Powertrain, generic · Auxiliary emission controls

P04F2 Catalyst Heater Supply Voltage Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

An electrically heated catalyst has a heating disc ahead of the catalyst so that it works soon after a cold start; Emitec's EMICAT runs on 12, 24 or 48 V and draws up to about 300 A. It is in series production on 48-volt mild-hybrid diesel vans since 2022. P04F2 concerns the heated catalyst of bank 1. The code monitors the supply voltage of the heater: if it lies outside its window when heating is requested, the heater cannot deliver its power and is limited or switched off, so the catalyst lights off later. A Subaru patent detects faults in the supply path when the heater is commanded. No manufacturer document available to us uses this code – Ford, for example, states that heated catalyst monitoring is not supported – so thresholds and warning-lamp logic are not documented; the description follows CARB and patents for the same part.

Likely causes

48 V or high-voltage supply weak (battery, charge, cold, DC-DC converter)
Fuse, relay or power switch faulty
Contact resistance or corrosion at high-current connector or ground
Module or voltage measurement faulty

Symptoms

  • Check engine light on (depending on the maker)
  • Higher emissions after a cold start
  • Otherwise usually unremarkable

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–3 h

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

Parts involved

catalyst heater bank1heater power switchheater wiringengine control module

Related codes

P04F3P04F4P04F6P0429

References: law.cornell.edu, patents.google.com, emitec.com.