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P0423 Generic (SAE) Basic entry

Heated Catalyst Efficiency Below Threshold

Updated 2026-10-02 · Auxiliary emission controls

Severity
Medium
Typical repair cost
300 – 2 000 EUR
System
Powertrain

What it means

P0423 is stored when the monitored efficiency of the Heated Catalyst falls below the threshold programmed by the manufacturer, usually measured by comparing sensors before and after the component.

This entry is compiled automatically from the standard definition and typical diagnostic practice for this type of fault. A detailed, reviewed article is in preparation.

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Symptoms

  • Check engine light on, often with no change in driveability
  • Failed emissions inspection
  • Slight increase in fuel consumption
  • Smell from the exhaust in some cases

Common causes

  1. Component worn out or poisoned by oil, coolant or fuel additives
  2. Faulty downstream sensor giving a false efficiency reading
  3. Exhaust leak before the rear sensor
  4. Upstream problem (misfire, rich mixture) damaging the component
  5. Incorrect or low-quality aftermarket part

How to diagnose

  1. Fix any other stored codes first, especially misfire and fuel trim codes
  2. Inspect the exhaust for leaks and rattling internals
  3. Compare live sensor data before and after the component
  4. Check the component temperature or pressure differential as applicable
  5. Replace the component only after confirming sensors and engine are healthy

Frequently asked questions

Can I keep driving with P0423?

Usually yes for a short time, but expect reduced performance or higher fuel use. Have it diagnosed within a few days.

Will clearing P0423 fix it?

No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.

Related codes

P0433

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