Powertrain, generic (extended)

P2B77 Coolant Heater B Control Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P2B77 means the command circuit of coolant heater B reads low: the line is pulled toward ground although the module drives or expects a higher level. With a second heating stage this usually costs part of the heating power rather than all of it, because stage A can still run. A short to ground in a harness branch routed along the coolant hoses, corrosion from coolant seeping into the connector or a defective input in the heater electronics are the usual suspects. No manufacturer documents P2B77 publicly, so the setting limits are unknown. CARB 13 CCR 1968.2 asks for circuit monitoring of outputs commanded by a powertrain computer. Open and high faults of heater B are P2B76 and P2B78; the engine coolant heater B family is P01EC–P01EF.

Likely causes

Harness branch along the coolant hoses shorted to ground
Coolant seepage corroding the heater connector
Heater B input shorted inside the heater electronics

Symptoms

  • Heating power noticeably lower than usual
  • Cabin and windows take longer to warm up
  • Possible traces of coolant at the heater connector

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–900
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

P2B76P2B78P2B73P01ED

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