U1700

Code

FIAT U — Network/User

CAN bus communication between engine control module and body computer

Brand: FIAT

Type: U — Network/User

Views: UK: 25 EN: 138 RU: 32

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Page language: EN

Open or shorted CAN wiring (damage, chafing, corrosion).
Poor connector contacts, pins pushed out, or water intrusion.
Missing or failed termination (open or wrong resistance).
Low battery voltage, poor ground, or intermittent supply to one or more modules.
Faulty body computer, ECM, or another node forcing bus into error state.
Software/firmware mismatch or required module reprogramming.

MIL on and multiple communication-related DTCs.
Loss of body functions controlled by ECM or body computer (e.g., instrument cluster, lighting, remote features).
Intermittent or permanent loss of data from one module (no live parameters).
Other modules reporting related U-codes or multiple modules failing to respond to scan tool.

Check battery voltage and good engine/vehicle grounds; low voltage can create communication errors.
Scan for U-codes and note which modules respond and which do not.
Visually inspect CAN wiring, connectors and junctions for damage or corrosion.
Measure bus idle voltages and bus resistance (termination) with key ON, engine OFF.
Wiggle test harness/connectors while monitoring live CAN traffic for interruptions.
If available, use an oscilloscope to view CAN_H and CAN_L waveforms for noise/dominant states.

CAN_H and CAN_L nominal idle ~2.5 V each (check vehicle spec).
Dominant state typical: CAN_H ~3.5 V, CAN_L ~1.5 V (differential ≈ 2.0 V).
Bus termination resistance ≈ 60 Ω measured across CAN_H and CAN_L (two 120 Ω terminators in parallel).
Common baud rates: 500 kbps or 250 kbps (verify vehicle specification).
Look for clean differential square waveforms without excessive ringing, distortion, or long dominant states.

Retrieve freeze frame, all stored U-codes and note which modules are not responding to the scan tool.
Verify battery voltage and main grounds; charge battery or connect maintained supply if low.
Visual inspection of wiring and connectors at ECM, body computer and intermediate junctions for damage/corrosion.
With key ON, measure CAN_H and CAN_L voltages at both ECM and body computer connectors; compare to expected values.
Measure termination resistance across CAN_H and CAN_L (engine OFF, both module connectors disconnected if required).
Use a scan tool or CAN bus analyzer to observe message traffic and error frames; perform a wiggle test at suspected sections.
Isolate the fault by disconnecting non-essential modules one at a time to identify a module causing bus-off conditions.
If a module is confirmed faulty, verify power/ground and replace or reprogram the module per manufacturer procedures.
After repairs, clear codes and verify restoration of normal CAN traffic and functionality; road test as needed.

⚠️ Status

U1700 — CAN communication error between ECM and Body Computer (missing or corrupted messages).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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