P0423Heated Catalyst Efficiency Below Threshold Bank 1
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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 petrol 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.
- What you may notice
- Check engine light on
- Higher emissions after a cold start
- Otherwise usually unremarkable
- Likely causes
- LikelyHeated catalyst aged or poisoned
- PossibleHeater not working, catalyst active too late
- PossibleSensor used for the assessment faulty
- PossibleCalibration (Ram: software update per bulletin)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €150 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.