P0423 — Heated Catalyst Efficiency Below Threshold Bank 1

P0423: Heated Catalyst Efficiency Below Threshold Bank 1 — what it means, the affected system and what to check before replacing parts.

Meaning, symptoms, causes and checks

Heated Catalyst Efficiency Below Threshold Bank 1. Basic catalogue definition; diagnostic details have not yet been reviewed.

Affected system: Powertrain

Symptoms

  • Reduced power, rough idle or higher consumption.

Possible causes

  • Degraded heated catalyst, faulty sensors or incomplete combustion.

What to check

  • Test sensor signals and wiring; diagnose combustion problems before replacement.

Also written as: P0423-00, P042300, PO423 (some scan tools add the fault-type byte, e.g. -00).