Powertrain, generic (extended)

P2B78 Coolant Heater B Control Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P2B78 flags the command line of coolant heater B as too high: voltage is present where the commanding module expects a low level, or its driver reports overcurrent. A short to a 12 V circuit is the classic cause; with a low-side driver, a heater input that has failed short can produce it as well. The risk is a stage that will not switch off on request, which is why heater designs such as US11628706B2 include independent overheat sensing in the housing. No manufacturer documents P2B78 publicly. The CARB regulation calls for an electrical check of such a line. If the fault lies in a shared harness, P2B74 for heater A may appear at the same time; measuring the request lines with the heater unplugged separates harness faults from heater faults.

Likely causes

Request wire for heater B shorted to a 12 V circuit
Heater B input failed short inside the heater
Damage in a harness section shared with heater A

Symptoms

  • Heater stage B off, or running although not requested
  • Unusual battery drain while parked
  • Lower heating power in winter

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 bwiring harnessecm

Related codes

P2B76P2B77P2B74P01EE

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