U0101 — CAN-Bus no Communication with Transmission Control Module

Causes

• Blown fuse or removed TCM power supply
• Faulty or intermittent TCM ground
• Damaged, corroded or loose TCM connector or wiring
• Open, shorted or high-resistance CAN bus wiring (CAN High / CAN Low)
• Failed TCM module
• Missing or incorrect bus termination (open or shorted terminator)

Symptoms

• Transmission will not shift properly or goes to limp mode
• Transmission warning light (transmission fault) and/or MIL illuminated
• No TCM listed in scan tool module list or TCM shows as unresponsive
• Other modules report related communication errors or degraded functions
• Possible engine limp mode or reduced drivability depending on inter-module dependencies

What to check

• Retrieve and record all U- and P- codes and freeze frame data with a dealer-grade or capable scan tool
• Check battery voltage (while cranking and key-on); low voltage can cause comms failures
• Inspect fuses and relays for the TCM power circuit
• Visually inspect TCM connector and harness for corrosion, bent pins, water ingress, or damage
• Using a scan tool, attempt to communicate with TCM; note presence/absence and any timeout messages
• Measure TCM power and ground pins at the connector (key ON) — expect steady battery voltage and good low-resistance ground

Signal parameters

• CAN bus idle (recessive): CAN High ≈ 2.5 V, CAN Low ≈ 2.5 V (differential ≈ 0 V)
• CAN bus dominant state: CAN High typically ≈ 3.5 V, CAN Low ≈ 1.5 V (differential ≈ 2.0 V)
• Typical powertrain CAN baud rate commonly 500 kbps (some vehicles use 250 kbps or others) — verify vehicle-specific rate with scan tool
• Termination: ~60 ohms measured between CAN-H and CAN-L at two-node network (two 120Ω terminators in parallel)
• Expected message behavior: TCM should transmit periodic messages (tens of Hz) and respond to diagnostic requests — absence of these indicates loss of communication

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