P0420 · Catalyst efficiency below threshold (bank 1)
The catalytic converter, the part that reduces harmful exhaust gases, is performing below the permitted limit.
- Worn or aged catalytic converter
- Faulty or slow-responding lambda sensor
- Leak in the exhaust system
Choose a point on the car or an area from the list
Red: stop or check immediatelyAmber: check soon
Fault code
Enter the code read with an OBD tester (for example P0420) and we will explain it in plain language.
Common codes
The code appears on the OBD tester or in apps connected to the car’s diagnostic socket (usually under the dashboard, on the driver’s side). If you do not have a tester, we can read it for you at the garage.
Fault-code (DTC) definitions follow SAE J2012 / ISO 15031-6, and reading through the OBD-II socket follows SAE J1979 / ISO 15031-5: confirmed codes (mode 03), pending (mode 07), permanent (mode 0A), freeze frame (mode 02) and live data (mode 01).
General guidance, not a diagnosis. The real cause can only be established by examining the car and reading its systems with a computer.