U3004: Accessory Power Relay

What it means

Fault on the accessory power relay that switches power to comfort/accessory loads (radio, infotainment, etc.) when the key is in ACC position.

At a glance

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

How it works

The accessory power relay switches power to selected accessories when the vehicle is in the accessory state. U3004 indicates a fault associated with that relay circuit or its control, so accessories such as infotainment may fail to power up or may behave inconsistently.

Common causes

Most likely first.

1. Relay contacts failed or coil open
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 the body and other relevant modules for relay, accessory-power, and communication codes, and review freeze-frame data for the conditions when the fault occurred.
2. Check whether the accessories supplied by the relay operate correctly in the accessory state and note whether the fault is intermittent.
3. Inspect the relay and its socket for poor fit, corrosion, heat damage, or loose terminals.
4. Check the relay's coil control, switched output, power feed, and ground according to the wiring diagram and manufacturer's specification.
5. If relay circuits pass, investigate related network or gateway faults using the manufacturer's procedure before replacing a module.

Common fixes

• Reseat the accessory power relay and repair loose or corroded socket terminals.
• Replace the accessory power relay if its coil or contacts fail the manufacturer's test.
• Repair an open or short in the relay's power, output, control, or ground wiring.
• Correct a verified gateway or network fault if it is preventing accessory relay control.

Can I keep driving?

The engine and essential driving systems are usually unaffected, but accessories powered through the relay may be unavailable. If the fault causes other electrical warnings or affects visibility or safety equipment, stop and address those functions before continuing.