U0189
Lost Communication with Auxiliary Heater Control Module
The HVAC controller has lost the auxiliary fuel-fired heater module. Pre-heating cannot be commanded; the heater stays inactive.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- Auxiliary heater unpoweredCommon
- Gateway module routing CAN messages incorrectlyLess likely
- CAN terminating resistor (120 Ω) drifted out of specLess likely
Parts involved
- Auxiliary Heater
- CAN Bus Wiring
U0189 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.