U0049: Vehicle Communication Bus C (+) Low

What it means

Bus C high-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.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Bus C carries data on paired conductors, including the positive (+) line. U0049 sets when that positive line's voltage is below the expected range, often because it is shorted to ground; modules may then lose messages carried over bus C.

Common causes

Most likely first.

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

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

How to diagnose it

1. Scan all modules and note communication failures and freeze-frame data so you can see which systems were affected when the fault set.
2. Identify the bus C (+) conductor from the vehicle wiring diagram and inspect its route and connectors for crushed, chafed, wet, or contaminated sections.
3. Check for other bus C codes that point to a shared gateway or network fault, and compare which modules respond to the scan tool.
4. With power isolated as directed by the manufacturer, test the positive line for a short to ground and check its continuity through connectors and splices.
5. Inspect bus C termination and the gateway connections, comparing test results with the manufacturer's network specifications.
6. If the wiring tests pass, isolate connected modules using the manufacturer's diagnostic procedure to find a module or gateway pulling the line low.

Common fixes

• Repair insulation damage or a short to ground on bus C (+).
• Clean or repair a wet, corroded, or damaged positive-line connector.
• Repair an affected splice or terminal that is grounding the conductor.
• Address a faulty gateway, termination, or module only after line testing identifies it as the source.

Can I keep driving?

Do not drive if communication loss affects braking, steering, engine, or transmission controls, or if the vehicle enters limp mode. If affected functions are limited to noncritical equipment and the vehicle is stable, keep any trip short and seek diagnosis promptly.