Common Checks
- Repair misfire, rich, or lean codes before condemning the catalyst.
- Inspect for exhaust leaks and oxygen-sensor wiring problems.
- Compare upstream and downstream oxygen-sensor activity.
Diagnostic trouble code
P0420 Catalyst System Efficiency Below Threshold, Bank 1
The PCM sees downstream oxygen-sensor behavior that suggests bank 1 catalyst efficiency is below threshold.
Catalyst Efficiency CodesCommon Repairs
- Repair the engine problem that damaged or confused the catalyst monitor.
- Repair exhaust leaks or oxygen-sensor faults.
- Replace the catalytic converter only after upstream causes are corrected.
Mustang Notes
- Long-running misfires can damage converters; a flashing MIL should be handled immediately.
Model-Year Applicability
Listed for 1996 and newer traditional gasoline Mustang pages in this first-pass catalog. Engine, calibration, and Ford-specific codes may narrow that range as more service data is added.