Comprehensive diagnostic guide for OBD-II code U0357

Disclaimer about the code meaning

• U codes are manufacturer/network diagnostic trouble codes. They point to issues on the vehicle's data communications network (CAN, LIN, K-line, etc.) rather than a single sensor input. The exact target module and the precise description of U0357 are vehicle-specific; OEM service information or a manufacturer- or vehicle-specific code definition is required for the exact wording and targeted module. For a general diagnostic approach, treat U0357 as a network communication fault affecting one or more modules on the vehicle's data bus. This placement aligns with how OBD-II U-codes are described in the OBD-II framework (Diagnostic Trouble Codes, U-codes) and the broader Powertrain/diagnostic code discussions on Wikipedia.

Symptoms

• MIL/Check Engine Light on with a U0357 stored
• Intermittent or complete loss of one or more module communications (example: no data from the PCM/ECM, TCM, ABS, instrument cluster, body control module, etc.)
• Scan tool shows "no communication" or "no response" with one or more ECUs while others respond normally
• Vehicle may exhibit erratic gauge readings, misbehavior of modules that rely on data from the network (transmission shifts, dash indicators, ABS/traction control indicators, etc.)
• Aftermarket devices or prior wiring work coinciding with the appearance of U0357

What you should know about U0357 (scope and interpretation)

• U-codes indicate network-level communication problems. They commonly arise from issues on the vehicle's data bus (CAN bus in most modern vehicles), including faulty modules, wiring harness problems, poor power/ground to modules, or bad network termination. Because the exact target module for U0357 is vehicle-specific, always check OEM service information for the precise module descriptor and any vehicle-specific notes.

Step-by-Step Diagnosis

1) Confirm the code context and related data

• Record the exact DTC: U0357. Note any related codes (P, B, C, U) that appear with it.
• Review freeze frame data to understand engine speed, load, voltage, ignition state, and other conditions at the time of fault occurrence.
• Check for recent repairs or aftermarket wiring that could affect the data bus.

2) Verify power, grounds, and basic electrical health

• Check battery voltage (vehicle should be in a healthy range, typically ~12.6-14.8 V when running).
• Inspect main and accessory fuses related to vehicle power, ignition, PCM/ECM, and other modules on the data bus.
• Inspect grounds to ECUs and the chassis; corrosion or loose grounds can create intermittent bus faults.
• If the vehicle was recentlyJump-started or battery disconnected, ensure all modules were restored to proper wake-up and that grounds are clean and tight.

3) Inspect the data bus wiring and connections

• Visually inspect CAN_H and CAN_L (and any other bus conductors used by the vehicle) for damage, chafing, heat damage, or aftermarket wiring that could load or short the bus.
• Check connector integrity for the suspected modules (pins, locks, and seal condition). Look for bent pins, bent terminals, or corrosion.
• Look for signs of contamination or water intrusion in connectors or harnesses near the PCM/ECM, ABS, instrument cluster, BCM, or other networked modules.
• If aftermarket devices were installed (alarm, remote start, radios, telematics), verify they are removed or isolated properly; these can cause bus conflicts.

4) Identify the likely affected module(s) on the network

• Use a capable scan tool to identify which modules respond and which do not. If some modules respond but one does not, the non-responding module or its bus segment is the likely root cause.
• If all modules fail to respond or the tool cannot identify any module on the bus, the problem could be one or more modules with severe load or a major bus fault (or a vehicle-specific fault on a secondary network).
• Note if the vehicle has multiple networks (high-speed CAN for powertrain, low-speed CAN for body/comfort; or separate networks). Identify which network the non-responsive module uses.

5) Perform live data and communication sanity checks

• With the scan tool, attempt to poll modules individually (or "ping" modules) to see which ones respond. Record response times and any error messages.
• Observe any data abnormalities during live bus operation (e.g., racing or unusual CAN_H/CAN_L activity, persistent dominant state, or irregular differential signals if you have an oscilloscope).
• If possible, capture CAN messages with a scope or specialized software to verify bus integrity and to identify if the bus is stable or if it shows dominant/recessive patterns inconsistent with normal operation.

6) Isolate and test the bus in a controlled manner

• If the bus shows a suspected fault, attempt to isolate sections or modules. This can involve:
  • Disconnecting suspected modules one at a time and observing if the fault code clears or if the system behavior improves.
  • Disconnecting non-essential modules to see if communications stabilize (do this carefully to avoid creating new faults in other subsystems).
• If isolating modules changes the fault state, the last disconnected module is a strong candidate for fault isolation.

7) Evaluate power/ground stability at each module

• With key on and engine off, measure supply voltage and ground integrity at the suspected modules and their connectors.
• Check for voltage dips during start or high electrical load (e.g., headlights on, A/C compressor cycling). A compromised supply to any one or more ECUs can cause loss of communication.

8) Review system software and firmware

• Some U-codes can be transient or caused by firmware/ Calibration mismatches. Check for OEM software updates or service bulletins related to U0357 or network faults for the exact vehicle.
• If a module has known firmware issues, an approved reflash or software update from the OEM or dealer may be required.

9) If no obvious hardware fault is found, perform controlled tests

• Reconnect all modules and clear codes. Drive the vehicle through multiple drive cycles to see if the code reappears and under what conditions.
• If the code returns under similar driving conditions or during certain electrical loads, re-check the bus wiring with a focus on the affected modules and any power-demanding devices on that network.

10) Final verification and documentation

• After a repair, re-scan the vehicle to confirm the code is cleared and no new codes appear.
• Perform a drive cycle and monitor live data to verify that module communications are stable and all previously dependent systems are functioning correctly.
• Document the fault scenario, parts replaced, wiring repairs, test results, and the final verification steps.

Probable Causes

Note: do not include NHTSA complaint data for U0357, so the probabilities below are informed by typical ASE-field diagnostic experience for network/communications DTCs and general vehicle-network fault patterns.

• Faulty or non-communicating module on the network (or a module not responding due to a bus fault): 40-55%

• Wiring/connector issues on CAN or other data-bus lines (damage, corrosion, loose connections, shorts to power/ground): 20-40%

• Power/ground or fuse-related issues affecting the data bus or a module's power supply: 5-15%

• Aftermarket devices or improper wiring causing bus interference: 5-15%

• Software/firmware mismatch or need for module reflash: 5-10%

• Unknown/less common hardware interactions (rare): 0-5%

Tools and tests to have on hand

• OBD-II scan tool capable of reading U-codes and showing live data and freeze-frame data
• Vehicle service manual or OEM/Werks service information for vehicle-specific U-coded definitions
• Multimeter or low-amp clamp for checking power and ground stability
• OBD-brick or CAN bus diagnostic tool for CAN-H/CAN-L analysis (scope is ideal for deep bus analysis)
• Oscilloscope or CAN bus analyzer (optional but highly helpful for diagnosing bus integrity)
• Visual inspection tools (flashlight, mirror, service pouches) for connectors and harnesses
• Clean, organized workspace and a repair manual for proper disassembly procedures

Safety Considerations

• Disconnect battery only when necessary and follow proper procedure to avoid electrical shock or system faults. Some modules rely on wake-up circuits and can reinitialize when power is restored.
• When inspecting or repairing high-voltage or high-current circuits, follow proper PPE and manufacturer procedures.
• Be mindful that disconnecting modules or altering wiring can cause other DTCs to appear; document and recheck everything after any changes.
• Avoid forcing wiring with connectors; damaged pins can worsen the fault or create new faults.

What to log and report when communicating with customers or service teams

• Exact DTC: U0357 with any related codes
• Freeze frame data (engine speed, vehicle speed, ambient conditions, voltage, etc.)
• Modules identified as responding or not responding during the scan
• Observed conditions during driving cycles when the fault occurs (speed, load, electrical load)
• Any aftermarket devices connected to the network or recent wiring changes
• Fuses and power/ground test results
• Step-by-step actions taken to isolate the fault and the results

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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