U0031: Vehicle Communication Bus A (+) Low

What it means

A control module detects voltage below the expected level on the positive conductor of vehicle communication bus A. It sets the code when the bus line remains below its calibrated voltage threshold.

How it works

Bus A's positive conductor carries network signals between connected modules. U0031 sets when the monitored voltage on that conductor stays below the expected range, which can happen if it is shorted to ground, pulled down by a module, or affected by low system voltage.

Common causes

Most likely first.

1. Short to ground on bus wire
2. Damaged or chafed wiring
3. Poor connector connection
4. Faulty module pulling the bus low
5. Low system voltage

Symptoms

• Communication warning lights
• Multiple warning messages
• Intermittent system operation
• Some systems unavailable

How to diagnose it

1. Scan all modules and record event data and system voltage, noting which bus A nodes report faults and whether the code coincides with a low battery condition.
2. Check battery and charging-system condition, then inspect the bus A positive wire and connectors for chafing, corrosion, or a possible contact with ground.
3. Check power and ground connections at modules on bus A that are missing or reporting faults.
4. Measure the positive conductor under the manufacturer's specified conditions and compare its voltage and continuity with the manufacturer's specification.
5. If the wiring checks out, isolate connected nodes one at a time using service information to find a module pulling the line low.

Common fixes

• Charge or repair a weak battery or charging-system fault if system voltage is low.
• Repair chafed bus A positive wiring or a short to ground.
• Clean, secure, or replace a poor connector or terminal.
• Repair a module's supply or ground, or replace a node only if isolation tests show it is pulling the line low.

Can I keep driving?

If multiple systems are intermittent, limit driving and arrange diagnosis promptly because more modules may drop offline. Stop if the affected functions include controls needed to operate the vehicle safely.