The majority of pressure washers come with a spray trigger that lets you control when the water flows out, while this is a great option as you can leave the machine running without spraying, you should never let this happen for more than a few minutes.
The reason being that the water doesn’t flow out of the spray nozzle, this water is instead redirected through a loop and using a part called a bypass valve. The water will continue to travel in this loop until you release the pressure by depressing the spray trigger.
As the water goes through the loop, it passes through the pump over and over, getting hotter and hotter as the majority of pressure washers use the cold water coming into the system to cool the pump. If the temperature of the water get’s too hot, it will no longer cool the pump, which can cause the seals to fail and, ultimately, the pump to fail. This is the same regardless of if you are using a cold or hot water pressure washer.
It’s because of this that you should only let your pressure washer work without releasing the pressure for a maximum of three minutes. Check your user manual as some manufacturers will recommend more or less time in the bypass loop. My recommendation is that if you need to move something or take a phone call, or even step away from the machine, turn it off. This will save you maintenance costs and also costly pump replacements or repairs.