Powertrain, generic · Auxiliary emission controls

P0423 Heated Catalyst Efficiency Below Threshold Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Heated catalyst aged or poisoned
Heater not working, catalyst active too late
Sensor used for the assessment faulty
Calibration (Ram: software update per bulletin)

Symptoms

  • Check engine light on
  • Higher emissions after a cold start
  • Otherwise usually unremarkable

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,000
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

electrically heated catalystcatalyst heaterengine control module

Related codes

P0424P0429P0433P0420

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov, emitec.com.