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DTC codes / U00xx · Network wiring and bus faults / U0035

U0035Vehicle Communication Bus A (-) High

U0035 means the vehicle's primary communication bus, a twisted pair of wires known as CAN (Controller Area Network) Bus A, has its negative or low side wire stuck at a voltage higher than the normal idle level. The engine control module or another module acting as the bus monitor detected that the CAN low line is being pulled toward battery voltage instead of resting near 2.5 volts, which disrupts digital messages between modules.

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

Symptoms you may notice

  • Multiple warning lights illuminated at once, including the check engine light, ABS, airbag, and traction control lamps
  • The engine may crank but not start, or may start and run poorly because modules cannot share data
  • The transmission may default to a limp mode with harsh or delayed shifting
  • The instrument cluster may show dashes or zeros instead of speed, rpm, and temperature readings
  • Non-critical features such as the radio, power windows, or climate control may stop working
  • Often no symptoms besides the warning light if the fault is intermittent and the bus recovers between drive cycles

Common causes, most likely first

  1. A chafed or shorted CAN low wire touching a power wire or battery voltage source somewhere along the harness
  2. Water intrusion into a connector on the communication bus, causing corrosion that bridges the low line to a higher voltage
  3. A failed module on the bus that is internally shorting the CAN low line high and dragging down the entire network
  4. A poor ground connection for one of the modules, causing the bus reference voltage to float high
  5. Aftermarket accessories such as alarms, radios, or telematics units improperly spliced into the CAN wiring
  6. A damaged or improperly terminated CAN bus splice pack or junction connector

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes from every module, then clear them and note which codes return first to identify the affected bus segment
  2. Measure resistance between CAN low and CAN high at the diagnostic link connector with the battery disconnected; a reading near 60 ohms confirms the bus is intact, while a very low or very high reading points to a wiring or module fault
  3. Backprobe the CAN low circuit at the diagnostic link connector with the key on and confirm the voltage is stuck above the normal idle level, then disconnect modules one at a time to see if the voltage returns to normal
  4. Inspect the harness along its known routing points for chafing, pinches, or contact with power wires, and check every bus connector for corrosion or water intrusion
  5. Disconnect aftermarket accessories one at a time to rule out a device that is backfeeding voltage onto the CAN low line
  6. Repair or replace the shorted section of wire, clean and reseal any corroded connector, and verify the bus voltage returns to its normal idle level before reassembling
  7. If a module is found to be the source of the short, replace or reprogram that module and perform a full network scan to confirm all modules communicate cleanly

Why one bad module can shut down the whole bus

  • A single module with an internal short on the CAN low line can pull the entire network down, so technicians often disconnect modules one by one until the bus voltage recovers rather than replacing parts at random
  • The code does not always mean the wiring is damaged; a failed module, a bad ground, or even a low battery can cause the bus low line to read high, so voltage and resistance tests come before any parts replacement
  • Aftermarket alarms and remote starters are a commonly missed cause because they are spliced into the CAN wiring and can backfeed voltage onto the low line without any visible damage to the factory harness
  • U0035 is often stored alongside U0001, U0002, U0100, and U0140 because when the bus goes down, every module that talks on it logs a communication loss code
  • Water intrusion into a connector under the carpet or in a door jamb can corrode the CAN low pin and bridge it to a neighboring power wire, which is easy to overlook when the harness looks fine from the outside

Can I keep driving with U0035?

Do not drive the vehicle with an active U0035 because the loss of communication can disable safety systems such as ABS and airbags and may cause the engine to stall or fail to restart. If the light appeared intermittently and the vehicle runs normally, drive only to a safe location or a repair shop and avoid highways.

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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