OBD-II code family
Catalyst Efficiency Faults (8 codes)
8 related fault descriptions. Use each full code and the exact vehicle service information to identify its circuit, monitored condition, and required tests.
Codes in this family
This is the complete published member list for this guide. Each code retains its own page; shared diagnostic context does not mean that every member has the same test or repair.
- P0420Catalyst System Efficiency Below Threshold (Bank 1)
- P0421Warm Up Catalyst Efficiency Below Threshold (Bank 1)
- P0422Main Catalyst Efficiency Below Threshold (Bank 1)
- P0423Heated Catalyst Efficiency Below Threshold (Bank 1)
- P0430Catalyst System Efficiency Below Threshold (Bank 2)
- P0431Warm Up Catalyst Efficiency Below Threshold (Bank 2)
- P0432Main Catalyst Efficiency Below Threshold (Bank 2)
- P0433Heated Catalyst Efficiency Below Threshold (Bank 2)
What these codes mean
P0420/P0430 describe catalyst-system efficiency for banks1/2. P0421/P0431 name warm-up efficiency, P0422/P0432 main-catalyst efficiency and P0423/P0433 heated-catalyst efficiency. The labels identify different monitoring categories; they are not eight instructions to replace a converter.
Locate the monitored assembly using the engine’s exhaust diagram. “Warm-up” does not universally identify a separate small manifold converter: Mitsubishi’s 2023 DGE P0421 procedure places the catalyst in the main converter assembly. Ford’s covered summary also allows a bank1 or Y-pipe P0420 configuration. Bank numbers are not a universal left/right map.
Monitor design and prerequisites matter. A flat downstream trace, similar front/rear traces, a surface-temperature difference or an apparent oxygen-storage value is not a stand-alone verdict for every engine. Sensor technology, validity checks, mixture control and the specified operating state determine interpretation. Efficiency is also separate from exhaust restriction.
Concrete manufacturer examples help prevent unnecessary parts. Toyota’s dated T-TT-0686-22 identifies exhaust leaks as possible P0420/P0430 triggers within its stated scope. GM24-NA-058 lists P0422 among possible software-anomaly codes on specified2024L5P HD trucks. Apply their actual limits and current vehicle service guidance; a bulletin is not proof that this vehicle has that cause.
Save the scan and monitor information, investigate relevant engine/sensor faults, inspect exhaust integrity and perform the prescribed catalyst test. Repair the confirmed cause and verify monitor evaluation. Review possible warranty coverage before approving major replacement. Seek help for severe running problems, unsafe power loss, overheating, flashing warnings or cabin exhaust fumes.
Identify the circuit on your vehicle
Match the full DTC, reporting module, and any failure-type suffix to the service information for your exact year, make, model, engine, and equipment. An identifier alone does not establish installed hardware count, location, connector pinout, or a universal measurement.
Where the manufacturer publishes its own meaning for this code, that is the one to work from — it describes the actual component on your vehicle, which the generic definition cannot. Keep the generic description for what it is good for: it tells you which subject area the standard assigned the code to, which is useful context when the two appear to disagree. If they do disagree, follow the manufacturer.