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Home / DTC / U1700 — CAN bus communication between engine control module and body computer

U1700 — CAN bus communication between engine control module and body computer

Detailed page for trouble code U1700.

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23,694specific
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U1700

ALFA ROMEO U — Network/User

CAN bus communication between engine control module and body computer

Brand: ALFA ROMEO
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Open or short in CAN High or CAN Low wiring between ECM and BCM
  • Poor ground or battery power to one or both modules
  • Faulty ECM or BCM (internal failure)
  • Missing or incorrect CAN termination resistors
  • Corroded or loose connector pins at ECM/BCM or intermediate junctions
  • Intermittent wiring harness damage (chafing, water intrusion)

Symptoms

  • MIL (check engine light) or multiple network warning lamps
  • Loss of certain body or powertrain functions (e.g., remote locking, instrument cluster messages)
  • Intermittent no-start or crank/no-start conditions if critical messages blocked
  • Erratic operation of systems shared across modules
  • Stored communication-related DTCs and possibly related module faults

What to check

  • Retrieve freeze-frame and module list with a factory-level or capable scan tool; note other U-codes and related P-codes
  • Check battery voltage (stable, fully charged) and main fuses/relays for ECM and BCM
  • Visually inspect ECM and BCM connectors, wiring harnesses, and any splice/junctions for corrosion, damage, or water ingress
  • Measure CAN bus voltages (CAN_H and CAN_L) at ECM and BCM with key ON, engine OFF
  • Measure bus resistance across CAN_H to CAN_L with power removed (expected ~60 Ω for two 120 Ω terminations in parallel)
  • Perform wiggle test on harness/connectors while monitoring live CAN traffic for interruptions

Signal parameters

  • Recessive idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (typical)
  • Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
  • Voltage tolerance: expect CAN_H ≈ 2.5–3.5 V and CAN_L ≈ 1.5–2.5 V depending on bus state
  • Termination resistance (power off): approx. 60 Ω across CAN_H and CAN_L for a network with two 120 Ω terminators
  • Common bus speeds: high-speed CAN typically 500 kbps (verify OEM spec for vehicle)

Diagnostic algorithm

  1. Confirm code and capture any additional U- and P-codes and freeze-frame data with a scan tool.
  2. Verify battery voltage and main power/ground integrity to ECM and BCM. Repair weak battery/grounds before further testing.
  3. Inspect physical wiring: follow CAN pair from ECM to BCM (and intermediate junctions). Look for chafing, pin corrosion, water traps, or damaged insulation.
  4. With ignition ON (engine off), measure CAN_H and CAN_L at the ECM and BCM connectors. Compare to signal parameter ranges. Note if one node shows strongly different voltages.
  5. With vehicle power off, measure resistance across CAN_H-to-CAN_L at multiple nodes; expect ~60 Ω. If much lower/ higher, suspect short/open or missing terminator.
  6. Using a capable scan tool, check for presence of ECM/BCM on the bus (module list). If a module is not responding, disconnect suspected faulty module and observe bus behavior (isolate to identify failing node).
  7. Perform a wiggle test on wiring/connectors while monitoring live CAN traffic for interruptions or error frames.
  8. If a specific connector or harness section is at fault, repair or replace wiring and connectors. Use OEM repair methods (solder/crimp, heat shrink) and protect against moisture.
  9. If wiring and power/grounds are good and one module is not communicating, consider reflashing or replacing the non-communicating module per OEM procedures.
  10. After repairs, clear codes, perform functional tests and a road test while monitoring the bus to confirm the fault does not return.

Likely causes

  • Broken or shorted CAN-L / CAN-H conductors between ECM and BCM
  • Bad ground at ECU/BCM or low battery voltage
  • Open/incorrect termination (missing 120 Ω resistors or short to ground)
  • Corroded connector at ECM/BCM or junction splice
  • Faulty body computer module

Fault status

⚠️ Status
Communication error on CAN bus between Engine Control Module and Body Computer; messages missing or corrupted. Check power/ground, wiring continuity, terminations, and module operation.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5 - 3.0 hours
89

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