A catalyst efficiency code usually allows normal driving for now, but arrange a check and stop when safe if the engine runs roughly or the check-engine light flashes.
Trouble codes · P0420
P0420 meaning
Catalyst System Efficiency Below Threshold Bank 1
The same first verdict Car Assist shows from this code alone, before a diagnosis. The code does not tell the app whether the warning light is flashing.
What this code doesn’t tell you
A code says where the car noticed a problem. It does not prove which part failed, whether the fault is still present, or what repair this car needs. The engine, other codes, freeze frame and live readings matter.
Usual causes, most likely first
The order is a starting point across cars; the exact engine and its readings can change it.
- Ageing or damaged catalytic converter
- Exhaust leak affecting the monitor
- Oxygen sensor fault or unresolved mixture or misfire fault
What you can check
Keep all stored codes and note recent exhaust work or unusual fuel use. Have mixture faults and exhaust leaks checked before replacing the converter. Usually rough running or a flashing check-engine light needs a stop when safe and the engine switched off.
Words the car uses
- Stored codes
- Faults the car has confirmed and saved. They help explain a check-engine light, but a code alone does not prove which part needs replacing.
- Freeze frame
- A snapshot of the engine's readings when a fault was recorded. It helps show the conditions that caused the fault, even if the car now seems fine.
- Readiness monitors
- The car's own checks of its emissions systems. An unfinished check means the car has not tested that system yet, so a clean code scan may miss a fault.
- Catalytic converter
- The catalytic converter cleans harmful gases from the exhaust. Faults here can affect emissions, and misfires can damage it.
- Oxygen sensor (O2)
- A sensor that measures oxygen in the exhaust. It helps control fuel use or check the catalytic converter; B1/B2 name cylinder groups and S1/S2 name sensor positions.
- Misfire
- A cylinder has failed to burn its fuel properly. It can cause shaking and loss of power; a flashing check-engine light usually means stop when safe and switch off the engine.
Find out what’s causing yours
Car Assist reads your car through an OBD-II adapter and your iPhone. Its assistant uses the codes, freeze frame and live readings to work through the likely cause with you.