Fault Tree: U0121 lost communication with ABS module

Decision flow

1. Confirm battery/charging stability
   • Low voltage events cause modules to drop off the network.
   Use: Battery/charging fault tree →
2. Scan network: who can you talk to?
   • If multiple modules are missing, suspect CAN physical layer or a common power/ground.
   • If only ABS is missing, focus on ABS power/ground and connector.
3. Check ABS power and grounds under load
   • Back-probe supply pins and do a voltage drop test on grounds while a load is present (pump/valves commanded if possible).
   Use: Voltage drop testing →
4. Inspect connector and mounting area
   • Water ingress and corrosion near ABS units is common.
   • Check for green crust, pushed pins, and damaged seals.
5. CAN physical layer sanity checks
   • Check CAN-H/CAN-L resistance (typical ~60Ω across the network at rest, varies by architecture).
   • Look for shorts to power/ground or a dominant bus state.
   Use: CAN physical layer checks →
6. Wiring diagram and splice points
   • Locate ABS network splice/connector points and check continuity only after load testing.
   Use: Wiring diagrams basics →
7. After the fix: verify
   • Confirm ABS module is present, no U-codes return, and ABS/traction warnings clear after drive.