Catalyst monitoringDiagnose promptlyReviewed July 15, 2026
P0420Catalyst system efficiency below threshold — Bank 1
The Bank 1 catalyst monitor calculated efficiency below its calibrated threshold during a qualifying test.
Conditions that can produce P0420
These are diagnostic branches, not a ranked parts list. Prevalence and testing order depend on the exact vehicle and failure scene.
- Aged, damaged, contaminated, or incorrect catalyst
- Unresolved misfire or fuel-control problem
- Oil or coolant consumption
- Exhaust leak or sensor installation problem
- Upstream/downstream sensor or calibration issue
What to inspect next
- Resolve active misfire and mixture faults first
- Inspect for exhaust leakage and contamination sources
- Review exact monitor and Mode $06 evidence when verified
- Use application-specific temperature, gas, or efficiency tests before replacement
Evidence that strengthens the answer
Freeze-frame and readiness stateVerified upstream/downstream responseMode $06 result with exact scalingMisfire, fuel trim, consumption, and exhaust-leak evidence
Verify the repair
Preserve the pre-repair record, document exactly what changed, then repeat the relevant operating condition safely. Confirm no current or pending fault returns, review permanent-code and readiness state where applicable, and state which tests or modules remained unavailable.
This page explains a commonly standardized code at a general level. Confirm the exact definition, module, status, test conditions, thresholds, wiring, and service process for the vehicle application using current official information.