Unloader valve or thermal relief valve? They do different jobs and people mix them up.
Two cheap parts that get confused constantly — and confusing them is how people end up replacing the wrong one twice.
What the unloader does
When you release the trigger, the pump keeps running but the water has nowhere to go. The unloader diverts that flow — either back to the pump inlet or to a bypass — so the pump is not deadheading against a closed gun. It is a flow management device.
When it fails you see pressure that comes and goes, a hard spike when you let off the trigger, or a machine that will not build at all.
What the thermal relief valve does
That bypassed water is going in circles, and water going in circles gets hot. A thermal relief valve opens at a set temperature and dumps the hot water so the pump is not cooking itself. It is a temperature safety device.
When it fails you usually see one of two things: it is stuck closed and your pump runs hot on long bypass periods, or it is stuck open and dribbles water constantly while you work.
Telling them apart in practice
Pressure symptoms point at the unloader. Temperature and dribbling point at thermal relief. If the machine is dumping water on the ground while you work and pressure is otherwise fine, you are looking at thermal relief. If pressure is erratic, you are looking at the unloader.
Both are cheap. Both protect a pump that is not. If you are in there replacing one and the other is original, replace both — you already have the machine apart.