What does this code mean?
The controller judged catalyst performance below its threshold under the monitor's test conditions. P0420 refers to bank 1 and P0430 to bank 2. Inspect exhaust leaks and related misfire or mixture codes; assess sensor signals and the vehicle-specific catalyst test. The code alone does not prove the catalyst or an oxygen sensor needs replacement.
Sources and interpretation [1]
A code identifies a detected condition, not a part that must automatically be replaced.
Manufacturer wording
Source wording can vary by vehicle and suffix. It is shown as written, not as a universal diagnosis.
P0420rear bank catalyst system efficiency below threshold [bank 1]P0420CATALYST SYSTEM EFFICIENCY BELOW THRESHOLDManufacturer communications
6Original wording in English
HONDA
Reported code forms P0420
"Owner Notification Letter - On some 2016-17 Pilot, 2017 Ridgeline, and 2018 Odysey models, the malfunction indicator light (MIL) may come on if one of the following diagnostic trouble codes (DTCs) are stored: - P0420…
HONDA
Reported code forms P0420
"Owner Notification Letter - On some 2017-19 Pilot, 2017-19 Ridgeline, and 2018-19 Odyssey models, the malfunction indicator light (MIL) may come on if one or more of the following diagnostic trouble codes (DTCs) are…
HONDA
Reported code forms P0420
"Service Bulletin - The PCM software may misinterpret sensor inputs as a deteriorated catalytic converter. Additionally, debris from the high pressure pump or the fuel injector machining process can cause internal wear…
HONDA
Reported code forms P0420
"Service Bulletin - The PCM software may misinterpret sensor inputs as a deteriorated catalytic converter. Additionally, debris from the high pressure pump or the fuel injector machining process can cause internal wear…
HONDA
Reported code forms P0420
Service bulletin - The MIL is on with DTC P0420 or DTC P0430 (catalyst system efficiency below threshold) with a possible DTC P2243 or DTC P2247 (A/F sensor 1 circuit high voltage). One or both primary A/F Sensor (Air…
HONDA
Reported code forms P0420
MALFUNCTION INDICATOR LAMP IS ON WITH DIAGNOSTIC TROUBLE CODE P0420 (CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD) IS STORED. *TT *MJ
Check vehicle applicability
6A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Sources and interpretation
These explanations are editorial summaries of the linked manufacturer material. PDF page numbers count from the first file page. Original publications are in English; translations are reference aids. A cited edition is not a claim that it is the manufacturer's latest instruction.
- 2024–2025 MY OBD System Operation Summary for Gasoline Engines ↗
Ford Motor Company · PDF page(s) 2, 3 ·
Source check date · 2026-10-27
Can I keep driving?
A code alone cannot establish whether driving is safe. A flashing malfunction light, severe shaking, overheating, oil-pressure warning or loss of braking or steering needs prompt attention. Stop in a safe place and seek assistance when control or engine safety is affected. Follow the vehicle handbook.