U-Codes (Network / Communication)

OBD-II U-codes are communication and network faults. They usually indicate a control module has lost communication with another module, or that the vehicle’s network (often the CAN bus) is seeing errors. U-codes can trigger multiple warning lights, limp mode, or intermittent no-start issues.

CAN bus Lost communication Multiple warning lights Voltage / wiring

Find Your U-Code

U-codes often appear alongside P-, B-, or C-codes. The most useful clue is usually which module is missing (PCM/ECM, TCM, ABS, BCM, instrument cluster, etc.).

If the vehicle won’t start or the dash is going crazy: Network/voltage issues can escalate quickly. If you have severe drivability symptoms, prioritize safe troubleshooting and consider professional diagnosis.

What U-Codes Usually Mean

Most U-codes fall into a few patterns:

  • Lost communication with a module (example: ECM can’t “see” the ABS module).
  • Invalid data received (a module is sending data that doesn’t make sense).
  • Bus off / bus error conditions (CAN bus short/open, noise, or a module pulling the network down).
  • Voltage events (weak battery, charging problems, or bad grounds causing modules to reset).

U-Code Sections (Browse by Root Cause)

Most Common
Low Voltage / Power & Grounds

Weak battery, alternator issues, corroded grounds, loose connectors causing module resets.

Often multiple modules show codes at once
 Network
CAN Bus Wiring & Shorts

Short to power/ground, open circuits, damaged twisted pair wiring, water intrusion.

Can cause “bus off” and no-start
 Modules
Module Failure / Sleep-Wake Issues

A failing module can pull down the network or fail to wake up, triggering intermittent U-codes.

Intermittent faults are common
 Tools
Scanning Strategy

Why you need a scanner that reads all modules and shows network topology where possible.

Best practice for “lost comm” codes

Low Voltage / Power & Grounds (Most Common Root Cause)

A large percentage of U-codes are triggered by low voltage events—especially after a weak battery, jump start, alternator problems, or poor grounds. When modules reset at different times, they report missing messages and store U-codes.

Troubleshooting tips — battery/charging checks and basics B-Codes hub — BCM/body module issues often tied to voltage C-Codes hub — ABS/ESC modules commonly involved in U-codes

CAN Bus Wiring & Shorts

CAN bus wiring is typically a twisted pair running between modules. Damage near common pinch points (door jambs, under seats, behind kick panels, or near the battery/fuse box) can create intermittent communication errors—especially after water intrusion.

Code Library — search your U-code and related companion codes Scanners & Tools — models that read network modules

Module Failure / Sleep-Wake Issues

A failing module can stop responding or can flood the bus with bad messages. Symptoms may include random warning lights, battery drain, or intermittent no-start. These issues can be tricky—multiple U-codes may be “symptoms” of one module causing disruption.

Scanner guide — full-system scans and module lists Search the library — identify the missing module referenced

Scanning Strategy (How to Diagnose U-Codes Faster)

For U-codes, start with a full module scan. Note which module is reporting the fault and which module is missing. If multiple modules report the same missing module, the missing module (or its power/ground/network wiring) is often the priority.

How to Use an OBD2 Scanner — stored vs pending and module scans Code Library — look up U-codes by number
Best practice: If you have U-codes plus lots of unrelated symptoms, check battery voltage and grounds first. Many communication errors disappear after stabilizing power—then the remaining codes point to the true fault.