CAN / communication faults
Communication codes can look dramatic. Most first-pass wins come from basics: battery/voltage stability, grounds, connectors, and water ingress - not replacing modules.
The 2-minute reality check
- Is the battery healthy? Low voltage during crank or a weak battery can trigger multiple U-codes.
- Any recent work? Battery change, jump start, stereo install, water leak, accident repair - these are common triggers.
- Is it intermittent? If it clears and drives fine, treat it as a connection/voltage event until proven otherwise.
Usually is
- Battery/charging instability (especially after sitting)
- Ground/connector issue (corrosion, loose pins, damaged loom)
- Water ingress to a module/connector (footwell, trunk, under-seat)
- A single module pulling the bus down (shorted or stuck awake)
Usually is not
- Multiple modules failing at once
- A random "bad ECU" without voltage/ground checks
- Something fixed by clearing codes repeatedly
A safe first-pass test order
- Voltage snapshot: measure battery resting voltage, then cranking voltage drop. If it dives, fix this first.
- Charging check: confirm alternator output and that voltage is stable with electrical loads.
- Scan pattern: note which module is "missing" (U0100 style) versus which module is reporting faults.
- Water/loom inspection: check known leak points, fuse boxes, and any connector sitting low in the car.
- Isolation step (if you have the knowledge/tools): disconnect suspect modules one at a time to see if the network recovers.
When to stop and get help
- If the car won't start and you have multiple U-codes, prioritise battery/grounds and a professional network diagnosis.
- If a module is overheating, smells, or a fuse keeps blowing - stop and investigate before damage spreads.
Trust note: CAN diagnosis is about patterns. The goal is to find the one root cause that explains the rest. Start with voltage and grounds every time.