Code
C1786
HUMMER
C — Chassis
Brake Control Module Communication Fault
Views:
UK: 14
EN: 17
RU: 10
AI status
Completed
Completed
100%
Causes
- Open/short in CAN bus wiring (CAN High/Low) or poor connector/cable condition
- Blown fuse or lost power feed to the brake control module
- Poor or corroded ground at the module or chassis ground strap
- Faulty or intermittent Brake Control/ABS module
- Module not programmed or incompatible after replacement
- Other module on network pulling bus low or interfering (short to B+ or ground)
Symptoms
- ABS, Traction Control and/or Stability lamps illuminated
- Brake warning or Service Brake message in instrument cluster
- Loss of ABS/ESC/traction control functionality (mechanical brakes remain)
- Stored communication-related DTC(s) and inability to communicate with BCM via scan tool
- Intermittent or persistent ABS self-test failures
What to check
- Read all stored codes and freeze frame data with a capable scan tool; note presence of other network codes (U-codes)
- Check battery voltage (key on) and state of charge; verify good charging system operation
- Inspect fuses/relays for ABS/BCM power and associated fusible links
- Visually inspect module connectors and wiring for corrosion, bent pins, water intrusion or physical damage
- Check chassis and module ground connections for corrosion and tightness
- Measure CAN bus termination resistance (~60 ohms across CAN H and CAN L with ignition off)
Signal parameters
- Vehicle battery voltage: nominal 11–14.5 V (key on); significant dips can cause comms loss
- CAN bus idle voltages: CAN H ≈ 2.5–3.5 V, CAN L ≈ 1.5–2.5 V (differential ~0–1 V idle; active differential up to ~2 V)
- Termination resistance between CAN H and CAN L: ~60 ohms (two 120 ohm terminators in parallel)
- Module ground continuity: low resistance to chassis (
- Fuse/ignition feed present: battery feed and ignition-switched feed should be present at the module connector with correct key state
Diagnostic algorithm
- Connect full-function scan tool, record DTCs, note freeze frame and any U-codes or related ABS codes.
- Verify battery voltage and charging system. If voltage
- Inspect ABS/BCM fuses and relays; replace or test as required.
- Visually inspect ABS module connector and harness for damage, moisture, corrosion or pushed-out pins; repair as needed.
- With ignition off, measure CAN termination resistance across CAN H and CAN L at module connector; expect ~60 ohms. If open or high, trace wiring to find open or disconnected terminator.
- With key on, measure CAN H and CAN L voltages at the module connector relative to ground. Compare to expected ranges; look for short to B+ or ground or missing bias voltage.
- If voltages and termination are correct, attempt to communicate with the module using scan tool. If no response and wiring checks OK, consider disconnecting other modules one at a time (or isolate branches) to find a bus device creating a fault.
- Perform wiggle test on harnesses while monitoring comms for intermittent faults. Repair damaged wiring, connectors or grounds.
- If wiring, power and grounds are confirmed good and module still fails to communicate, replace or reflash/program the Brake Control Module per manufacturer procedures. Ensure correct part number and any required calibration/programming.
- Clear codes and perform a functional test / road test to verify the fault does not return. Document any intermittent occurrences for further investigation.
Likely causes
- Disconnected or corroded connector at the brake control module
- Missing/failed fuse or relay supplying module battery/ignition power
- CAN bus termination open or short (incorrect resistance between CAN H and CAN L)
- Faulty module hardware or internal communication processor failure
- Intermittent wiring harness damage (pinched, rubbed through, water intrusion)
- Battery voltage below 11 V or unstable during key on
Fault status
Status
Stored when the Brake Control (ABS) Module fails to communicate on the vehicle data bus. ABS/ESC functions may be disabled until communication is restored.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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