Network, generic

U0425 Invalid Data Received From Auxiliary Heater Control Module

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0425 concerns the auxiliary heater control module, the controller of an extra heat source such as a fuel-fired coolant heater on a diesel. The HVAC or engine controller sets it when the heater's status frames arrive with implausible content. No manufacturer documents U0425 publicly; GM's 2013 bulletin PIP5094 merely includes it in a long list of codes for which terminal fit should be checked before circuit tests. Ford's patent US 10,124,649 shows how such a heater is tied into the network: it talks to the HVAC unit over the bus and to the engine controller through a bus interface, receives ambient temperature from it and runs its own fuel pump. GM's 6.6 L Duramax ECM (2014) watches messages from this module, stating lost contact as U0166, Special Type C. U0325 is software. The heater stays off; no MIL is expected.

Likely causes

Auxiliary heater control unit faulty or locked after an internal fault
Low supply voltage at the heater, for example from a weak battery
Loose terminals or corrosion in the heater's bus connector

Symptoms

  • Auxiliary or parking heater does not start
  • Slow warm-up and weak cabin heat in cold weather

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€50–900
Time0.5–2.5 h

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

Parts involved

auxiliary heater module

Related codes

U0166U0325U0424

References: patents.google.com, gsi.ext.gm.com, static.nhtsa.gov.