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]Manufacturer communications
5Original wording in English
HONDA
Reported code forms P0420
Customer letter - In some cases, the electronic control module (ECM) misinterprets sensor inputs as a warm-up three-way catalytic converter (TWC) deterioration and turns the malfunction indicator lamp (MIL) on with…
HONDA
Reported code forms P0420
Customer letter - In some cases, the electronic control module (ECM) misinterprets sensor inputs as a warm-up three-way catalytic converter (TWC) deterioration and turns the malfunction indicator lamp (MIL) on with…
HONDA
Reported code forms P0420
Dealer message - Today, June 21, 2018, American Honda is announcing a Warranty Extension for 2006-07 Accord Hybrid vehicles to address the MIL illuminating with DTC P0420 or P0430. Basic Problem - The ECM may…
HONDA
Reported code forms P0420
Service bulletin - In some cases, the ECM misinterprets sensor inputs as a warm-up three-way catalytic converter (TWC) deterioration and turns the MIL on with DTC P0420 or DTC P0430 (catalyst system efficiency below…
HONDA
Reported code forms P0420
Service bulletin - In some cases, the ECM misinterprets sensor inputs as a warm-up three-way catalytic converter (TWC) deterioration and turns the MIL on with DTC P0420 or DTC P0430 (catalyst system efficiency below…
Check vehicle applicability
2A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
| Make | Model | Model year |
|---|---|---|
| Honda | ACCORD HYBRID | 2007 |
| Honda | ACCORD HYBRID | 2006 |
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.