U2106

Code

HUMMER U — Network/User

Lost Comm with Trans Control System

Brand: HUMMER

Type: U — Network/User

Views: UK: 16 EN: 23 RU: 16

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Page language: EN

Causes

Open or shorted CAN bus wiring (CAN High or CAN Low) between TCM and network
Corroded, loose, or damaged TCM connector or pins
Loss of TCM power or ground (blown fuse, bad relay, damaged wiring)
Failed or intermittent TCM (internal fault)
High bus capacitance or missing/failed termination resistor(s)
Intermittent interference from aftermarket electronics or accessories

Symptoms

MIL (Malfunction Indicator Lamp) or Transmission warning lamp illuminated
Transmission stuck in limp or default gear, erratic shifting, or no shift response
Transmission-related messages missing from scan tool network list
No communication with TCM when attempting to access it with a scan tool
Reduced drivability, torque management, or start/stop of engine inhibited

What to check

Connect a factory-level or professional scan tool. Verify U2106 and any related U-codes or transmission codes; view module list and network status
Check freeze frame and occurrence data to see when the fault sets (key on, driving, cold start)
Visually inspect TCM connector and wiring harness for corrosion, bent pins, water intrusion, or damage
Verify battery voltage and measure TCM supply voltage and ground integrity (key ON)
Check fuses and relays that supply the TCM and any gateway/BCM powering the bus
With key ON, check that TCM is powered and awake; attempt to communicate with TCM via scan tool

Signal parameters

CAN High idle voltage: ~2.5 V (typical 2.5–3.5 V depending on vehicle)
CAN Low idle voltage: ~2.5 V (typical 1.5–2.5 V depending on vehicle)
Differential idle: near 0 V; during data transfer differential ~1–3 V (bus dependent)
Expected bus bit rate: depends on network (commonly 250 kbps or 500 kbps); verify vehicle-specific rate
Termination resistance (across CAN H and CAN L with key off): ~60 ohms (two 120Ω in parallel)

Diagnostic algorithm

Verify the DTC: connect a scan tool, record U2106 and any related codes, note freeze frame and occurrence count.
Attempt communication: try to access the TCM with the scan tool. If no response, confirm the condition is present.
Check power/ground: with key ON, verify battery voltage at TCM power pins and check a good chassis ground. Repair any blown fuses or open power feeds.
Visual inspection: inspect TCM connector and wiring for corrosion, damage, or water ingress. Repair or replace damaged connectors/wiring.
Bus physical layer checks: measure CAN High/Low voltages at the TCM connector. If voltages are abnormal, trace wiring for shorts to battery/ground or open circuits.
Termination and resistance: with ignition OFF, measure resistance across CAN H and CAN L. If not near expected ~60Ω, locate missing or failed terminator and repair.
Isolate and test: if safe, disconnect suspect modules (one at a time) from the bus to identify a module that causes bus errors. Re-check communication after each disconnect.
Capture waveforms: if available, use an oscilloscope to capture CAN signals while attempting communication to identify noise, stuck dominant bits, or missing frames.
Verify other module operation: check gateway/BCM/ECM module communications; a gateway failure can block TCM messages.
Repair wiring or replace connectors as required. Replace the TCM only after confirming the bus and power/ground are good and other modules are functioning.
After repair, clear DTCs and perform a functional test drive and re-scan to confirm the fault does not return. If module replacement or programming is done, perform required re-programming/coding per manufacturer procedure.

Likely causes

Loose or corroded connector at the TCM
Blown fuse or poor battery/ignition power feed to TCM
Damaged CAN pair (pinched, chafed, rodent damage) producing open/short
Failed TCM hardware
Failed gateway/body control module that routes messages to other networks
Missing or failed CAN termination resistor(s) causing bus errors

Fault status

⚠️ Status

Lost communication with the Transmission Control System (TCM). The vehicle did not receive valid messages from the TCM on the network for the required timeout period. This may be intermittent or permanent and can cause transmission warnings or limited functionality.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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