P0420 — Catalyst system efficiency below threshold (bank 1)
Generic powertrain DTC · MEDIUM priority for follow-up (not a certified safety rating)
What the code means
The monitoring system reports catalyst efficiency below its calibrated threshold on bank 1.
What you might notice
Check-engine light; sometimes no noticeable driving symptoms.
Possible causes — not a diagnosis
Exhaust leakage; incorrect fuel mixture; aging catalyst; oxygen-sensor response; unresolved misfires.
Useful diagnostic checks
Check for misfire and mixture codes first; inspect exhaust leaks and fuel trims; follow manufacturer catalyst monitor procedures.
Next steps
Do not replace a converter based on this code alone; eliminate upstream faults first.
Related OBD-II codes
These generic definitions are explanatory summaries of commonly standardized OBD-II powertrain conditions. Vehicle-specific code logic and service procedures must be verified against manufacturer documentation. EPA: On-board diagnostics.