U1109 — No communication with the automatic gearbox fault: Automatic gearbox ECU absent

Causes

• TCM (automatic gearbox ECU) removed, disconnected or defective
• Blown fuse or disabled power/ground supply to the TCM
• Open or short in CAN or gateway wiring (broken wire, corroded connector)
• Faulty gateway or CAN bus termination
• Low battery or insufficient supply voltage during key-on
• Module lost due to configuration/programming or immobiliser/security lockout

Symptoms

• Transmission in limp/limited-gear mode or no shifting
• No gearbox-related menus or ECU present in dealership scan tool network list
• Transmission warning lamp or gearbox fault indicator illuminated
• Possible reduced driveability or inability to engage gears
• Related systems that rely on TCM data may show faults or limited function

What to check

• Confirm code details and freeze frame with a capable diagnostic tool (manufacturer-level if available)
• Verify battery voltage (should be >12.4 V with ignition on) and check recent battery events
• Check fuses and relays for TCM power/ignition circuits
• Visually inspect TCM connectors and harness for damage, corrosion, or disconnection
• Scan the vehicle network to see if the TCM appears in module list or if bus activity is present
• Measure CAN bus voltages at a connector: idle and during activity; check termination resistance (~60 ohm across CAN_H–CAN_L)

Signal parameters

• CAN idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V; differential ~0 V at rest, ±1–2 V differential during activity
• Expected CAN differential during messages: ~2 V peak-to-peak (varies with network type)
• Termination: ~60 ohms measured between CAN_H and CAN_L at vehicle wiring (two 120Ω in parallel)
• Wake/ignition supply to TCM: nominal vehicle battery voltage when key on (~12–14 V)
• K-line/LIN (if present for wake): typically 0 V when inactive, pulled to supply when active (manufacturer-specific)

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