What does P0420 mean?

Catalyst system efficiency below threshold on bank 1. It means the post-catalyst oxygen sensor readings are too close to the pre-catalyst ones — commonly a failing catalytic converter, but also caused by a bad O2 sensor, an exhaust leak or an untreated misfire.

Does a P code always mean the engine?

It means the powertrain, which includes the engine, transmission and emissions systems. B, C and U codes cover body, chassis and network faults respectively.

Can I clear a code myself?

Yes, with any OBD-II scanner — but clearing without fixing the fault only resets the light until the monitor runs again, and it also erases the freeze-frame data a mechanic would use to diagnose it.

What is the difference between a generic and manufacturer code?

Generic codes (second character 0) are defined by SAE and mean the same on every vehicle. Manufacturer codes (second character 1) are defined by the carmaker and need brand-specific documentation.

OBD-II Trouble Code Decoder

Look up OBD-II diagnostic trouble codes and understand what each one means.

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What is the OBD-II Trouble Code Decoder?

An OBD-II diagnostic trouble code is a five-character identifier such as P0420 that a vehicle's engine control unit sets when it detects a fault, where each character encodes the affected system, whether the code is standardised, and the specific fault.

The structure is fixed. The first letter is the system: P for powertrain, B for body, C for chassis and U for network and communication. The second character is 0 for a generic code defined by the SAE standard and shared across manufacturers, or 1 for a manufacturer-specific one. The third indicates the subsystem — fuel and air metering, ignition, emissions, speed control, computer output — and the last two are the specific fault number.

So P0420, the most-searched code in existence, reads as a generic powertrain emissions fault: catalyst system efficiency below threshold, bank 1. And, importantly, it names a symptom rather than a part. That reading — 'the sensors downstream and upstream look too similar' — has many causes: a failed catalytic converter, but equally a worn oxygen sensor, an exhaust leak or a lingering misfire.

That gap between code and cause is the single most useful thing to understand about OBD-II. A code narrows the search; it does not identify the part to replace.

OBD-II Trouble Code Decoder at a glance

Standard: OBD-II · SAE J2012 · mandatory on US cars from 1996
First letter: P powertrain · B body · C chassis · U network
Second digit: 0 = generic SAE · 1 = manufacturer-specific
Connector: 16-pin DLC, within reach of the driver's seat

How to use the OBD-II Trouble Code Decoder

1Enter the code exactly as your scanner reported it, for example P0301.
2Read the system, standardisation and subsystem breakdown along with the fault meaning.
3Treat the result as a starting point for diagnosis, not a parts order.

When you would reach for it

Understanding a check-engine code before booking a garage visit.
Deciding whether a fault is emissions-related and will fail an inspection.
Learning what an inherited scan-tool report actually said.

OBD-II Trouble Code Decoder — frequently asked questions

Last reviewed 25 July 2026 · questions or corrections: contact@decoder.tools