Powertrain, generic · Fuel and air metering

P01F9 Engine Coolant Heater B Control Circuit Driver Current/Temperature Too High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

The engine module switches an electric coolant heater B – a second heater or heating stage – through a power stage (driver). This code means the driver itself has reported excessive current or overheating and has shut down to protect itself – it concerns the control circuit, not the coolant temperature. Such heaters warm the coolant around a cold start, shortening warm-up and providing cabin heat sooner; a GM patent describes how the module recognises a pre-heated engine so other temperature diagnostics do not misfire. No manufacturer using this code family was found in GM, Ford or FCA documents; the description rests on CARB's requirements for output components. Siblings for heater B are P01EC–P01EF.

Likely causes

Short or partial short in heater element B
Wiring or connector chafed to ground, moisture in connector
Driver stage in the module or relay faulty or overheated

Symptoms

  • Check engine light or warning message (manufacturer-dependent)
  • Slow warm-up, little heat in cold weather

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–600
Time1–3 h

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

Parts involved

engine coolant heater bheater driverwiring harness

Related codes

P01ECP01EDP01EEP01EFP01F8

References: law.cornell.edu, patents.google.com, en.wikipedia.org.