U0046: Vehicle Communication Bus C

What it means

Generic fault on bus C (third bus, often chassis CAN or diagnostic CAN). 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 is a vehicle communication network that links the modules assigned to that bus; its role varies by vehicle and may include chassis or diagnostic systems. U0046 signals a general fault on bus C, meaning the monitored network is not communicating as expected without identifying a particular wire or module.

Common causes

Most likely first.

1. Wiring or node fault on bus C
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. Record all codes and freeze-frame data, then use the vehicle wiring information to identify which modules are on bus C and which report communication faults.
2. Check scan-tool responses from bus C modules to determine whether the fault affects the whole network or one node or segment.
3. Inspect accessible bus C wiring, connectors, and module power and grounds for looseness, corrosion, damage, or water entry.
4. With power isolated as directed by the manufacturer, test bus C conductors for opens and shorts and check termination against the service information.
5. If the wiring checks pass, test gateway routing and isolate nonresponsive bus C modules using the manufacturer's diagnostic procedure.

Common fixes

• Repair damaged bus C wiring or restore loose connector terminals.
• Clean or repair corroded connections and restore affected module power or grounds.
• Replace a bus C terminating resistor only if testing confirms it is outside specification.
• Repair gateway routing or replace a bus C module only after network tests identify it as faulty.

Can I keep driving?

If bus C communication loss affects steering, braking, stability control, or another essential function, stop driving and arrange recovery. Otherwise, drive cautiously and avoid relying on affected functions until the network fault is diagnosed.