U2105

Code

OPEL U — Network/User

CAN-Bus no Communication with ECM (Engine Control Module)

Brand: OPEL

Type: U — Network/User

Views: UK: 4 EN: 9 RU: 9

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

ECM is powered down, disabled or has internal fault
Blown fuse or interrupted power/ground to the ECM
Open, shorted or damaged CAN bus wiring or connectors between ECM and gateway/bus
Poor or corroded connector pins at ECM, gateway, or junctions
Bus termination missing or incorrect resistance (improper or damaged terminators)
Another module driving the bus incorrectly or bus-off condition

Symptoms

MIL/Check Engine lamp illuminated
No communication with ECM in diagnostic scanner (ECM not detected)
Engine cranks but will not start or starts and immediately stalls
Reduced performance or limp mode
Loss of engine-related data on dash (tachometer, engine temp, warnings)
Other CAN modules report errors or limited functionality

What to check

Connect scan tool and attempt to communicate with ECM; record status, freeze frame and any additional U-codes
Check for related U-codes and bus-off or network errors from other modules
Inspect ECM power and ground circuits; verify fuses and relay feeding ECM
Visually inspect ECM connector and wiring for corrosion, damage, loose pins, or water ingress
Check for presence of CAN bus activity with a scan tool or CAN bus monitor (message list and frequency)
Measure CAN_H and CAN_L voltages with key-on engine-off and during crank (see signal_params)

Signal parameters

Key ON, engine OFF: CAN High ≈ 2.5 V, CAN Low ≈ 2.5 V (recessive level)
Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Termination resistance across CAN_H to CAN_L ≈ 54–66 Ω (two 120 Ω terminators in parallel ≈ 60 Ω)
Expected message rates: engine-related IDs typically transmit from 1 Hz up to tens of Hz; specific ID/mapping depends on vehicle model
Bus voltage supply to ECM: normal battery voltage at power pins (verify against chassis battery)

Diagnostic algorithm

Record code(s) and attempt to communicate with ECM using a dealer-level or capable scan tool. Note whether the ECM responds and any additional U-codes present.
Check battery voltage and system power. Verify fuses and relays that feed ECM; restore any blown fuses and retest communication.
Inspect ECM connector and nearby wiring for mechanical damage, corrosion, water ingress, pin push-out or bent pins; repair or reseat connector as required.
Verify ECM ground(s) are clean, tight and low resistance to chassis; clean and retighten ground straps.
Use scan tool or CAN bus monitor to check for bus traffic. If no traffic, check wiring continuity between ECM and gateway/module/junctions.
Measure CAN_H and CAN_L voltages at ECM connector with ignition ON. Compare to expected signal parameters. If levels are incorrect, trace wiring for short to battery or ground.
Measure termination resistance across CAN lines with ignition OFF. If resistance is open or far from ~60 Ω, inspect terminators and wiring for open circuits.
If intermittent, wiggle test wiring harness while monitoring communication to reproduce fault (safe to do with ignition OFF or under controlled conditions).
If a short is suspected, isolate segments by disconnecting downstream modules one at a time until bus activity returns; repair or replace faulty segment.
If wiring and power/ground are good but ECM still does not communicate, check for module bus-off status via scan tool and attempt to clear codes and reset modules. Reflash or restore ECM software only if advised by manufacturer procedures.
If manufacturer TSBs or module configuration steps exist after replacement, follow reprogramming/coding procedure. Consider ECM replacement only after wiring and bus checks confirm module is at fault.

Likely causes

Blown fuse or missing power/ground to the ECM
Damaged/loose connector at the ECM or wiring harness chafed near firewall/engine
CAN bus short to battery or ground (damage after collision/repair)
Failed ECM or gateway putting bus into bus-off state

Fault status

⚠️ Status

No CAN communication detected with Engine Control Module. ECM not responding on vehicle CAN network; possible wiring, power/ground, termination, or module failure.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-4.0 hours

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U2105 — CAN-Bus no Communication with ECM (Engine Control Module)