P0423: Heated Catalyst Efficiency Below Threshold (Bank 1)

What P0423 means

P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 4, places it in the auxiliary emission controls subsystem; the last two digits identify the specific fault.

The catalytic converter stores and releases oxygen as it cleans the exhaust. The engine computer judges its health by comparing the upstream O2 sensor (which switches rapidly) with the downstream one (which should stay comparatively steady while the converter is working). A downstream signal that starts mirroring the upstream one means the converter is storing less oxygen than required.

Bank 1 is the side of the engine that contains cylinder 1. Inline engines have only bank 1, so a bank 2 code on an inline engine usually indicates a manufacturer-specific use.

What “Efficiency Below Threshold” means here

The catalyst monitor judged the heated converter on bank 1 to be storing less oxygen than the threshold requires. It is a measurement of performance, so it can be caused by an ageing converter, but also by exhaust leaks, a slow downstream O2 sensor, or upstream problems such as misfires and fuelling errors that have damaged it.

What you may notice

Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0423:

• Often no drivability symptom
• Rotten-egg smell
• Rattle from a broken substrate
• Loss of power if the converter is blocked
• Failed inspection

Where to start diagnosing P0423

1. Fix any misfire or fuelling codes first; they damage converters and can cause false efficiency readings.
2. Check for exhaust leaks near the O2 sensors.
3. Compare upstream and downstream O2 sensor activity on a warm engine at steady cruise.
4. Confirm the downstream sensor itself is responding before condemning the converter.

Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.

Live data to watch

These standard OBD2 values relate to P0423. Support varies by vehicle; a value your car does not report will simply be missing.

Related codes

Codes from the same catalytic converter family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.

Further reading

• It failed the emissions test
• Something is rattling underneath
• O2 sensor switching and waveform analysis
• OBD2 readiness monitors explained

Frequently Asked Questions