Code
U0073
ISUZU
U — Network/User
Control Module Communication Bus Off
AI status
Completed
Completed
100%
Causes
- Open or shorted CAN bus wiring (CAN_H or CAN_L)
- Damaged connector or corrosion at modules or splice points
- Failed control module or CAN transceiver
- Missing or incorrect termination (open/mismatched 120 Ω resistors)
- Short to battery or chassis ground on CAN lines
- Low or unstable battery/charging system voltage
Symptoms
- Multiple modules report loss of communication or show U-codes
- Instrument cluster, ABS, traction control, or engine control messages missing or inoperative
- Check Engine Light or multiple warning lamps illuminated
- No crank/no start if PCM loses comms or is bus-off
- Erratic instrument behavior or frozen displays
- Intermittent electrical faults that clear after key cycle
What to check
- Connect a factory-level or professional scan tool; record all U and related codes and freeze frame data
- Check battery voltage and charging system; ensure >12.4V with ignition off and >13.5V running
- Visually inspect CAN connectors, modules, and splice points for corrosion, bent pins, water intrusion, or damage
- Check for recent repairs or aftermarket module installations
- Measure DC voltage on CAN_H and CAN_L relative to chassis ground with ignition ON (see signal params)
- Measure resistance across CAN_H to CAN_L with ignition off (approx. 60 Ω expected for a properly terminated two-terminator bus)
Signal parameters
- Recessive state (idle): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ≈ 0 V)
- Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
- Typical differential voltage when active: ~1.5–2.5 V
- Bus termination resistance (two 120 Ω in parallel): ≈ 60 Ω across CAN_H to CAN_L with ignition OFF
Diagnostic algorithm
- Read and document all DTCs and freeze frame data with a professional scan tool. Note which module reported U0073 and any related U-codes.
- Verify battery and charging voltages; recharge or replace weak battery before continuing.
- Visually inspect wiring, connectors, and module mounting points for corrosion, water entry, damage, or loose pins (focus on gateway/BCM/ECM connectors and splice boxes).
- With ignition ON, measure CAN_H and CAN_L voltages at the module that reported the code and at other accessible nodes. Compare to expected signal parameters.
- With ignition OFF, measure resistance between CAN_H and CAN_L near each end of the bus. Expect ≈60 Ω if both termination resistors present. If open or much lower, investigate termination resistors or intentional shorts.
- Check for shorts to 12V or ground on CAN lines by measuring continuity/voltage to battery rails. Repair any shorts discovered.
- If voltages and resistance are normal but communications are lost, use an oscilloscope to view waveforms and look for noise, missing dominant/recessive transitions, or bus-off patterns.
- Isolate network segments: disconnect non-critical modules one at a time (starting with recently serviced or suspicious modules) while monitoring the bus to identify which module insertion causes bus-off.
- If a specific module's transceiver is faulty, confirm with bench tests or module replacement only after confirming wiring integrity.
- Clear codes and perform a road/drive test to confirm repair and that the bus remains stable; rescan for reappearance of U0073 or related codes.
Likely causes
- Corroded connector at gateway/BCM or ECM causing intermittent contact
- Broken/damaged CAN_L or CAN_H between modules or at splice
- Failed CAN transceiver in a module (module enters bus-off state)
- Missing/failed termination resistor(s) at network ends
- Short to 12V on a CAN line after recent repair or component replacement
Fault status
Status
Control Module Communication Bus Off — one or more control modules have entered a bus-off state and communications on the vehicle CAN network are interrupted. Investigate wiring, termination, power/ground, and individual module transceivers.
Repair difficulty: Hard
Diagnostic time: 2-4 hours
Repair manuals
Manual library for ISUZU
229
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