Powertrain, generic (extended)

P2B79 Coolant Heater B Control Circuit Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modeNo

P2B79 says that coolant heater B is wired correctly but does not do what it is told: after a request the expected warming, current or status feedback fails to appear. In layouts with several stages, as in Hyundai and Kia patent US11378000B2, the controller switches the stages individually and shuts them down when outlet or housing surface temperature runs too high – a stage disabled by that protection can look like a performance fault from outside. No manufacturer documents P2B79 publicly. CARB 13 CCR 1968.2 (e)(15.2.2) demands functional checks on output components. Low coolant, trapped air, a weak heating element or an internal fault in the heater electronics are typical causes; the over-temperature code of heater A is P2DA6.

Likely causes

Low coolant or trapped air, so protection shuts the stage down
Heating element of stage B weak or open
Internal fault in the heater electronics for stage B

Symptoms

  • Heating power lower than expected
  • Heater outlet temperature rises slowly in live data
  • Fault may only appear at low outside temperatures

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,500
Time1–4 h

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

Parts involved

coolant heater bwiring harnessecm

Related codes

P2B76P2B75P2DA6P01EF

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