U2106

Code

OPEL U — Network/User

CAN-Bus no Communication with Transmission Control Module

Brand: OPEL

Type: U — Network/User

Views: UK: 7 EN: 13 RU: 10

AI status

Completed

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Page language: EN

Causes

Blown fuse or relay supplying TCM power
Open, shorted or corroded CAN-H/CAN-L wiring or connectors
Faulty TCM (power/ground or internal failure)
Missing or damaged CAN termination resistor(s)
Low battery or unstable vehicle voltage
Other module driving the bus stuck or causing bus errors (short to battery/ground)

Symptoms

TCM not shown or not responding on a diagnostic scanner
Transmission may default to limp mode or fail to shift correctly
Transmission-related warning lamp or general MIL illuminated
No gearbox data available on dash or scan tool (gear, temp, etc.)
Intermittent communication faults or recurring U2106 code
Other CAN modules reporting multiple communication errors

What to check

Connect a manufacturer-level scan tool and confirm U2106 and any related codes; check freeze frame and occurrence history
Verify presence of the TCM in the module list; note if it does not enumerate or appears intermittently
Visually inspect TCM power, ground and CAN connectors for corrosion, bent pins, water damage or loose terminals
Check fuses/relays for TCM power supply and measure key-on voltage at TCM power pin
With ignition on, measure CAN bus voltages at a known-good node and at the TCM connector: CAN_H and CAN_L (see signal parameters)
Measure DC resistance across CAN_H and CAN_L with ignition off — expect ~60 ohms (two 120 Ω terminators in parallel)

Signal parameters

CAN bus typical idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each)
Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (typical differential ≈ 2.0 V)
Resistance across CAN_H and CAN_L (ignition OFF): ≈ 60 Ω (two 120 Ω terminators in parallel)
Common powertrain CAN bitrate: 500 kbps (varies by vehicle; some body networks use 250 kbps)

Diagnostic algorithm

Read all diagnostic trouble codes and freeze-frame data with a capable scan tool; record related communication codes.
Confirm whether the TCM appears on the scan tool module list. If missing, attempt a bus rescan and check other modules on same CAN network.
Check TCM power and ground circuits: verify fuses, relays, and measure key-on and cranking voltages at the TCM connector pins.
Visually inspect CAN wiring, splice points and connectors for damage, corrosion, or water ingress. Repair any obvious faults.
With ignition ON, measure CAN_H and CAN_L at the TCM connector and at a known-good node. Compare to expected idle voltages and look for stuck-high/low or excessive noise.
With ignition OFF, measure resistance across CAN_H and CAN_L to confirm termination ≈60 Ω. If open or very low, isolate which terminator or module is causing it.
Check for shorts to battery or ground on each CAN line (ohm and voltage tests). Disconnect suspected modules to see if bus communication is restored.
If wiring and power/ground check good, swap or bench-test the TCM only if a known-good unit and correct programming are available. Do not replace module solely based on code; confirm failure by elimination.
After repairs, clear codes, reprogram/recode module if replaced, and perform a road test while monitoring communications and transmission behavior.
If intermittent or unresolved, consult OEM wiring diagrams and module network topology; use an oscilloscope for in-depth bus waveform analysis.

Likely causes

Power to TCM lost (blown fuse, bad connector, poor ground)
Open/short in CAN wiring between TCM and junction splice or gateway
Bad/low-resistance termination (open or short across CAN H/L)
Faulty TCM or gateway module
Interrupted or misconfigured CAN network after recent repairs or module replacement

Fault status

⚠️ Status

No communication received from the Transmission Control Module on the CAN bus. Condition may be persistent or intermittent; may be caused by loss of power/ground to TCM, wiring faults, bad terminator, or failed module.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 4.0 hours

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U2106 — CAN-Bus no Communication with Transmission Control Module