U0425

Code

Generic U — Network/User

Invalid Data Received From Auxiliary Heater Control Module

Brand: Generic

Type: U — Network/User

Views: UK: 33 EN: 58 RU: 29

AI status

Completed

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Page language: EN

Causes

Loss of communication or corrupted CAN/LIN messages from the auxiliary heater control module.
Faulty auxiliary heater control module (hardware or firmware).
Intermittent or low power supply or poor ground to the auxiliary heater module.
Damaged or corroded connectors or wiring between the module and the network.
Bus wiring short, open, high resistance, or missing termination.
Software/calibration mismatch or missing programming for the module.

Symptoms

Auxiliary heater may not operate, operate intermittently, or remain in limp mode.
HVAC cabin heating reduced or delayed when auxiliary heater should assist.
One or more communication-related warning lamps or messages in the instrument cluster.
Stored network-related DTCs for heater or other modules (U0xxx family).
Possible battery drain or unexpected module behavior if module is malfunctioning.

What to check

Use a capable scan tool to read U0425 plus any related DTCs and view freeze frame and manufacturer-specific data.
Verify the auxiliary heater module is present on the network (module ID/serial responds to scanner).
Check battery voltage with engine off and running; confirm adequate supply (typically ~12V nominal).
Visually inspect connectors and wiring at the auxiliary heater module and along the harness for damage, corrosion, or water intrusion.
Check module power and ground circuits with a multimeter for continuity and proper voltage.
Scan CAN/LIN bus for other network errors or fault counters; inspect related modules for similar communication faults.

Signal parameters

Module communicates over vehicle data bus (CAN or manufacturer-specific serial bus). Expect periodic status messages; absence or corrupted payload triggers U0425.
Typical CAN idle voltages: CANH ~2.5–3.5 V, CANL ~1.5–2.5 V; differential idle ~0 V (recessive) and ~2 V differential when dominant. Use scope to verify waveform integrity.
Message frequency: periodic heartbeat/status message (manufacturer-specific; commonly 5–20 Hz). Missing or intermittent heartbeats indicate communication fault.
Payload includes status/command bits and diagnostic/fault fields; values should be within manufacturer-defined ranges and any checksum/CRC must validate.
Power supply to module: nominal battery voltage ~12 V (range depending on vehicle). Ground must be low resistance (

Diagnostic algorithm

Retrieve and record U0425 plus all current and pending codes and freeze-frame data with a scan tool. Note whether code is continuous or intermittent.
Confirm whether the auxiliary heater module responds to diagnostic requests. If it does not appear on the bus, suspect module power/ground or wiring.
Check battery voltage and inspect power/ground at the auxiliary heater module connector. Repair poor connections before further testing.
Visually inspect the module, connector, and harness for damage, corrosion, rodent chew, or water entry. Repair or replace damaged wiring/connector pins.
Use a scan tool or oscilloscope to monitor the bus while reproducing the fault. Look for missing messages, corrupted payloads, or erratic voltage levels on CANH/CANL.
Measure termination resistance across CANH and CANL with key off (~60 ohms). If open or out of range, locate and repair missing/failed terminator.
Isolate the fault by disconnecting the auxiliary heater control module (or suspect nodes) and observing whether bus integrity returns and the code clears or changes.
If wiring and bus are good but invalid data persists, reflash or update module software per manufacturer procedure or replace the module if diagnostics indicate internal failure.
After repairs, clear codes, re-scan for related faults, and perform a road test or functional test of auxiliary heater to confirm proper operation.
If intermittent, perform extended monitoring with data logger to capture the event; inspect for correlation with vehicle events (temperature, vibration, key cycles).

Likely causes

Connector corrosion or loose pin at auxiliary heater control module.
Defective auxiliary heater control module.
CAN bus fault (short to voltage/ground or missing termination resistor).
Low battery voltage or poor chassis ground affecting module communications.
Module firmware/software incompatibility after module replacement or update.

Fault status

⚠️ Status

Control module received invalid or corrupted data from the auxiliary heater control module (messages failed integrity checks or contained out-of-range values). Communication may be absent, intermittent, or the module may be providing malformed messages.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours