U0040: Vehicle Communication Bus B (+) Low

U0040 reports the (+) line held at too low a voltage, usually because it is shorted to ground on vehicle communication bus B. SAE J2012 gives each vehicle bus a block of codes: the bus itself, performance, then open, low and high for the (+) and the (-) line and a short between them; which physical network carries which letter is set by the manufacturer. On CAN networks (+) normally stands for CAN-High and (-) for CAN-Low. Modules on the affected bus may stop exchanging data, so lost-communication codes and warning lights in several systems usually appear together.

Symptoms

• Several warning lights at once
• Lost-communication codes stored in other modules
• Functions of modules on this bus inoperative
• Scan tool cannot reach some modules

Most likely causes

• (+) wire shorted to ground
• Harness chafed through
• Failed bus transceiver in a module

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the battery and terminals, and read the codes from all control modules with a multi-system scanner
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Identify which control module is not responding and check its fuses, power supply, and grounds
4. Disconnect add-on accessories, such as alarms or trackers, and read the codes again
5. Inspect the network connectors for moisture or bent pins
6. Measure the network's resistance and signal using the manufacturer's wiring diagram

Good to know

• Repair difficulty: hard
• It can trigger limp mode.

What each character of U0040 means

• U: Communication network
• 0: Generic code: same on every make
• 0: Subsystem depends on the code range
• 40: Specific fault in that system

Frequently asked questions

Can I keep driving with code U0040?

It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.

What causes code U0040?

The most common causes are: (+) wire shorted to ground; harness chafed through; failed bus transceiver in a module.

Related codes

• U0037Vehicle Communication Bus B
• U0038Vehicle Communication Bus B Performance
• U0039Vehicle Communication Bus B (+) Open
• U0041Vehicle Communication Bus B (+) High
• U0042Vehicle Communication Bus B (-) Open
• U0043Vehicle Communication Bus B (-) Low
• U0044Vehicle Communication Bus B (-) High
• U0045Vehicle Communication Bus B (-) shorted to Bus B (+)

Sources

• github.com: codes
• github.com: obd2 codes
• en.wikipedia.org: CAN bus
• hella.com: en u codes

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.