Code
U0001
HUMMER
U — Network/User
Controller Area Network (CAN) Bus Communication (ECM)
AI status
Completed
Completed
100%
Causes
- Damaged or shorted CAN high/low wiring
- Loose, corroded, or poor connector pin contact at ECM or other modules
- Missing or failed CAN termination resistor(s)
- Power or ground fault to the ECM (blown fuse, bad relay, poor ground)
- Low battery voltage or unstable charging system
- Faulty ECM or other network node interfering with the bus
Symptoms
- Check Engine Light or multiple network-related warning lights
- Scan tool cannot communicate with ECM or intermittent communication
- Multiple U-codes stored across different modules
- Crank/no-start or poor drivability if ECM is not available
- Instrument cluster or other modules showing errors or reduced functionality
- Engine runs but certain systems disabled or limited
What to check
- Verify battery voltage is stable (12.6V+ at rest) and charging system is healthy
- Scan with a capable tool; record all U-codes and module addresses/availability
- Visually inspect CAN wiring harness, connectors, and ECM harness for damage, corrosion or pin push-out
- Check power and ground circuits to ECM (fuses, relays, ground strap continuity)
- Measure DC resistance between CAN H and CAN L with ignition OFF (expected ~60Ω)
- Probe CAN H and CAN L with a scope or DVM with ignition ON; check idle voltages and differential signaling
Signal parameters
- CAN idle common-mode voltage: ~2.5 V on both CANH and CANL (with respect to chassis ground)
- Dominant bit voltages: CANH ~3.5 V, CANL ~1.5 V (approx.) during active transmission
- Approx. bus differential voltage when idle: ~0 V; dominant differential ~2.0 V
- Bus resistance between CAN H and CAN L (ignition off): ~60 Ω (two 120Ω terminators in parallel)
- Expected no DC shorts to battery or ground on either CAN line
Diagnostic algorithm
- Record freeze-frame and all stored U-codes from all modules. Note if multiple modules report communication errors.
- Verify battery state-of-charge and charging system; charge battery and retest communications.
- Inspect ECM connectors and nearby harness for pin damage, corrosion, or water ingress. Repair as needed.
- Check ECM power and ground circuits: verify fuses, relays, and continuity to chassis ground. Repair faults before continuing.
- With ignition OFF, measure resistance between CAN H and CAN L at a convenient connector — expect ~60 Ω. If open or high, inspect termination resistors and wiring segments.
- With ignition ON, use a scope (preferred) or DVM to view CANH/CANL signals at ECM connector: verify idle voltages (~2.5 V), proper dominant/recessive transitions, and absence of noise or voltage offsets.
- If abnormal voltages or shorts are present, isolate bus sections by unplugging connectors/modules to find the segment or node causing the fault.
- Replace or repair damaged wiring, connectors, or faulty modules identified. If a module replacement is performed, ensure correct software calibration and reflash/program as required by manufacturer procedures.
- After repairs, clear codes and perform a functional test/road test. Re-scan to ensure U0001 and related codes do not return.
Likely causes
- Open, short, or high-resistance in CAN H/L pair between ECM and nearby module(s)
- Bad ECM power or ground (fuse/relay, battery connection)
- Failed termination resistor or open/shorted termination at one end
- Corroded connector pins at ECM or gateway module
- Faulty module placed on the bus that drags bus voltages out of range
Fault status
Status
CAN Bus Communication (ECM): Network messages to/from the Engine Control Module are missing, corrupted, or intermittent. Verify wiring, power/ground, termination, and module integrity before replacing the ECM.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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Repair manuals
Manual library for HUMMER
138
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