A chafed or pinched Bus A (+) wire shorting to ground against the chassis or engine block; Corrosion or water intrusion in a connector on the Bus A circuit, creating a partial short; A failed control module on Bus A that is internally shorting the positive line to ground; A loose or damaged ground connection shared by modules on the bus, causing a voltage drop; Aftermarket accessories or alarm wiring tapped incorrectly into the Bus A (+) circuit; A damaged wiring harness from rodent chewing or prior collision repair

DTC codes / U00xx · Network wiring and bus faults / U0031

U0031Vehicle Communication Bus A (+) Low

Q: What does code U0031 mean?

Vehicle Communication Bus A (+) Low. U0031 means the vehicle's main communication network, known as Bus A, has a positive voltage line that is reading lower than the manufacturer's expected threshold. The control modules that share this bus have detected a short-to-ground or excessive voltage drop on the Bus A (+) circuit, so they can no longer reliably exchange data.

U0031 means the vehicle's main communication network, known as Bus A, has a positive voltage line that is reading lower than the manufacturer's expected threshold. The control modules that share this bus have detected a short-to-ground or excessive voltage drop on the Bus A (+) circuit, so they can no longer reliably exchange data.

SystemNetwork

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaNetwork wiring and bus faults

Symptoms you may notice

Multiple warning lights illuminated at once, often including the check engine light, ABS, and airbag lamps
The engine may crank but fail to start, or start and run poorly because modules cannot share data
Intermittent or complete failure of the instrument cluster, radio, or climate controls
A no-start condition where the scan tool cannot communicate with any module on Bus A
Erratic transmission shifting or a default limp-home mode
Often no symptoms besides the warning light when the fault is intermittent

Common causes, most likely first

A chafed or pinched Bus A (+) wire shorting to ground against the chassis or engine block
Corrosion or water intrusion in a connector on the Bus A circuit, creating a partial short
A failed control module on Bus A that is internally shorting the positive line to ground
A loose or damaged ground connection shared by modules on the bus, causing a voltage drop
Aftermarket accessories or alarm wiring tapped incorrectly into the Bus A (+) circuit
A damaged wiring harness from rodent chewing or prior collision repair

How to diagnose and fix it

Connect a scan tool and record which modules can and cannot communicate to map the extent of the Bus A failure
Measure voltage on the Bus A (+) wire at the diagnostic link connector (DLC) with the key on and compare to the manufacturer's specification
Inspect the DLC pins and nearby harness for corrosion, pushed terminals, or loose wires
Disconnect modules one at a time from Bus A and recheck voltage to isolate a module that is shorting the line
Perform a resistance and continuity test on the Bus A (+) wire from the DLC to each module, looking for a short to ground
Repair or replace the damaged section of harness, connector, or faulty module found during isolation
Clear the code and verify that all modules communicate and the code does not return after a road test

What Bus A voltage tests really reveal

A low Bus A (+) reading is frequently caused by a single module holding the line down, so unplugging modules one by one is faster than chasing the whole harness.
The code does not always mean the wiring is damaged; a weak battery or bad ground can pull bus voltage below threshold and set U0031 without any short circuit.
Aftermarket remote starters and alarm systems are a common hidden cause because installers sometimes tap the wrong wire on the communication bus.
Some vehicles set U0031 when a module on the bus loses power or ground, not just when the data line itself is shorted, so check module power feeds before condemning the harness.
A scan tool that cannot communicate at all on Bus A points to a short near the DLC or a dead module, while intermittent communication points to a chafed wire that moves with the vehicle.

Can I keep driving with U0031?

Do not drive the vehicle when U0031 is active, because the modules that control safety and powertrain functions may not communicate and the car can stall or fail to restart. If the code is intermittent and the vehicle runs normally, drive only to a repair shop and avoid rough roads that could worsen a wiring short.

Tools that help

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