All codes
P0423Emissions System
Heated Catalyst Efficiency Below Threshold (Bank 1)
P0423 indicates the heated-catalyst monitor reports efficiency below the calibrated threshold on Bank 1. Learn what to inspect before replacing parts.
What you may notice
Symptoms
- 01Warning light may illuminate
- 02Intermittent or implausible live data
- 03Symptoms vary by vehicle
Where to look first
Common causes
- 01Catalyst degradation, incorrect mixture, misfire, exhaust leakage, or a sensor signal problem can affect the test.
- 02Wiring or connector fault
- 03Vehicle-specific operating condition
A sensible sequence
Best diagnostic order
- 01Save codes and freeze-frame information
- 02Resolve misfire and mixture faults first, then verify the relevant sensor signals and catalyst monitor using vehicle-specific procedures.
- 03Compare live data with vehicle specifications
- 04Confirm the repair under the conditions that set the code
Step by step
What to check and repair
- 01Resolve misfire and mixture faults first, then verify the relevant sensor signals and catalyst monitor using vehicle-specific procedures.
- 02Repair the confirmed cause and verify the result
Read the signals
Live-data clues
Affected sensor or switch input
- Typical behavior
- Plausible and consistent with operating conditions
- Possible clue
- A fixed or intermittent value can help localize the circuit fault
| Parameter | Typical behavior | What it can suggest |
|---|---|---|
| Affected sensor or switch input | Plausible and consistent with operating conditions | A fixed or intermittent value can help localize the circuit fault |
What P0423 means
The heated-catalyst monitor reports efficiency below the calibrated threshold on Bank 1. This code describes an observed condition and does not prove that the sensor itself has failed.
Diagnostic direction
Resolve misfire and mixture faults first, then verify the relevant sensor signals and catalyst monitor using vehicle-specific procedures.
Before clearing the code
Save stored and pending codes and freeze-frame data. Use the procedure for the exact vehicle, repair the verified cause, and confirm the fault does not return.