P1120

Code

OPEL P — Powertrain

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

Brand: OPEL

Type: P — Powertrain

Views: UK: 5 EN: 10 RU: 9

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Page language: EN

Causes

ECM not powered or no battery/ignition supply
Blown fuse or faulty relay supplying ECM
Damaged or disconnected CAN-H / CAN-L wiring or connectors
Short to battery or ground on CAN lines
Open network (missing termination resistors) or incorrect termination
Faulty ECM or internal ECU processor/communications fault

Symptoms

MIL (check engine light) illuminated or flashing
No response from ECM on diagnostic tool / "No Communication"
Engine may not start or run poorly if ECM is not responding
Dashboard gauges or other modules acting erratically
Loss of certain functions controlled by ECM (fuel, ignition control)
Multiple network-related DTCs present

What to check

Verify battery voltage (12.4–12.8 V resting; >13.5 V with engine running) and ensure good battery connections
Check fuses and relays for ECM and CAN power circuits
Attempt to communicate with other modules (ABS, BCM) to confirm partial network operation
Inspect ECM connector for corrosion, bent pins, water ingress or loose terminals
Measure CAN bus voltages with key ON, engine OFF: CAN-H and CAN-L absolute voltages and differential
Measure resistance across CAN-H to CAN-L with power off (expected ~60 ohms for two 120Ω terminators in parallel)

Signal parameters

Recessive (idle) voltages: CAN-H ≈ 2.5 V, CAN-L ≈ 2.5 V (both near mid-rail); typical idle spread small
Dominant state during traffic: CAN-H can go up to ≈ 3.5 V, CAN-L down to ≈ 1.5 V
Differential voltage during dominant bits ≈ 2.0 V
Termination resistance (power off) across CAN-H/CAN-L ≈ 60 Ω (two 120 Ω terminators in parallel)
Common CAN bus speeds on passenger cars: 250 kbps or 500 kbps (verify factory spec for vehicle)

Diagnostic algorithm

Record all stored codes and freeze frame data from the scan tool; note whether only ECM is non-responsive or multiple modules fail.
Check battery state and recharge if low. Retest communication with known-good scan tool.
Check fuses/relays that supply the ECM and CAN power. Replace any blown fuses and retest.
Visually inspect ECM connector and CAN wiring for damage, corrosion, improper repairs or aftermarket taps.
With ignition ON (engine OFF), measure CAN-H and CAN-L voltages at ECM connector and at an accessible junction. Compare to expected values.
With ignition OFF and battery disconnected, measure resistance across CAN-H and CAN-L to confirm termination ≈60 Ω. If open or abnormal, trace wiring for broken terminator or open circuit.
Check continuity from ECM CAN pins to other module connectors to locate open/short. Isolate segments by disconnecting modules to find which segment causes failure.
Check for shorts to battery + or ground on CAN lines using an ohmmeter (power off). Repair any faults found in wiring or connectors.
Use an oscilloscope (preferred) or scan tool data to verify CAN frames and signals while cranking/idle. Look for noise, dominant state stuck, or missing traffic.
If wiring and termination are good and other modules communicate, suspect ECM hardware/software fault. Verify ECM supply voltages and grounds at the unit.
Only after confirming ECM is at fault, replace or refl ash/reprogram ECM following manufacturer procedures (immobilizer pairing may be required).
Clear codes and test drive. Confirm normal CAN communication and that no related codes return.

Likely causes

ECM power/ground fault or blown fuse
Broken/disconnected CAN wiring or corroded connector at ECM
Missing or damaged CAN termination (bad resistor or short)
Faulty ECM or module interfering with the network

Fault status

⚠️ Status

No communication with ECM over CAN bus: diagnostic tool cannot communicate with the Engine Control Module. Network may be down due to power/ground, wiring, termination, or ECM fault.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-4 hours

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P1120 — CAN-Bus No Communication with ECM (Engine Control Module)