P0423: Heated catalytic converter efficiency below the computer's limit (Bank 1)
Heated catalyst efficiency below threshold, bank 1
Severity: Severity not verified — if the car runs badly, have it inspected
The sources define the code but none rates how urgent it is. The catalyst monitor reports reduced emissions cleanup rather than a failed driving component, but no source says so for this code, so the severity is not verified.
P0423 in short
On the code lists, P0423 means the heated catalytic converter on bank 1 is working below the efficiency the computer requires. TroubleCodes.net lists the probable causes as the catalytic converter, wiring and the rear oxygen sensor. A scan tool test helps tell which part failed.
The computer tests how well the bank 1 converter cleans the exhaust. When the result falls below its limit, it stores P0423.
TroubleCodes.net lists the catalytic converter, wiring and the rear oxygen sensor (HO2S 2) as the probable causes, and says bidirectional scan tool tests are needed to confirm which part actually failed rather than which circuit reported it.
Bank 1 is the side of a V or flat engine that does not hold cylinder 1; check the vehicle's service information for the bank layout.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Catalytic converter on bank 1 worn or damaged
Common general cause- Why it ranks here
- The converter is the part this monitor is built to judge, and TroubleCodes.net lists it first among the causes. It ranks first only if sensor, leak and misfire checks come back clean.
- What would change this
- Rises if the rear oxygen sensor switches like the front one with no leaks or misfire codes. Falls if a leak or sensor fault is found.
Cause 2: Oxygen sensor or its wiring
Common general cause- Why it ranks here
- The code is judged from the oxygen sensor signals, and TroubleCodes.net lists the sensor and wiring as probable causes, so a lazy sensor can mimic a bad converter.
- What would change this
- Rises if other oxygen sensor codes are stored or the sensor responds slowly. Falls if both sensors respond normally.
Cause 3: Exhaust leak ahead of the converter
Common general cause- Why it ranks here
- Extra oxygen from a leak skews the sensor readings, which TroubleCodes.net says to rule out before blaming the converter.
- What would change this
- Rises if a leak is found at a flex joint, gasket or rusted section. Falls if the exhaust is sealed.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all stored codes and freeze-frame data. Note any misfire, fuel-trim or oxygen sensor codes stored with P0423.
IfMisfire, fuel-trim or oxygen sensor codes are also stored
It meansRepair those first, then clear codes and recheck; the converter can't be judged until they are fixed.
IfP0423 is the only code
It meansLook for exhaust leaks next.
Step 2: Have the exhaust inspected for leaks between the engine and the converter: flex joints, gaskets and rusty spots.
Warning:The exhaust is very hot after driving and carbon monoxide leaks out of any break. Inspect only when cold and never run the engine in a closed garage.
IfA leak is found
It meansExcess air skews the oxygen sensor readings; repair it and recheck.
IfNo leak is found
It meansCompare the sensors before and after the converter.
Step 3: Watch the oxygen sensor readings before and after the converter on a scan tool, at idle and at steady highway speed.
IfThe sensor after the converter switches like the one before it instead of holding steady near 0.5 volt
It meansThe converter is likely compromised.
IfThe rear sensor is steady but slow or erratic elsewhere
It meansSuspect the sensor or its wiring.
Symptoms
- Check engine light on
How to fix
Step 1: With the exhaust cold, look for loose or rusted exhaust joints and any damaged wiring near the oxygen sensors.
Step 2: Have other stored codes, especially misfire, fuel trim and oxygen sensor codes, diagnosed and repaired before any converter work.
Parts and cost context
- Catalytic converter:Replace the converter only after sensors, leaks and misfires are ruled out. Get a quote.
- Oxygen (O2) sensor:A slow rear sensor can mimic a failing converter; check its response first.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P0423 yet. The causes and tests above apply across vehicles.
Related codes
Sources and review
- TroubleCodes.net pages for P0423, P0424, P0432, P0433 and P0434
- On-Board Diagnostic Trouble Codes (generic code title list)
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error