A: Before assuming the thermostat is to blame for a cooling system problem, check the coolant level and temperature gauge operation. If the engine seems slow to warm up, the thermostat might be stuck open and needs replacement. If the engine runs hot and the upper radiator hose isn't hot, the thermostat could be stuck closed and must be replaced. Caution: Don't drive the vehicle without a thermostat as emissions and fuel economy may suffer. If the hose is hot, it indicates the coolant is flowing, and other cooling system diagnosis may be required. To replace the thermostat, disconnect the cable from the negative battery terminal, drain the cooling system, and locate the thermostat housing cover by following the upper radiator hose. Loosen the hose clamp, detach the hose, and remove the housing cover fasteners and cover. Take note of the thermostat and gasket installation, remove material from the staked areas if necessary, and clean all traces of the old gasket from the surfaces. Install the new thermostat with the jiggle pin at the 12 o'clock position, and the spring end directed into the engine. Use RTV sealant if appropriate, and install a new gasket. Reattach the cover, hose, and all previously removed components, reconnect the battery, refill the cooling system, and start the engine to check for leaks and proper thermostat operation.