U1711

Code

ALFA ROMEO U — Network/User

CAN bus communication between engine control module and transmission

Brand: ALFA ROMEO

Type: U — Network/User

Views: UK: 8 EN: 15 RU: 9

AI status

Completed

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Page language: EN

Causes

Open or short in CAN wiring between ECM and TCM
Corroded, loose or damaged connectors at ECM, TCM or splice points
Missing or incorrect CAN termination (open/shorted terminator resistors)
Faulty ECM or TCM (internal module failure)
Low or intermittent battery/ignition supply or poor ground to one of the modules
Aftermarket device interfering with CAN bus (alarm, radio, tuner)

Symptoms

MIL/engine warning lamp illuminated
Transmission may go to limp/backup mode or shift erratically
Loss of transmission-related functions or incorrect gear indications
No/limited communication with TCM via diagnostic scanner
Other modules show related communication errors or reduced functionality
Intermittent faults that may appear/disappear or be persistent

What to check

Connect a diagnostic scanner and capture U-codes and freeze frame data; attempt communication with ECM and TCM
Visually inspect CAN wiring, seals and connectors between ECM and TCM for damage, corrosion or water ingress
Measure resistance across CAN_H to CAN_L with ignition off — expected ≈60 Ω for a properly terminated bus
With ignition on, measure CAN_H and CAN_L voltages (see signal parameters) and check for expected recessive/dominant levels and no stuck high/low
Check battery voltage and module power/ground circuits at both ECM and TCM
Disconnect aftermarket devices that interface with the bus and re-check communication

Signal parameters

Recessive (idle) state: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both near mid-supply)
Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Bus differential: ~0 V when recessive, ~2 V when dominant
Termination resistance: ≈ 60 Ω measured across CAN_H to CAN_L (two 120 Ω terminators in parallel)
Common bit rates: typically 250 kb/s or 500 kb/s depending on vehicle network

Diagnostic algorithm

Read all stored U-codes and freeze-frame data with a capable scanner; note time stamps and related codes.
Attempt direct communication with both ECM and TCM. If one module cannot be reached, record that behavior.
Visually inspect connectors and wiring between ECM and TCM. Repair any physical damage or corrosion before further testing.
With ignition off, measure resistance between CAN_H and CAN_L at convenient connector point — expect ~60 Ω. If open or very low, trace/repair short/open.
With ignition on, measure CAN_H and CAN_L voltages at connector pins under no-traffic and while exercising bus (e.g., crank or operate related systems). Compare to signal parameters.
If voltages are abnormal, check module power and ground circuits (fuses, relays, battery voltage at harness pins). Repair as needed.
Isolate the fault by disconnecting sections/modules (one at a time) and re-measuring bus resistance/behavior to find which segment causes the fault. Avoid disconnecting modules while engine running unless manufacturer permits.
If wiring and power/grounds are good but communication still fails, consider reflashing software or replacing the module that fails to communicate, after confirming with a known-good module or bench test where possible.
Clear codes and perform a controlled road test to verify the repair and that no communication errors return.

Likely causes

Damaged or chafed CAN wires between ECM and TCM
Corroded/loose connector at one of the control modules
Open or shorted CAN termination resistor(s) (wrong resistance between CAN_H and CAN_L)
Intermittent power/ground to ECM or TCM
Faulty transmission control module

Fault status

⚠️ Status

CAN communication fault between Engine Control Module (ECM) and Transmission Control Module (TCM): messages are not exchanged or are corrupted. Check bus wiring, termination, power/ground and module operation.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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