* Failed Fuel Pump Relay: This is the most common cause. The relay is an electrically operated switch that controls the power to the fuel pump. A faulty relay might remain energized, constantly supplying power to the pump.
* Short Circuit in the Wiring: A short circuit in the wiring harness leading to or from the fuel pump can bypass the normal control system and send constant power to the pump. This could be due to damaged wiring, corrosion, or a rubbed-through wire.
* Faulty Inertia Switch: Some vehicles have an inertia switch (a safety device that cuts power to the fuel pump in a collision). If this switch is stuck in the "off" position *and* there's a short elsewhere, the short could still draw power while bypassing the switch's normal function. However, this scenario is less likely to cause a constant run.
* Faulty PCM (Powertrain Control Module): In later model years, the PCM controls the fuel pump. A malfunctioning PCM could send a continuous signal to the pump. This is less common but possible.
* Grounding Issue: A poor ground connection could cause erratic behavior, potentially leading to the fuel pump receiving constant power.
Troubleshooting Steps (Do these with the car OFF):
1. Check the Fuel Pump Relay: Locate the relay (often in a fuse box under the hood or in the passenger compartment) and visually inspect it for any signs of damage. You can also try swapping it with a known good relay of the same amperage and type (if you have one).
2. Inspect the Wiring: Carefully examine the wiring harness leading to the fuel pump for any signs of damage, chafing, corrosion, or loose connections. Pay close attention to the area near the fuel tank, where the wiring is more susceptible to wear.
3. Check the Inertia Switch (if equipped): Locate the inertia switch (usually under the dash or in the trunk) and reset it. See if this resolves the problem.
4. Test the Fuel Pump Relay Circuit: Use a multimeter to check for power at the relay's terminals when the ignition is OFF. Constant power with the ignition OFF points to a wiring fault or a shorted relay.
Important Safety Note: Working with automotive electrical systems involves risks. Disconnect the battery's negative terminal before performing any electrical checks or repairs. If you are not comfortable working with automotive electrical systems, take your vehicle to a qualified mechanic. A constantly running fuel pump is a serious safety hazard and a drain on your battery. Do not attempt to start the engine while troubleshooting this issue; you risk further damage or fire.
