Failed accessory power relay (burned contacts, stuck armature, or open coil); Blown fuse or fusible link in the relay's power feed circuit; Corroded, loose, or backed-out terminals at the relay socket or inline connector; Damaged wiring between the relay, the controlling module, and the accessory load (chafed, pinched, or broken wire); Faulty ground connection for the relay coil or the accessory circuit; Internal fault in the body control module (BCM) or other module that controls the relay; Water intrusion or corrosion in the relay box or module connector causing high resistance

DTC codes / U30xx · Network wiring and bus faults / U3004

U3004Accessory Power Relay

Q: What does code U3004 mean?

Accessory Power Relay. U3004 is a generic body/control code indicating that the module responsible for monitoring the accessory power relay has detected a fault in that relay's control circuit or its feedback signal. The computer is not seeing the expected voltage state when it commands the relay on or off, so it flags the accessory power relay circuit as out of range.

U3004 is a generic body/control code indicating that the module responsible for monitoring the accessory power relay has detected a fault in that relay's control circuit or its feedback signal. The computer is not seeing the expected voltage state when it commands the relay on or off, so it flags the accessory power relay circuit as out of range.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork wiring and bus faults

Symptoms you may notice

Often no symptoms besides the warning light, especially if the relay only fails intermittently
Accessories such as the radio, power windows, or interior lighting may not work when the ignition is off or in accessory mode
The accessory power relay may click repeatedly or not click at all when the key is turned
Battery may drain overnight if the relay sticks closed and keeps accessories powered
Intermittent operation of multiple accessories that share the relay's output circuit
Other control modules may store communication or low-voltage codes if the relay interrupts their power feed

Common causes, most likely first

Failed accessory power relay (burned contacts, stuck armature, or open coil)
Blown fuse or fusible link in the relay's power feed circuit
Corroded, loose, or backed-out terminals at the relay socket or inline connector
Damaged wiring between the relay, the controlling module, and the accessory load (chafed, pinched, or broken wire)
Faulty ground connection for the relay coil or the accessory circuit
Internal fault in the body control module (BCM) or other module that controls the relay
Water intrusion or corrosion in the relay box or module connector causing high resistance

How to diagnose and fix it

Verify the code with a scan tool and check for related body or communication codes before touching any parts
Locate the accessory power relay in the underhood or interior fuse box and inspect the relay and socket for corrosion or loose terminals
Test the relay coil resistance and contact continuity with a multimeter, or swap it with an identical known-good relay to see if the code changes
Check the fuse and fusible link that feed the relay's power circuit, and confirm battery voltage at the relay socket with the key on
Measure voltage drop and continuity on the control wire from the module to the relay, and on the ground circuit, while wiggling the harness
Inspect the controlling module connector for water intrusion, corrosion, or pushed pins, and check module grounds
If all wiring and the relay test good, follow the manufacturer's diagnostic flow for the controlling module before condemning it

What the accessory relay code hides

A relay that tests fine on the bench can still set U3004 because the module monitors the load side, so a shorted accessory or bad ground downstream will mimic a failed relay.
Many technicians replace the relay first and the code returns because the real fault is a corroded ground stud behind a kick panel or a chafed wire in the door jamb boot.
U3004 is often stored alongside communication codes when the relay interrupts power to a module on the same bus, so clearing it without fixing the power feed leads to repeat visits.
A weak battery or low charging voltage can cause the module to misread the relay feedback signal, so always check system voltage before chasing the relay circuit.
On some vehicles the accessory power relay is controlled by the body control module through a low-side driver, and a shorted driver inside the module will keep the relay energized even with the key off.

Can I keep driving with U3004?

You can usually drive the vehicle, but if the relay has failed open you may lose accessories and, on some cars, the engine may not start or may stall if the relay feeds the ignition circuit. Avoid leaving the vehicle parked for long periods if the relay sticks closed, because it can drain the battery.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.