Code
U1129
HUMMER
U — Network/User
Loss of Communications with AMP
AI status
Completed
Completed
100%
Causes
- Blown fuse or lost power to the amplifier module
- Open or corroded ground connection for the amplifier
- Damaged or disconnected wiring harness or connector to the AMP
- Faulty AMP module
- Network bus fault (CAN/LIN) such as short, open, or missing termination
- Software/firmware issue or module not configured
Symptoms
- No audio or partial audio function from speaker system
- Infotainment or radio warnings/errors
- One or more U-codes stored related to lost communication
- Certain features controlled by the AMP may be inoperative (e.g., subwoofer, zone audio)
- Intermittent function or total failure of audio system
What to check
- Read all stored U-codes and freeze frame with a scan tool; note dependant and network codes
- Verify fuses and relays supplying the amplifier (inspect and test continuity)
- Visually inspect amplifier connector and harness for corrosion, bent pins, water intrusion, or damage
- Verify good battery voltage at module power pin with key on
- Check module ground(s) for low resistance to chassis
- Use scan tool to see if AMP module appears on network and if it responds to requests
Signal parameters
- CAN bus idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ~0 V). Dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
- Expected CAN bus termination: ~60 Ω between CAN_H and CAN_L (two 120 Ω terminators in parallel)
- LIN bus idle voltage: near battery voltage through pull-up (varies by vehicle, typically ~12 V); low when driven by master
- Module supply: battery voltage at power pin with key on (typically 12–14.5 V)
- Module ground: near 0 V with good continuity to chassis
Diagnostic algorithm
- Connect a capable scan tool and record U1129 and any related codes. Note freeze frame data and time of occurrence.
- Verify vehicle battery voltage is healthy and stable. Low battery can cause module comm faults.
- Inspect fuses/relays for the amplifier circuit; replace if open and re-test.
- Visually inspect AMP connector and harness for corrosion, water, damage, or poor seating. Repair or reseat as needed.
- With ignition on, verify power and ground at the AMP connector (multimeter): battery voltage at power pin,
- Use an oscilloscope or CAN bus tester at the AMP connector to check CAN_H/CAN_L waveforms and differential activity while operating the network (radio on, start/stop events).
- Measure termination resistance between CAN_H and CAN_L with ignition off (should be ~60 Ω). If open or incorrect, trace bus for missing terminator or open splice.
- If CAN/LIN wiring checks good, attempt to communicate with AMP via scan tool. If no response and power/ground OK, consider performing a module reset/reflash per service info.
- If module still fails to respond after software attempts, substitute a known-good AMP or bench-test the module if available, or consult manufacturer service for module programming/compatibility.
- After repair, clear codes, exercise system, and confirm code does not return and affected functions are restored.
Likely causes
- Blown or missing fuse to the audio amplifier
- Poor ground or power supply to the AMP
- Disconnected or corroded connector at the AMP
- Open or short on the CAN/LIN network wiring for the AMP
- Failed amplifier module
Fault status
Status
Loss of communications with AMP module — AMP not responding on vehicle network or not present.
Repair difficulty: Medium
Diagnostic time: 0.5-3.0 hours
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Repair manuals
Manual library for HUMMER
138
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