What it does
The thermostat is a valve between the engine and the radiator. It stays closed while the engine warms up, then opens to let coolant flow to the radiator so the engine stays at the right temperature.
Stuck open
- The engine takes a long time to warm up, and the temperature gauge stays low
- Weak heat from the heater, which you'll notice fast in a Northland winter
- Worse gas mileage
- A check engine light, often code P0128
Stuck closed
- The temperature climbs fast and the engine overheats, often soon after starting
- Coolant leaking around the thermostat housing from the extra pressure
A temperature gauge that jumps up and down can also mean the thermostat is failing.
What to look for
- Watch the gauge: on a normal drive it should climb to its usual spot within the first several minutes and stay there.
- Hose check: on a warmed-up engine, the upper radiator hose should be hot. If the engine is hot but that hose stays cool, the thermostat may be stuck closed. Be careful, hoses get very hot.
- Never open the radiator cap on a hot engine.
Could be something else
Overheating can also be low coolant, a bad water pump, a cooling fan or a head gasket. No heat can be low coolant or a heater problem. A low gauge reading can be a bad temperature sensor.
Is it safe to drive?
Stuck open isn't an emergency, but no heat or defrost in winter is a safety problem of its own. Stuck closed can ruin an engine. If the gauge climbs, pull over and shut it off.
Related: Bad water pump symptoms · Heater fan only works on high?
Think it's your thermostat? Northwoods can check it right where your vehicle sits, anywhere in the greater Twin Ports area.