Powertrain, generic · Auxiliary emission controls

P04F8 Catalyst Heater Circuit Intermittent 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. P04F8 concerns the heated catalyst of bank 1. The code means the heater circuit shows brief or repeated circuit faults that do not persist – typically a loose contact at the high-current connectors or a wire broken by vibration and heat. 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

Loose contact at the heater or module connector
Wire broken near the exhaust
Corrosion at ground or connector
Internal contact point of the heating disc

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€50–800
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

P04F9P04F2P04F4P0429

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