P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

System: Emissions
Dashboard light: Check engine light (MIL)
Check-engine blink code: —
Applies to: Generic OBD-II meaning. Not every code is used by this car.
Description: Catalyst System Efficiency Below Threshold (Bank 1)
Detection / symptoms: 1.Check engine light on 2.Symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in…
Possible causes: 1.A code 2.P0420 3.Has happened 4.Leaded fuel was used where unleaded was called for (unlikely) 5.A damaged or failed oxygen / O2 sensor 6.Downstream oxygen sensor (HO2S) wiring damaged or connected improperly 7.The engine coolant temperature sensor is not working properly
What to check: 1.Read the code and live data with an OBD-II scanner 2.Look for exhaust or vacuum leaks and check the wiring of the sensor or valve 3.Test the part before replacing it 4.Fix the cause, clear the code and test-drive
Confidence: medium (generic OBD-II definition, not checked against the factory manual for this car)

Generic OBD-II meaning (obd-codes.com), not checked against this car's factory manual. Clear the code and test-drive to confirm it returns. Search all fault codes · Common problems · Warning lights