U01B9: Lost Communication With Switch Input Module C

What it means

Communication with Switch Input Module C was not received as expected. The code does not specify which switches the module serves.

At a glance

• Usually lights the warning light of the system that lost communication

How it works

Switch Input Module C reads switch signals and reports their status to other modules over the vehicle network. This code sets when the expected messages from Module C are not received; it does not reveal which switches Module C serves.

Common causes

Most likely first.

1. Module unpowered
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 module codes and freeze-frame data, noting whether other modules also report communication loss that could indicate a shared network problem.
2. Check with a compatible scan tool whether Switch Input Module C responds and whether its network messages drop out.
3. Inspect Module C’s power and ground connections, connectors, and wiring for looseness, corrosion, damage, or water entry.
4. Check the communication wiring and connectors along Module C’s network branch for opens, shorts, or poor contact.
5. If the module has sound power, ground, and wiring, test gateway routing and CAN termination against the manufacturer’s specification.

Common fixes

• Clean and secure a poor power or ground connection at Module C.
• Repair damaged network wiring or connector terminals on Module C’s branch.
• Correct a confirmed gateway routing or configuration problem.
• Repair or replace a CAN terminating resistor if it fails testing against the manufacturer’s specification.

Can I keep driving?

If the vehicle drives normally and only functions served by Module C are affected, limit driving to what is necessary and check that essential controls work. Do not drive if the missing functions or accompanying warnings compromise safe operation.