Fault Code Search
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Fault-Code Lookup
Search an OBD-II code and get the practical first pass.
Start with the code from your scan tool, then work through the symptoms, likely causes, and next checks before you start replacing parts.
OBD-II fault codes are starting points. Confirm the root cause with basic testing and vehicle-specific service information before replacing parts.
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P0420
Catalyst system efficiency below threshold, bank 1
The catalyst monitor sees bank 1 oxygen sensor activity that suggests the converter is no longer storing oxygen effectively.
Severity
Moderate
Moderate on its own, but higher priority when paired with other running issues.
Common symptoms
Likely causes
Quick diagnostic steps
Check for stored or pending misfire, fuel-trim, and oxygen-sensor codes first
Review upstream and downstream oxygen sensor patterns on a fully warm engine
Inspect the exhaust for leaks ahead of the rear oxygen sensor
Confirm the engine is not consuming oil or coolant before condemning the converter
Driveability
Usually driveable, but do not ignore it if there are companion misfire or rich-running codes that can overheat the converter.
Recommended next checks
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