A mechanic will check for exhaust leaks, test the oxygen sensors, and look at live data to see if the downstream sensor is switching too much. If the converter is truly bad, replacement is the fix; aftermarket converters are cheaper but may not last as long as OEM. In some states, a P0420 will fail an emissions test.
- Check for exhaust leaks first—they're cheap to fix.
- Test oxygen sensors; a lazy downstream sensor can mimic a bad cat.
- Look for misfire codes (P0300 series) that may have damaged the converter.
- Avoid cheap aftermarket converters if you need to pass emissions.
- Clear the code and see if it returns; if it does, diagnose further.
- Address any oil burning or rich running to protect the new converter.
Common mistakes
- Replacing the catalytic converter immediately without checking oxygen sensors or exhaust leaks.
- Ignoring other codes like P0300 that could have caused the converter to fail.
- Assuming the code means the converter is completely dead—it may still work partially.
