P0420
Catalytic converter efficiency below the computer's limit (Bank 1)
Severity: MonitorCatalytic converter (catalyst efficiency) codes
The catalytic converter cleans exhaust by converting harmful gases into less harmful ones. The engine computer judges how well it works by comparing an oxygen sensor before the converter with one after it. When the after-sensor starts to look like the before-sensor, the converter is no longer storing and using oxygen well, and a catalyst efficiency code sets.
How these codes differ
- P0420 and P0430 are the same test run on different sides of the engine: P0420 is bank 1 (the side with cylinder 1) and P0430 is bank 2. A four-cylinder engine has only bank 1, so it can only set P0420.
- One bank failing alone points to something on that bank (that converter, its downstream oxygen sensor, or an exhaust leak near that sensor). Both banks failing together makes it worth checking first for something both banks share, such as misfire history, fuel that is too rich, or oil or coolant being burned, because these can damage or fool both converters.
- A catalyst code is a conclusion drawn from oxygen sensor readings, so a sensor fault code (for example a heater or circuit code) on the same bank should be fixed first; the efficiency test may not be valid while a sensor is faulty.
Codes that show up together
- P0420 with P0430: both converters judged inefficient; look for a shared upstream cause before replacing either converter.
- P0420 or P0430 with any misfire code (P0300 to P0306): fix the misfire first. Misfire sends unburned fuel into the converter, which overheats it, and the regulation requires the light to flash when misfire is bad enough to damage the catalyst.
Catalytic converter (catalyst efficiency) codes
P0430
Catalytic converter efficiency below the computer's limit (Bank 2)
Severity: MonitorP0421
Warm-up catalytic converter not cleaning well enough
Severity: MonitorP0422
Main catalytic converter not cleaning exhaust well enough
Severity: MonitorP0423
Heated catalytic converter efficiency below the computer's limit (Bank 1)
Severity: Severity not verifiedP0424
Heated catalytic converter not reaching its expected temperature (Bank 1)
Severity: Severity not verifiedP0431
Bank 2 catalytic converter warm-up efficiency is below the limit
Severity: MonitorP0432
Main catalytic converter efficiency below the computer's limit (Bank 2)
Severity: Severity not verifiedP0433
Heated catalytic converter efficiency below the computer's limit (Bank 2)
Severity: Severity not verifiedP0434
Heated catalytic converter not reaching its expected temperature (Bank 2)
Severity: Severity not verified
Sources and review
- Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II), 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles (2015 amendments)
- 40 CFR §86.1806-04 On-board diagnostics (2011 edition page, includes the end of §86.1806-01(k) malfunction criteria)
- OBD: Frequently Asked Questions (EPA420-F-02-014, rev. September 2012)
- Diagnosing Catalytic Converter Problems
- What Causes a Catalytic Converter to Fail
- How to Fix P0420 Emissions Code
- What's really wrong when a catalytic converter 'fails' (Mike Mavrigian)
- Oxygen Sensors (technical PDF)
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error