P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
Your car's computer has decided the catalytic converter on bank 1 (the side of the engine containing cylinder 1) is no longer cleaning exhaust gases as effectively as it should.
What this means for driving
P0420 itself is an emissions fault, not usually a safety-critical one: the engine typically keeps running normally. The risk of continued driving is mostly to the vehicle — a failing converter can get worse, and if the underlying cause is an engine misfire or a rich-running condition, unburned fuel can overheat and damage the converter further. It is worth having assessed promptly rather than ignoring it for months, partly because the car will fail an emissions test in most states while the light is on.
When in doubt, have a mechanic assess it promptly. If any of the red-flag symptoms below are present, stop driving and arrange a tow rather than risking it.
Important: This page is general information only — not professional mechanical advice and not a diagnosis for your specific vehicle. Fault causes and severity vary by make, model, year, and condition. A qualified mechanic should confirm any diagnosis before you make driving decisions or authorize repairs.
Red-flag symptoms: stop and get help
- Check-engine light flashing (not just steady)
- Noticeable loss of power or the engine struggling
- Strong rotten-egg (sulfur) smell combined with poor running
- Overheating or unusual noises from underneath the car
Symptoms of P0420
- Check-engine light on (often the only symptom)
- Reduced fuel economy
- Slight loss of engine performance
- Rotten-egg smell from the exhaust
- Failed emissions test
Most likely causes, ranked
- Failing catalytic converter — the most common confirmed cause once other items are ruled out
- Faulty downstream oxygen sensor giving a false reading
- Exhaust leak ahead of or near the converter
- Engine misfire or rich/lean running condition damaging the converter over time
- Engine oil or coolant contamination reaching the exhaust
How a mechanic diagnoses it
- A mechanic checks live oxygen-sensor data: a healthy converter makes the downstream sensor read fairly steady, while a failed one makes it mirror the upstream sensor.
- Exhaust system inspected for leaks, damage, or rattling inside the converter.
- Engine checked for misfires or fuel-trim problems that could be the root cause — replacing a converter without fixing those just kills the new one.
Likely fixes
- Replace downstream oxygen sensor
- Repair exhaust leak
- Fix underlying misfire or fuel-trim problem
- Replace catalytic converter (Shop job)
Repair costs vary by vehicle, region and shop. Get a quote from a local mechanic.
Related codes
- P0430: Catalyst System Efficiency Below Threshold (Bank 2)
- P0300: Random/Multiple Cylinder Misfire Detected
- P0171: System Too Lean (Bank 1)
- P0174: System Too Lean (Bank 2)
CheckMyCarCode content is general information only and is not a substitute for professional diagnosis. Trouble-code meanings are standardized for generic (P0xxx) codes, but causes, severity, and repairs vary by vehicle. If your car shows a red-flag symptom, stop driving and get professional help.