Diagnosis
The humming sound indicates the electric motor is receiving power but cannot overcome mechanical resistance to begin spinning. For the Greenworks GPW1804, this is almost always caused by the pressure washer pump's internal components being seized or stuck due to mineral deposits or corrosion from residual water left inside after the last use. It can also be caused by insufficient voltage reaching the motor.
How to Fix It
First, attempt to "bump" the motor free by following these steps:
- Ensure the pressure washer is unplugged and the water supply hose is disconnected.
- Plug the unit directly into a grounded 120V wall outlet (do not use an extension cord).
- Turn the power switch to the ON (I) position for just 1-2 seconds, then immediately turn it off. You should hear a brief hum.
- Wait 10 seconds, then repeat this bump-start process 3-4 times. The goal is for the short jolts of power to break the pump piston(s) free.
- If the motor starts and runs normally, reconnect your water supply and test with the spray gun trigger open to flush the system.
If That Didn't Work
Voltage loss is preventing the motor from developing enough starting torque. Confirm the power source: ensure no other high-draw appliances are on the same circuit and that you are plugged directly into the outlet. If the issue persists, the pump seizure is more severe. Remove the pump outlet and inlet hoses. Using a flat-head screwdriver, carefully try to manually turn the pump's brass coupling that connects to the motor shaft. If it will not turn, the pump is locked and likely needs replacement.
Heads Up
Never let the motor hum for more than 5-7 seconds continuously, as this can overheat and burn out the windings. To prevent future seizure, always follow the proper storage procedure: disconnect hoses, trigger the gun to release pressure, and run the pressure washer for 30 seconds with the water supply disconnected to purge most water from the pump.