Powertrain, generic (extended)

P2B74 Coolant Heater A Control Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P2B74 is the opposite of P2B73: the command circuit of coolant heater A sits at or near supply voltage while the module expects it low, or the output stage measures excess current and shuts down. A wire rubbed through onto a 12 V feed, a crushed harness next to the heater's own supply cable or a shorted input in the heater electronics are typical. Depending on the wiring, the heater may run without a request or not at all. Designs with integrated control, such as Hanon's patent US11628706B2, add their own overheat sensing so that an unintended run does not destroy the element. No manufacturer documents P2B74 publicly. CARB wants such outputs monitored for electrical faults. Internal module and over-temperature faults have their own codes, P2DA4 and P2DA6.

Likely causes

Command wire shorted to a 12 V circuit
Harness crushed or melted near the heater supply cable
Heater electronics input shorted to its supply
Output stage of the commanding module faulty

Symptoms

  • Coolant heater off, or running without request
  • Higher 12 V or high-voltage battery drain when parked if the heater runs on
  • Weak cabin heat in cold weather

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,000
Time0.5–3 h

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

Parts involved

coolant heater awiring harnessecm

Related codes

P2B72P2B73P2B78P2DA4P2DA6

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