P0423 code meaning
Heated Catalyst Efficiency Below Threshold Bank 1
Powertrain system Generic code P04xx codes Check engine light
What P0423 means
An electrically heated catalyst has a heating disc ahead of a support catalyst so that it works soon after a cold start (Emitec EMICAT, 12–48 V); it was tried on Californian gasoline cars in the 1990s and is used today mainly on diesels, for example Audi's 3.0 TDI from 2014 and 48-volt diesel vans. P0423 means the heated catalyst of bank 1 (the side with cylinder 1) no longer converts enough; CARB requires it to be assessed like any other catalyst. The only manufacturer reference is two Ram software bulletins for the 6.7-litre Cummins HO (model years 2025/26), which list P0423 without saying which part is meant.
P0423 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyHard
- Typical labor0.5 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P0423
- Check engine light on
- Higher emissions after a cold start
- Otherwise usually unremarkable
Common causes of P0423
- Most likelyHeated catalyst aged or poisoned
- PossibleHeater not working, catalyst active too late
- PossibleSensor used for the assessment faulty
- PossibleCalibration (Ram: software update per bulletin)
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Electrically heated catalystCatalyst heaterEngine control module
What each part of P0423 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 23 | The exact fault in that area. |
What to do next
- Note P0423 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0423
What does P0423 mean?
P0423 means "Heated Catalyst Efficiency Below Threshold Bank 1". An electrically heated catalyst has a heating disc ahead of a support catalyst so that it works soon after a cold start (Emitec EMICAT, 12–48 V); it was tried on Californian gasoline cars in the 1990s and is used today mainly on diesels, for example Audi's 3.0 TDI from 2014 and 48-volt diesel vans. It is a generic code, so it means the same on every make.
Is it safe to drive with P0423?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0423 fail an emissions test?
Yes, it can. P0423 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0423 to fix?
Difficulty: hard. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P0423 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.