Comprehensive diagnostic guide for OBD-II code U0373

Important Notes

• Class: U-coded DTC (vehicle network/communication fault). U-codes describe data communication issues between modules on the vehicle's data network (e.g., CAN). Exact OEM definitions for U0373 can vary by manufacturer; the generic interpretation is a network/communication problem rather than a single sensor fault. For OEM-specific definitions, consult the vehicle service information and standard code dictionaries.
• Sources and context: This guide draws on general OBD-II concepts from the Wikipedia OBD-II articles (Diagnostic Trouble Codes, Powertrain Codes) to frame how DTCs are generated and interpreted, and to align with the notion that modern diagnostics monitor network communications among control modules. See: Wikipedia - OBD-II, Diagnostic Trouble Codes; Powertrain Codes. Where applicable, the guidance also reflects standard practices implied by the general definitions of U-codes as network-related.
• NHTSA data note: No NHTSA complaint data for U0373 is provided you shared. Where possible, probabilities for causes are informed by ASE field experience and typical network-diagnosis patterns; if and when real NHTSA data exist, they should supersede these estimates.

1) Quick definition and what to expect

• U0373 is a network/communications-related DTC. It indicates that the vehicle's on-board diagnostic network is not functioning as expected between one or more modules. The issue is typically not a single sensor fault but a data-link problem, which can be caused by wiring, connectors, power/ground, or a malfunctioning module or gateway.
• OEM definitions vary; verify with OEM service information and use standard code dictionaries for exact wording per vehicle.

2) Common symptoms a customer might report

• MIL (Malfunction Indicator Lamp) illuminated or flashing intermittently.
• Other modules failing to report data or behaving erratically (e.g., speedometer, ABS/traction control, transmission control, ECU data not updating in the scan tool).
• Intermittent loss of communication to the PCM or other modules; scan tool shows DTCs for multiple modules or "no data" on several live PID readings.
• Vehicle runs/starts sporadically poorly or with hesitation when the data network is unstable.
• Intermittent error messages on the scan tool such as "no response," "bus-off," or "data not available" from certain modules.

3) Probable causes and their provisional likelihood

• Loose, damaged, or corroded CAN bus wiring or connectors (30-50%)
• Faulty or intermittent control module (PCM/ECU, gateway, or other NCAM/vehicle network node) (15-30%)
• Ground or power supply anomalies affecting one or more network nodes (battery/alternator issues, poor chassis ground, power supply instability) (10-20%)
• Aftermarket devices, bad fuses, or rogue wiring creating network interference (5-15%)
• Software/firmware issues on one or more modules or gateway (5-15%)
• Intermittent impedance or short to battery/ground on a data line (5-10%)

Note: These ranges are approximate and reflect typical network-diagnostic patterns observed in the field. OEM-specific definitions and conditions can shift these probabilities.

4) Required tools and data

• OBD-II scan tool capable of reading U-codes and viewing live data; preferably with CAN bus support and the ability to monitor multiple modules on the network.
• Multimeter (digital preferred) and a data-logger/oscilloscope or breakout box for measuring CAN_H and CAN_L activity if available.
• Service information for the vehicle (OEM wiring diagrams, connector pinouts, and any known TSBs or reflash/ECM update notes).
• Electrical spray/wire-resistance check tools, contact cleaner, and flashlight for visual inspection of connectors and harnesses.
• Personal protective equipment as appropriate (gloves, safety glasses, disconnect procedures).

5) Diagnostic flow (systematic approach)

Phase A - Confirm and scope

• Record all DTCs: Note all U-codes present, plus any related DTCs from other modules. Check for concurrent issues (e.g., powertrain DTCs, body/electrical DTCs).
• Check vehicle condition: Battery voltage healthy (typically ~12.6 V with engine off; higher with engine running). Prior to diagnosing a CAN network issue, ensure the vehicle's electrical system is solid.
• Review recent work or changes: Aftermarket radios, amplifiers, telematics devices, alarm systems, or poor-quality wiring may introduce bus interference.

Phase B - Visual and mechanical inspection

• Inspect major harnesses and connectors for the PCM and gateway modules, including:
  • CAN bus harnesses for damage, pin corrosion, pin back-out, or wedging with other connectors.
  • Evidence of moisture, corrosion, or heat damage on connectors.
  • Any aftermarket wiring that could intersect the CAN lines.
• Inspect grounds and power supply:
  • Verify battery negative ground points are clean and secure.
  • Check for corrosion on battery terminals and chassis grounding points.
  • Confirm fuses related to power to CAN network and gateway modules are intact.
• Look for aftermarket devices or poor installations that could drop voltage or inject noise on the data lines.

Phase C - Data verification and isolation

• Use the scan tool to observe live data from as many modules as possible. Look for:
  • Missing or inconsistent data across modules that should be present.
  • Modules reporting "no data" or failing to respond when polled.
• If the vehicle has a gateway or central data controller, observe the data paths:
  • Note whether the gateway shows communication with some, but not all, modules.
  • If a single module is not responding, suspect that module or its local wiring, rather than the entire network.
• CAN bus voltage verification (if accessible with equipment):
  • Check CAN_H and CAN_L differential activity during normal operation and during simulated load; abnormal voltages can indicate a module fault, wiring issue, or grounding problem.
  • Look for chatter, abrupt voltage drops, or leakage that could indicate shorts.

Phase D - Fault isolation and targeted testing

• If accessible, test the suspected module or gateway:
  • Disconnect the suspect module and re-scan (observe if DTCs related to U0373 clear or if other modules begin to report normal data). If the DTC clears when a module is disconnected, the module is a likely fault source.
  • Inspect the suspect module's power and ground pins for continuity and voltage when connected.
• Check for known OEM issues:
  • Review TSBs or service bulletins for gateway/module issues related to data-line stability in the specific vehicle.
  • Confirm there are no recalled or updated ECUs for this model year that address network communication issues.

Phase E - Reproduction and verification

• After repairs or changes, clear DTCs and perform a controlled test drive while monitoring data streams.
• Confirm the U0373 does not reappear and that data exchange among modules remains consistent under varying load conditions (idle, light throttle, and moderate driving).
• Recheck for any newly generated DTCs after testing.

6) Representative diagnostic steps you can follow (condensed)

• Step 1: Retrieve all DTCs with ignition on, engine off; note U0373 and any related codes.
• Step 2: Inspect power, ground, fuses to all networked modules; correct any obvious issues.
• Step 3: Visually inspect CAN bus wiring and connectors for damage, corrosion, or improper routing; repair as needed.
• Step 4: Test connectors on PCM/gateway; reseat and, if needed, replace damaged connectors or harness sections.
• Step 5: Check for aftermarket devices that could inject noise; remove or properly isolate them.
• Step 6: Monitor live data; identify modules that fail to respond or provide inconsistent data.
• Step 7: If possible, isolate modules by disconnecting and observing effects on DTCs; prioritize the gateway/PCM and the most critical data-line modules.
• Step 8: After any repair, clear codes and perform road test while re-scanning to verify stability.

7) Repair options (typical when U0373 is confirmed)

• Repair or replace damaged wiring/pins on CAN bus lines; ensure correct routing and proper strain relief.
• Replace faulty module(s) implicated by the isolation tests (e.g., PCM, gateway, or a non-communicating node).
• Correct power/ground integrity issues, including repairing ground straps, battery connections, or power supply wiring.
• Update or reflash affected modules if OEM service information indicates software issues; ensure software compatibility among networked modules.
• Remove or correctly install aftermarket devices that interfere with the data bus; ensure any added hardware is properly grounded and isolated.

8) Validation and follow-up

• After repair, clear all DTCs, perform a comprehensive drive cycle, and re-scan for any reoccurrence of U0373 or related codes.
• Confirm that all modules are communicating as expected and that live data is consistent across the network.

9) Safety considerations

• Disconnect the battery only with appropriate procedures and PPE; avoid shorting or accidental energizing of circuits while inspecting or wiring.
• When probing data lines, use proper tools and non-conductive access methods to avoid shorts or shocks.
• Handle modules with care; avoid static discharge and follow manufacturer guidelines for disconnect/reconnect procedures.
• If airbag or high-voltage systems are present, exercise additional caution; follow standard safety procedures and vehicle-specific guidelines.

10) References and notes

• General DTC and OBD-II framework: Wikipedia - OBD-II, Diagnostic Trouble Codes; Wikipedia - OBD-II, Powertrain Codes. These sections describe how trouble codes are generated, the role of DTCs, and the broad categories of codes used in modern vehicles.
• Standard code information (classification): U-codes are network/communication related in the OBD-II framework. OEM meanings can vary; see OEM service info and GitHub definitions for standardized interpretations of U0373. (General concept drawn from OBD-II code structure described .)
• If available, consult vehicle-specific OEM service information and TSBs for U0373 to obtain the exact OEM definition and any model-year-specific diagnostic steps.

Closing guidance

• Because U0373 is a network/communication code, the most common root causes are wiring/connectors and module/gateway faults or power/ground issues. A methodical approach to inspect, measure, and isolate the CAN network is essential. OEM-specific definitions for U0373 can differ, so always cross-reference with the vehicle's service literature and standardized code dictionaries to confirm the exact OEM wording and recommended procedures for your particular make/model.
• If more precise OEM diagnostics become available, update this guide with the OEM-recommended steps and test values for U0373 on the target vehicle.

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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