OBD-II code U3004 indicates a fault detected in the Accessory Power Relay circuit. This relay supplies switched 12-volt power to vehicle accessories such as the radio, power outlets, and other convenience features, and the code is set when the powertrain or body control module observes an electrical problem on that circuit (open, short, or out-of-range voltage). The exact definition and affected components vary by manufacturer, so confirming the vehicle-specific description in service information is important.
The relay itself is the most common failure point. Internal contacts wear, the coil opens, or the relay sticks, producing the voltage anomaly the module detects.
A fuse in series with the relay output or coil frequently opens due to a fault or simply age, and corroded fuse-box terminals produce the same symptoms as a bad relay.
Broken wires, chafed insulation, loose pins, or corroded connectors between the relay, battery, and the controlling module are common, especially in areas exposed to moisture or movement.
A shorted radio, power outlet, or harness powered through the relay can pull the circuit voltage out of range and trigger U3004 along with related codes.
The module's transistor that energizes the relay coil can fail, preventing proper relay operation. This is less common than relay or wiring faults but does occur.
An internal logic or voltage-monitoring fault inside the controlling module can set U3004. This is a rare last-resort diagnosis; the relay, fuse, wiring, and downstream loads should be ruled out first.
Usually the vehicle will still run, but accessory functions may be lost and a parasitic drain can kill the battery. Address it promptly to avoid being stranded.
Often yes, because the relay is the most common failure. However, a shorted downstream accessory, damaged wiring, or a blown fuse can produce the same code, so verify the circuit before replacing parts.
No. In the vast majority of cases U3004 is caused by the relay, its fuse, wiring, or a downstream accessory, not the powertrain or body control module. Modules are expensive and should only be considered after every external cause has been ruled out by direct electrical testing.
A hard fault that returns immediately means the underlying electrical problem is still present, usually a stuck relay, a persistent short, or a connector that is not making contact. The vehicle needs targeted circuit testing, not just repeated code clearing.