U0D92

Lost Communication With Auxiliary Heater Control Module

U0D92 is a generic OBD-II network diagnostic trouble code: Lost Communication With Auxiliary Heater Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0D92
Group
Network
System
Network
Severity
Warning (MIL on)
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What U0D92 means

U0D92 is stored when the body or climate control module loses CAN bus communication with the auxiliary heater control module. The auxiliary heater provides supplemental cabin heating independent of the main engine cooling circuit and is common on diesel vehicles, hybrid powertrains, and vehicles sold in cold-climate markets. Loss of communication with this module means the auxiliary heater cannot receive commands and its status cannot be monitored by the climate control system.

The fault commonly results from a loss of power or ground to the auxiliary heater module, a broken CAN bus wire in the harness routed to the heater unit (which is often located under the vehicle or in the engine bay), or a failed module. Corrosion at the module connector caused by condensation or road moisture is a frequent finding, especially on vehicles where the heater module is mounted in an exposed undercarriage location.

In cold climates this fault has a direct comfort and defrost impact, as the auxiliary heater may not activate. On some hybrid and electric vehicles, the auxiliary heater is also critical for battery thermal management, making timely diagnosis important beyond simple comfort considerations.

Common causes

Most-frequently reported root causes when U0D92 is logged.

  • 1
    Loss of battery voltage or ground feed to the auxiliary heater control module.
  • 2
    Broken or shorted CAN bus wiring routed to the heater unit location.
  • 3
    Corroded or moisture-damaged connector at the auxiliary heater module.
  • 4
    Failed auxiliary heater control module no longer responding on the network.
  • 5
    Blown fuse in the auxiliary heater power supply circuit.
  • 6
    Wiring harness damage from heat, chafing, or road debris near the heater mounting location.
  • 7
    Module removed or disconnected for service and not reconnected.

Symptoms drivers notice

Auxiliary heater inoperative; cabin may take longer to warm up in cold conditions.
Climate control system shows auxiliary heat unavailable or a heater warning message.
On hybrid or electric vehicles, battery heating function may also be affected.
Scan tool cannot communicate with the auxiliary heater module.
No driveability impact but comfort and defrost capability is reduced in cold weather.

How to diagnose U0D92

A typical diagnostic flow when this code is present.

  1. 1
    Scan all modules and retrieve DTCs; look for co-set power supply or configuration codes.
  2. 2
    Locate the auxiliary heater module and inspect its power supply fuse and relay.
  3. 3
    Check for battery voltage and a good ground at the module connector.
  4. 4
    Inspect the module connector and harness for corrosion, moisture, or physical damage.
  5. 5
    Measure CAN bus differential voltage at the module connector to verify the network signal is present.
  6. 6
    Attempt direct communication with the module using the scan tool; absence of any response with correct power supply points to a failed module.
  7. 7
    Repair wiring or replace the module as indicated; note that some modules require programming after replacement.

Related network codes

Frequently asked questions

Is U0D92 only relevant in cold climates?

Loss of communication is logged regardless of climate, but the functional impact is most significant in cold weather when the auxiliary heater is needed for cabin warmth and defrost. On hybrid platforms the battery thermal management relevance applies year-round.

Can a dead auxiliary heater module battery backup cause this code?

Some auxiliary heater modules have an internal memory supply, but loss of the main supply voltage is the primary cause. Check the main fuse and power feed before assuming the module has failed.

Will this code affect the main engine heater circuit?

No. The auxiliary heater is a supplemental system. The main engine coolant heater circuit operates independently. However, on some platforms the ECM may adjust its warm-up strategy if auxiliary heat is unavailable.

Is the auxiliary heater the same as the parking heater?

They are often the same unit. Many auxiliary heaters also serve as parking heaters, allowing cabin pre-heating before driving. The control module governs both functions and loss of communication disables them both.

Disabling U0D92 in software

RaceTune can permanently disable U0D92 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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