Catalyst system efficiency below threshold — Bank 1
P0420 means the Bank 1 catalyst monitor calculated efficiency below its calibrated threshold during a qualifying test. It does not identify a failed part or establish a repair price by itself.
Conditions that can produce P0420
These are diagnostic branches, not a ranked shopping list. Testing order depends 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
What strengthens the answer
Prove what changed
Preserve the pre-repair record, document exactly what changed, and repeat the relevant operating condition safely. Confirm that current or pending faults do not return, review permanent-code and readiness state where applicable, and state which tests or modules remained unavailable.
P0420 questions
What does P0420 mean?
The Bank 1 catalyst monitor calculated efficiency below its calibrated threshold during a qualifying test.
Does P0420 identify the broken part?
No. The catalyst may be degraded, but misfire, mixture problems, oil or coolant consumption, exhaust leaks, incorrect sensors, temperature issues, or calibration can influence the result and can damage a replacement catalyst. Confirm the exact vehicle definition and test the relevant branches before ordering a part.
Is it safe to drive with P0420?
Avoid postponing diagnosis. Reduce unnecessary driving if operation changes, and stop for a flashing warning lamp, severe drivability, smoke, fuel odor, overheating, braking or steering changes, or electrical heat.
How much does P0420 cost to repair?
The code alone cannot produce a defensible repair price. Cost depends on the confirmed cause, exact vehicle, parts choice, labor time, local labor rate, tax, supplies, and whether related damage is present.
What should I record before clearing P0420?
Record the reporting module, stored or pending status, freeze-frame data, readiness state, related codes, symptoms, battery voltage, and unavailable evidence. Clearing first can erase the most useful failure-scene context.
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 using current official information.
See how this page earns trust.
These links are primary-source starting points for the page's scope. They do not replace the current manufacturer procedure, wiring, specification, or service information for a specific vehicle.
Publisher
ScanWrench Editorial is the working byline for this library. Joshua Black, founder and publisher, is responsible for publication decisions, disclosures, and corrections.
A reviewed date means an editorial scope and safety check. It is not an ASE credential, OEM authorization, or vehicle-specific repair approval.
Primary sources
- SAE J2012 — Diagnostic Trouble Code DefinitionsSAE International · Standardized DTC structure, definitions, and manufacturer-controlled ranges.
- SAE on-board diagnostics standards indexSAE International · Primary standards family for regulated OBD services, identifiers, and transport context.
- OBD frequently asked questionsU.S. Environmental Protection Agency · Public explanation of regulated emissions diagnostics and the malfunction indicator light.
Revision history
- Reviewed edition: claim boundaries, safety language, internal links, and cited source scope checked.
No material correction has been recorded after this edition. If that changes, the correction and review date will be listed here.
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