Code
U1305
HUMMER
U — Network/User
Class 2 Data Link Low or High
Views:
UK: 11
EN: 15
RU: 11
AI status
Completed
Completed
100%
Causes
- Open or short in Class 2 wiring harness
- Short to battery voltage or ground on the Class 2 wire
- Corroded, loose or damaged connectors at modules or junctions
- Faulty or intermittent module pulling the bus high or low
- Poor or missing ground(s) or battery connections
- Aftermarket device or improperly installed module loading the bus
Symptoms
- Multiple modules show communication faults or do not respond to scan tool
- Intermittent or complete loss of vehicle functions controlled by networked modules
- Illuminated MIL or other warning lamps related to affected systems
- Erratic operation of body features (locks, windows, HVAC, instrument cluster)
- Possible no-start or crank issues if powertrain modules affected
What to check
- Connect a capable scan tool and confirm U1305 and any other U1xxx/U0xxx codes; record affected modules
- Visually inspect Class 2 wiring, connectors, and module grounds for corrosion, damage, or moisture
- Check battery voltage and charging system; ensure strong battery and chassis ground connections
- With ignition on, measure bus voltage at a known good connector (reference ground) using a DMM or scope
- Check for aftermarket modules or recent repairs/splices on the Class 2 circuit
- Perform a pin-to-pin wiggle test and try disconnecting modules one at a time to see if the fault clears
Signal parameters
- Physical: Single-wire Class 2 serial data bus (1-wire)
- Idle voltage: approximately battery voltage (≈12V) when bus is idle
- Active low: data is transmitted as pulses toward ground; low pulses near 0V
- Baud rate: low-speed asynchronous serial (approx. 8.192 kbps)
- Fault condition: bus held continuously low (near 0V) or pulled high/erratic voltages outside expected idle/pulse behavior
Diagnostic algorithm
- Verify and record all stored codes and freeze frame data with a scan tool; note any modules not responding
- Visually inspect connectors, junctions, and wiring along the Class 2 route for damage, corrosion, or water intrusion
- Check battery and main ground integrity; clean and tighten battery terminals and engine/body grounds
- With ignition ON (engine off), measure Class 2 voltage at a known good connector: verify idle ~battery voltage and that data pulses occur when modules communicate (use scope if available)
- If bus is held low or high, disconnect suspected modules (one at a time) or unplug non-essential modules/aftermarket devices to isolate the load that clears the fault
- Perform resistance checks between Class 2 wire and ground/battery to find shorts (key off); look for low resistance to ground or B+
- Repair or replace damaged wiring, corroded connectors, or faulty modules found during isolation; reseal and protect repaired areas
- After repairs, clear codes, reconnect all modules, and verify communication restoration with scan tool and road test to ensure no intermittent faults remain
Likely causes
- Corroded or loose connector at a control module or splice
- Short to ground or battery on the Class 2 wire (pinched or chafed harness)
- Faulty module on the Class 2 network dragging the bus low or high
- Poor vehicle ground or low battery voltage affecting bus idle level
Fault status
Status
Class 2 Data Link voltage out of range (low or high). Communication disrupted between networked modules.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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