Code
U1056
HUMMER
U — Network/User
Loss of Communications with RSS
AI status
Completed
Completed
100%
Causes
- RSS module not powered or ground open
- Faulty RSS module
- Damaged or corroded connector at the RSS module
- Wiring open, shorted, or high resistance on the vehicle data bus (CAN/GMLAN) to the RSS
- Failed network termination or bus wiring fault causing communication loss
- Intermittent battery/ignition power or blown fuse supplying RSS or gateway
Symptoms
- U1056 stored and confirmed in scan tool
- Inability to communicate with RSS using diagnostic scanner
- Related warning lamp(s) or reduced functionality of features controlled by RSS
- Intermittent or permanent loss of RSS-related features (e.g., occupant sensing, seat controls if applicable)
- Other modules reporting network-related codes or multiple U1xxx codes present
What to check
- Connect a scan tool and read all stored and pending U-codes; note freeze frame/snapshot data
- Check for other network communication codes (gateway/BCM) to identify broader bus issues
- Verify vehicle battery voltage is within specification and stable during testing
- Visually inspect RSS connector and wiring for corrosion, bent pins, or damage
- Verify fuses and relays that supply RSS module power and ignition feed
- Check power and ground at the RSS connector with a DVOM
Signal parameters
- CAN bus idle voltage ~2.5 V on both CAN_H and CAN_L (difference near 0 V with bus asleep)
- Dominant state: CAN_H ~3.5 V, CAN_L ~1.5 V (typical differential ~2.0 V)
- Expected periodic RSS message ID(s) and update frequency — check OEM data for exact IDs and timing
- No response to diagnostic request (no reply frame) when communicating to RSS
- Bus skew/intermittent frames or CRC/frame errors visible on scope if wiring problem exists
Diagnostic algorithm
- Verify vehicle and tool info: confirm code U1056 and capture freeze frame/last data. Note whether code is permanent, intermittent, or active.
- Check for related codes in gateway/BCM and other modules to determine if issue is module-specific or network-wide.
- Visually inspect RSS module connector and harness for contamination, loose pins, or damage. Unplug and re-pin-check; reconnect securely.
- Verify power and ground at RSS connector with key on per wiring diagram; check fuses/relays feeding the module and replace if blown.
- Check CAN/GMLAN bus presence at the RSS connector using a multimeter (idle voltages) and, if possible, an oscilloscope to verify message activity and waveform integrity.
- Attempt to communicate with the RSS using a factory scan tool. If the RSS responds intermittently, monitor while performing wiggle tests on harness and connector.
- If no bus activity at RSS but present elsewhere, trace/repair wiring between RSS and gateway/termination point. Check continuity and for shorts to battery/ground.
- If bus activity is absent vehicle-wide, focus diagnostics on gateway/BCM, bus termination resistors, and power distribution.
- If power/ground and bus wiring are confirmed good and the RSS still does not respond, consider replacing the RSS module. Re-program/initialize per OEM procedures after replacement.
- After repairs, clear DTCs, perform relearn/init procedures if required, and verify the fault does not return and that RSS functions are restored.
Likely causes
- Blown fuse or loose power/ground at RSS connector
- Corroded or disconnected RSS harness connector
- Broken CAN/LIN bus wire near RSS or pinched wiring in door/seat area
- Failed RSS module
- Faulty gateway/BCM preventing routing of RSS messages
Fault status
Status
No communication received from RSS module on vehicle network. Module not responding to diagnostic requests or expected periodic messages absent.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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