Comprehensive diagnostic guide for OBD-II code U0055

What This Code Means

• What the code represents in general: U-codes are network/communications codes in OBD-II. They indicate issues on the vehicle's on-board data networks where modules fail to communicate or report properly. This family of codes is distinct from the P, B, and C families (which are powertrain, body, and chassis subsystems). (
• OEM specificity: The exact definition of U0055 (the precise module-to-module communication fault) is OEM-specific. do not include an explicit, universal definition for U0055. When diagnosing, you should consult the vehicle's service information for the OEM's definition and required test procedures. (

Important Notes

• This guide uses the general interpretation of U-codes (network/communication issues) and provides a practical diagnostic flow that applies to most make/model systems. The exact root cause of U0055 will depend on the vehicle's network topology (which modules participate, how many networks exist, and where terminations are located) per OEM documentation.
• Since do not include NHTSA complaint statistics for U0055, probability weights are and common patterns seen with CAN/vehicle-network faults. If NHTSA data later becomes available, you should adjust the probability estimates accordingly.
• Real user complaints: U-codes typically present as intermittent or persistent "cannot communicate" situations, often with multiple modules appearing not to respond on scans, or the scanner reporting "no data from module," "bus-off," or "lost communication." Expect the vehicle to show a lit Malfunction Indicator Lamp (MIL) in some cases and no obvious engine fault in others. The symptom set described here reflects common user-reported experiences with network diagnostic codes in general (not a vehicle-specific dataset ).

Symptoms

• Intermittent or persistent loss of communication with one or more control modules on the vehicle network (e.g., ECM/PCM, BCM, TCM, ABS, instrument cluster, SRS, etc.).
• When scanning, some modules report inability to read or communicate; the primary scan shows U0055 with or without accompanying codes from other modules.
• MIL may be on (or not) depending on OEM behavior and which modules are affected.
• Vehicle may exhibit operational quirks tied to network behavior (e.g., cruise control behaving oddly, dash indicators flickering, or other modules behaving inconsistently).
• After full diagnostic scans or specific module work, the same U0055 may reappear, suggesting a non-persistent bus issue or intermittent fault.

Probable Causes

• Faulty or intermittent CAN bus wiring/connectors (damaged harness, loose grounds, corroded connections): ~25%
• Power/ground problems supplying modules on the network (low battery, failing alternator, grounds not solid, battery negative-to-chassis integrity): ~25%
• One or more control modules on the network failing to respond or having corrupted software/firmware (ECU/TCM/BCM/etc.). Sometimes a single module pulling the bus down or incorrect baud rate/firmware mismatch can cause multiple modules to drop off: ~20%
• Aftermarket devices or incorrect wiring causing bus contention or improper termination (alarms, radios, telematics, remote start, etc.): ~15%
• Network termination or physical layer issues (damaged terminations, damaged CAN High/Low wiring topology, impedance issues): ~10%
• OEM software/communication protocol issue or required service bulletin (less common, but possible in some platforms): ~5%

Diagnostic Approach

1) Confirm scope and gather data

• Retrieve all codes across the vehicle with a CAN-capable scanner. Note any pattern (which modules report not communicating; any related codes in addition to U0055).
• Collect freeze-frame data (if available) and note vehicle speed, ignition state, battery voltage, and any module-specific data present at the time of the fault.
• Record vehicle make/model/year, engine type, and any aftermarket devices installed.

2) Stabilize electrical power and grounding

• Check battery condition, charging voltage, and voltage drop under load. A weak or fluctuating supply can cause intermittent module communication failures.
• Inspect and clean main grounds (engine block to chassis, battery negative strap, and major modular grounds) and verify good continuity.
• Check for corrosion, damaged insulation, or pinches along primary CAN routing paths.

3) Visual and mechanical inspection of the network

• Inspect CAN High (CAN-H) and CAN Low (CAN-L) wiring for damaged insulation, chafing, or pinched harnesses near the engine, undercarriage, and behind instrument panels.
• Inspect connector pins for corrosion, bent pins, or loose fits; reseat critical connectors if accessible.
• Look for aftermarket wiring that could interfere with the network (alarm systems, remote starters, radios, telematics modules). Temporarily disconnect suspect aftermarket devices to see if U0055 behavior changes.

4) Isolate and test the network

• Identify the vehicle's CAN topology (which modules are on the same bus, how many buses exist, where terminations are). If you have OEM wiring diagrams, verify end-term resistors and any bus splitters.
• If feasible, perform a controlled test:
  • Disconnect individual modules (one at a time) that are known to be on the affected network and observe whether U0055 clears or returns. A module that, when disconnected, eliminates the fault suggests the module or its interface is the root cause.
  • Check for proper 12V power presence and ground on each module's supply pins. Some modules require stable power to maintain bus integrity; a failing module that powers down intermittently can cause bus dropouts.
• Check for proper CAN signal quality with the scanner or oscilloscope if available. Look for excessive bit-stuffing, frames with abnormal error counts, or persistent error frames that indicate bus contention or a faulty node.

5) Software and OEM service considerations

• Check for OEM service bulletins or firmware/Calibration updates related to the vehicle's network / control modules. Some U-codes resolve after software/firmware updates or reprogramming of one or more modules.
• If a specific module is identified as suspect, consider OEM-recommended reflash or reprogram procedure, ensuring the vehicle battery is protected and the ignition is in the correct state during programming.

6) Resolution paths (typical fixes)

• Repair or replace damaged wiring/terminals on the CAN bus; re-seat or replace affected connectors; repair insulation as needed.
• Replace or reprogram a malfunctioning module that is causing bus communication problems; ensure compatibility with other modules and correct baud rate settings per OEM specs.
• Remove or properly isolate aftermarket devices that are causing bus contention or impedance changes; restore proper bus termination if aftermarket devices affected network behavior.
• Correct power/ground issues; replace failing battery, alternator, or grounding straps as needed.
• Apply OEM software/firmware updates if a compatibility or bug in the network stack is identified.

What data to collect during testing

• Exact vehicle information (make/model/year), engine type, and network topology if known.
• A list of all modules that show communication problems or fail to respond.
• Battery voltage in key-on and running states; voltage drop measurements on critical grounds.
• Visual inspection findings (wiring damage, ground corrosion, loose connectors, aftermarket devices disconnected).
• OEM service information references (bulletin numbers, module reprogramming instructions) if available.

Safety Considerations

• Do not undermine safety systems (e.g., SRS/Airbags) while probing networks. Disconnecting or removing modules associated with safety-critical systems should be approached with caution and ideally performed by qualified personnel.
• When working around the vehicle's electrical system, ensure the ignition is off when disconnecting modules unless OEM procedures specify otherwise; protect yourself from unexpected module power resets or airbag system issues.
• Follow proper ESD precautions when handling control modules and connectors.

How to document your findings

• Create a diagnostic journal entry with: vehicle data, symptoms, all codes read (including U0055 and any related codes), data from live scans, test results, and step-by-step actions taken.
• Note final repair actions and verification steps (e.g., "U0055 cleared after replacing module X and repairing CAN wiring; multiple drive cycles show no reoccurrence; OEM software version updated to Y").

Summary

• U0055 is a network/communications fault code within the U family. The exact OEM meaning of U0055 is vehicle-specific, so you should confirm the OEM definition in the service information for the particular vehicle. The diagnostic plan should focus on ensuring network integrity (power/ground, wiring, connectors, and module health) and removing any interference from aftermarket devices or software faults. A systematic approach-verify power, inspect the CAN bus, isolate modules, and apply OEM-recommended software updates-will give you the best chance to identify and fix the root cause.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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