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DTC codes / U00xx · Network wiring and bus faults / U0025

U0025Low Speed CAN Communication Bus (-) Low

U0025 indicates that the vehicle's low-speed Controller Area Network (CAN) data bus has a fault on the CAN negative (CAN-) circuit, which the modules use to communicate. The computer detects an abnormal voltage or signal condition on the CAN- wire, disrupting normal data exchange between control units.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaNetwork wiring and bus faults

Symptoms you may notice

  • Multiple warning lights on the dashboard, such as check engine, ABS, and airbag, due to lost communication between modules
  • Intermittent or complete failure of electronic features like power windows, radio, or climate control
  • The engine may start but run poorly or stall if critical modules cannot exchange data
  • Often no symptoms besides the warning light, especially if the fault is intermittent
  • Transmission may default to limp mode or fail to shift properly
  • Non-start condition if the bus fault prevents the immobilizer or engine control module from communicating

Common causes, most likely first

  1. Damaged or chafed CAN- wire in the low-speed bus, often due to rubbing against metal or pinching
  2. Corroded or loose connector pins at a control module or inline splice
  3. Failed control module internally shorting the CAN- line to ground or power
  4. Water intrusion into a connector or module causing corrosion and signal degradation
  5. Aftermarket electronics improperly tapped into the CAN bus, introducing noise or a short
  6. Open circuit in the CAN- wire from a broken wire or failed splice
  7. Terminating resistor failure inside a module, altering bus voltage levels

How to diagnose and fix it

  1. Use an OBD-II scan tool to read all stored codes and identify which modules are not communicating
  2. Obtain the vehicle-specific wiring diagram and locate the low-speed CAN bus connectors and modules
  3. Visually inspect CAN- wiring and connectors for damage, corrosion, or loose pins, especially at modules that are offline
  4. Measure resistance and voltage on the CAN- circuit with a digital multimeter (DMM) to check for opens, shorts, or incorrect bias voltage
  5. Disconnect modules one at a time to see if the bus fault clears, isolating a faulty module
  6. Repair or replace damaged wiring, connectors, or the faulty module as needed
  7. Clear codes and perform a road test to verify the repair and ensure all modules communicate properly

Don't overlook the simple wiring faults

  • A common mistake is replacing control modules before checking the CAN- wire for a short to ground, which is often caused by a pinched harness in the door jamb or under the dash.
  • Intermittent U0025 codes frequently trace to a loose ground connection for a module, which can mimic a bus fault and mislead diagnosis.
  • Aftermarket alarms or remote starters tapped into the CAN bus are a frequent culprit, so always check for non-factory wiring before condemning a module.
  • The low-speed CAN bus often uses a single wire for some functions, so a fault on CAN- may not disable all communication, leading some technicians to overlook it.
  • A failed module can pull the entire bus down, but disconnecting it may restore communication temporarily; this does not mean the module is good, as it may have an internal short.

Can I keep driving with U0025?

If the vehicle drives normally with only a warning light, you can drive cautiously to a repair shop, but avoid long trips or heavy electrical load. If multiple systems fail or the engine runs poorly, stop driving and have the vehicle towed to prevent unsafe operation.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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