Powertrain, generic (extended)

P2B73 Coolant Heater A Control Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P2B73 is the low case on the command circuit of coolant heater A: the commanding module reads the line at or near ground when it expects a higher level. Usual reasons are a chafed wire touching body ground, coolant or water in a connector, or an input in the heater that pulls the line down. Hyundai and Kia patent US11378000B2 shows why the line carries only a request: the heater has its own controller, which switches several sheath elements one by one and protects them. No manufacturer documents P2B73 publicly, and no thresholds are known. Under CARB rules, commanded outputs like this must be watched for circuit faults. The heater then normally stays off; an electrified car may start its engine more often for cabin heat. Open circuit is P2B72, high voltage P2B74.

Likely causes

Command wire chafed through and shorted to ground
Coolant or water in the heater connector
Input of the heater electronics shorted internally
Commanding module cannot drive the line high

Symptoms

  • Coolant heater does not switch on, weak cabin heat
  • Engine runs more often in the cold (hybrids)
  • Fuse for the request circuit may blow repeatedly

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 awiring harnessecm

Related codes

P2B72P2B74P2B77P01E9

References: law.cornell.edu, patents.google.com, static.nhtsa.gov.