U0014: Medium Speed CAN Communication Bus (-) High

What it means

A high signal fault has been detected on the negative line of the medium-speed CAN communication bus. The affected functions depend on the vehicle network layout.

At a glance

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

How it works

The medium-speed CAN bus carries messages between modules, using CAN-L as the negative signal wire. U0014 indicates CAN-L is abnormally high, consistent with a short to battery voltage or a network electrical fault; a termination fault may also affect bus signaling, depending on the vehicle design.

Common causes

Most likely first.

1. CAN-Low shorted to battery
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 and save freeze-frame data and communication codes to identify the medium-speed nodes affected when CAN-L was reported high.
2. Inspect MS-CAN wiring and connectors for CAN-L chafing against a powered wire, melted insulation, or a poor repair joining it to supply.
3. With the vehicle powered down as specified by the manufacturer, check CAN-L for a short to battery or another supply and compare with the manufacturer's specification.
4. Inspect relevant connectors and any network termination points identified by the manufacturer for corrosion, loose terminals, or damage.
5. Use the manufacturer's isolation procedure to determine whether a module is driving CAN-L high or the fault lies in the harness.
6. Test the bus termination only using the vehicle's specified procedure and limits, then assess gateway routing if wiring and nodes are sound.

Common fixes

• Repair CAN-L insulation shorted to a power wire.
• Correct a damaged connector terminal bridging CAN-L to supply.
• Repair or replace a damaged MS-CAN harness section.
• Replace a confirmed faulty module transceiver that holds CAN-L high.
• Repair a confirmed termination fault or correct gateway routing according to the manufacturer's procedure.

Can I keep driving?

If affected systems are limited to body or comfort functions, cautious short-distance driving may be possible while avoiding reliance on them. Stop if safety-related systems are affected; an MS-CAN fault can disable connected functions and warnings.