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DTC codes / U00xx · Network wiring and bus faults / U0040

U0040Vehicle Communication Bus B (+) Low

U0040 is a generic network communication code indicating that the vehicle's main communication bus, often called Bus B or the high-speed CAN (Controller Area Network) bus, has a low voltage condition on its positive line. The computer that set this code detected that the bus voltage stayed below the normal threshold for a set period, which prevents modules from exchanging data reliably.

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

Symptoms you may notice

  • Multiple warning lights illuminated on the dashboard, often including the check engine light, ABS, and airbag lights
  • The engine may start and run, but other electrical systems like the radio, windows, or climate control may act erratically or not work
  • Intermittent or complete failure of dashboard gauges and indicator lamps
  • The vehicle may enter a limp mode or fail to start if the bus is completely down
  • Often no symptoms besides the warning light, especially if the fault is intermittent
  • Scan tool may be unable to communicate with some or all control modules

Common causes, most likely first

  1. A short to ground or chafed wire on the Bus B positive circuit, often caused by rubbing against metal or a pinched harness
  2. A faulty control module internally shorting the bus positive line to ground
  3. Corroded or damaged connector pins at any module on the bus, leading to high resistance or intermittent contact
  4. Water intrusion into a connector or module, causing corrosion and a low-voltage condition on the bus
  5. A failed termination resistor inside a module, altering bus voltage levels
  6. Aftermarket accessories improperly tapped into the bus wiring, creating a partial short

How to diagnose and fix it

  1. Use an OBD-II scan tool to read all codes and note which modules are communicating and which are not, as this narrows the search area
  2. Visually inspect the Bus B wiring and connectors at each module for chafing, corrosion, or loose pins, starting with the modules that are offline
  3. Measure the voltage on the Bus B positive line with a digital multimeter with the ignition on; a reading significantly below the normal 2.5 to 3.5 volts indicates a short or excessive load
  4. Disconnect modules one at a time to see if the low voltage condition clears, isolating a faulty module that is shorting the bus
  5. Perform a resistance check on the bus with the battery disconnected; look for readings outside the normal 60-ohm range that would indicate a wiring or termination fault
  6. Repair or replace the damaged section of harness or the faulty module, then clear codes and verify communication with all modules
  7. If the fault is intermittent, use a graphing multimeter or oscilloscope to monitor bus voltage while wiggling harnesses and connectors to locate the intermittent short

What most technicians overlook with U0040

  • A low voltage on Bus B does not always mean the bus itself is faulty; a module that is failing to power up can drag the bus low, so check powers and grounds at each module before condemning the wiring.
  • Aftermarket alarms, remote starters, or phone chargers plugged into the diagnostic connector can backfeed the bus and set this code, so always ask about recent electrical work.
  • The code can be set by a weak battery or poor charging system, as low system voltage can cause modules to misbehave and pull the bus down; load-test the battery and check charging voltage first.
  • Corrosion inside a connector that looks fine from the outside is a common culprit, especially in vehicles with leaks or in humid climates; disassemble and inspect the terminals closely.
  • U0040 often appears alongside other U-codes, but the root cause is frequently a single module or a shared power/ground point, not multiple failed modules.

Can I keep driving with U0040?

If the vehicle runs and drives normally, you can drive it to a repair shop, but avoid long trips and be prepared for the possibility of multiple systems shutting down unexpectedly. If the engine stalls, the transmission shifts erratically, or the dashboard goes dark, pull over safely and have the vehicle towed.

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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