Applicable Systems
OBD-II
Symptoms
The vehicle alarm system may not arm or function. Communication with the alarm module is lost. DTC U0491 is stored by a control module that lost communication with the Alarm module.
Possible Causes
- CAN bus wiring to alarm module fault
- Alarm module power supply fault
- Alarm module internal failure
- Alarm siren wiring fault
Troubleshooting Steps
- Step 1: Connect a diagnostic scanner and read all module DTCs to confirm U0491
DTC U0491 is present; note which module logged the code - Step 2: Check CAN bus wiring between the affected modules for continuity, shorts, and termination resistor values
CAN high and low lines intact, 60 ohms across the bus - Step 3: Verify power and ground supplies to the affected module
Battery voltage at module power pin, less than 0.1V drop on ground side - Step 4: If wiring is intact, the affected module may need replacement and programming
Communication restored after module replacement and programming
How to Reset
Clear the DTC after repair using a diagnostic scanner. The code will clear automatically if communication is restored and no fault is detected for 3 consecutive drive cycles.
Common Replacement Parts
- varies: Alarm control module (if confirmed faulty) (Confirm part number via VIN before ordering; programming typically required)
Frequently Asked Questions
- Q: What does code U0491 mean?
This is a network communication DTC indicating lost communication with the Alarm module. The CAN bus wiring, module power supply, and the module itself should be checked.
- Q: Can I drive with code U0491?
Driving may be possible but the affected system may not function. If the code relates to ECM, transmission, or brake communication, have it diagnosed promptly for safety.
Source: SAE J2012 Standard OBD-II Diagnostic Trouble Codes
Category: OBD-II