U0037: Vehicle Communication Bus B

What it means

A fault is detected on vehicle communication bus B. The code identifies the bus but does not specify the cause.

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

Vehicle communication bus B carries messages between the modules connected to that network. The control modules monitor bus B for valid communication and set U0037 when they detect a general bus fault; the code alone does not identify a failed wire, module, gateway, or terminator.

Common causes

Most likely first.

1. Wiring or node fault on bus B
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 check which modules on bus B are reporting communication faults to establish whether the problem is network-wide or limited to one node.
2. Inspect accessible bus B wiring, connectors, and module grounds for damage, corrosion, loose pins, or signs of water entry.
3. Use a scan tool to check which bus B modules respond and whether communication faults return while monitoring live network status.
4. With the vehicle powered down as specified by the manufacturer, check bus B continuity and for shorts between its lines or to power and ground.
5. If the wiring checks pass, test bus B termination and gateway message routing against the manufacturer's specifications, isolating modules only as the service information directs.

Common fixes

• Repair damaged or loose bus B wiring and connector terminals.
• Restore poor module ground or power connections affecting bus B communication.
• Replace a bus B terminating resistor only if testing confirms it is out of specification.
• Repair or replace a gateway or bus-connected module only after wiring and network tests identify it as the fault.

Can I keep driving?

If communication loss affects steering, braking, stability control, or other essential functions, stop driving and arrange recovery. Otherwise, limit driving because connected systems may become unavailable or enter limp mode without warning.