U0048: Vehicle Communication Bus C (+) Open

What it means

Bus C high-side line open. Same diagnostic as U0039 on bus C. 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 uses paired conductors to carry messages between connected control modules; the (+) conductor is one side of that pair. U0048 indicates that the bus C positive line is detected as open, which can stop modules from communicating and disrupt functions that depend on their messages.

Common causes

Most likely first.

1. Wire broken
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 for codes and record which ones fail to respond, along with freeze-frame data, before clearing faults.
2. Use the wiring diagram to locate the bus C (+) path, its connectors, gateway connections, and termination points.
3. Inspect the positive conductor and connectors for a break, pulled pin, corrosion, or prior repair, paying attention to harness bends and rub points.
4. With power isolated as specified by the manufacturer, check continuity along bus C (+) and test for an unintended short to ground or voltage.
5. Check bus termination and the paired conductor against the manufacturer's network specifications to rule out a shared network fault.
6. If the open is not in the harness, follow the manufacturer's procedure to isolate gateway or module connections and identify where continuity is lost.

Common fixes

• Repair or replace the broken bus C (+) wire.
• Reseat or repair a loose, corroded, or damaged connector on the positive line.
• Repair a failed splice or connector terminal found during continuity testing.
• Correct a termination or gateway fault only when network testing confirms it.

Can I keep driving?

Avoid driving if engine, transmission, braking, steering, or other safety related systems show warnings or lose function; dependent modules may enter limp mode or stop operating. If only noncritical features are affected and the vehicle behaves normally, limit driving and arrange prompt diagnosis.