U0043: Vehicle Communication Bus B (-) Low

Bus B low-side line voltage below expected range. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.

Medium severity. Can I keep driving?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.

Symptoms

Multiple warning lights may illuminate
Affected modules may go into limp mode or shut down
Specific functions inoperative depending on module
Diagnostic tools may show communication faults

Most likely causes

Wire shorted to ground
Gateway module routing CAN messages incorrectly
CAN terminating resistor (120 Ω) drifted out of spec

What to check, in order

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

Check the battery and terminals, and read the codes from all control modules with a multi-system scanner
With the connector unplugged, check whether the signal wire is making contact with ground.
Identify which control module is not responding and check its fuses, power supply, and grounds
Disconnect add-on accessories, such as alarms or trackers, and read the codes again
Inspect the network connectors for moisture or bent pins
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 U0043 means

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

Frequently asked questions

Can I keep driving with code U0043?

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 U0043?

The most common causes are: wire shorted to ground; gateway module routing CAN messages incorrectly; cAN terminating resistor (120 Ω) drifted out of spec.

Related codes

Sources

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.