U01B8: Lost Communication With Switch Input Module B

What it means

Communication with Switch Input Module B 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 B reads switch signals and sends their status to other vehicle modules over the communication network. This code sets when the expected messages from Module B are missing; it does not identify which switches that module 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, then check whether other modules also report lost communication, which can point to a shared network or gateway fault.
2. Use a compatible scan tool to see whether Switch Input Module B responds and whether its messages appear intermittently.
3. Check the module’s power, ground, connectors, and wiring for loose connections, corrosion, damage, or signs of water entry.
4. Inspect the network wiring and connectors on the module’s communication path for shorts, opens, or poor connections.
5. If power, ground, and wiring are sound, have the gateway routing and CAN termination checked against the manufacturer’s specification.

Common fixes

• Restore a loose or corroded power or ground connection at Module B.
• Repair damaged wiring or connector terminals on Module B’s network branch.
• Correct a gateway configuration or routing fault if confirmed by diagnostics.
• Repair or replace a CAN terminating resistor if testing shows it is out of specification.

Can I keep driving?

If only switch-related functions are affected and the vehicle otherwise operates normally, drive only as needed and verify that essential controls, such as lights and signals, work. Avoid driving if warning lights or inoperative controls make the vehicle unsafe, or if other modules report faults.