Fault Tree: U0073 CAN Bus Off
U0073 usually means a module saw CAN communication collapse (bus off). The cause is often low voltage, a short/open in CAN wiring, or a module/connector problem pulling the network down.
Quick triage & tools
- Scan first: read codes from as many modules as possible. Note which modules are missing/offline.
- Power basics: check battery health and charging voltage. Network faults often happen when voltage is unstable.
- Have ready: multimeter, wiring diagram access, and ideally a scan tool that can show module status.
Use: CAN bus physical layer checks → and Voltage drop testing →
What you need (minimal)
- Scan tool (preferably that shows module list / network health).
- Multimeter (DC volts + ohms) and basic back-probing capability.
- Wiring diagram for CAN H/CAN L routing and module connector pinouts.
Related: Wiring diagrams basics →
Decision flow
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Confirm whether this is a real “network down” event
- If multiple modules show comms codes and some are missing from the scan tool, treat it as a network issue.
- If only one module has a comms code, focus on that module’s power/ground and its specific network branch.
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Battery + charging voltage (don’t skip)
- Low voltage or unstable charging can push modules “offline” and cause bus-off conditions.
- Fix basic voltage issues first (poor grounds, loose terminals, weak battery, charging faults).
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Check CAN physical layer resistance (key off)
- With ignition off, measure between CAN-H and CAN-L at the OBD port (or a convenient connector).
- ~60Ω is typical on many vehicles (two 120Ω terminators in parallel). If it’s far off, suspect a termination issue or open/short.
- If you don’t know your vehicle’s expected reading, use the wiring diagram to confirm termination design.
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Check for shorts to power/ground (key off)
- If CAN-H or CAN-L is shorted to ground or battery, the network can collapse instantly.
- Common causes: chafed loom, water ingress at connectors, corrosion in splice packs, damaged module connector pins.
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Look for “known trouble” areas
Under-battery trays, wheel arch looms, bulkhead grommets, and areas with prior repairs. Water ingress at BCM/ABS connectors is common.
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Isolate the branch safely
- Use wiring diagrams to identify network branches and junctions. Work from easy access connectors outward.
- If unplugging modules/connectors, do it key off and one at a time. Re-check resistance to see when it returns to normal.
- If you find a branch that changes the resistance dramatically, inspect that branch for wiring damage or connector corrosion.
See also: U0100 comms patterns → -
Power/ground at the suspected module
- A “dead” module can appear as a comms failure but the root cause is missing power or a bad ground.
- Verify feeds and do a quick voltage drop test under load where possible.
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Final confirmation
- Clear codes, re-scan module list, and reproduce the conditions that triggered the fault.
- If the fault only happens with vibration/heat, suspect intermittent connector pin fitment or loom damage.
Related flows
Print / save checklist
Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.
- Freeze-frame captured / conditions noted
- Battery voltage checked (resting + cranking)
- Basic visual checks (connectors, grounds, fuses, water ingress)
- Resistance checked between CAN-H and CAN-L (key off)
- Re-scan modules and confirm fix under the same conditions
What to do next
Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.
Find another symptom flow
Jump to the symptom selector to locate the closest decision tree.
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