Q: How to testing, remove, and replace a thermostat in a Nissan Frontier's cooling system?A: To properly test a thermostat, suspend the (closed) thermostat on a string in a container of cold water, with a thermometer and heat the water on a stove. Write down the temperature when the thermostat begins to open and when it is fully open. If it doesn't work as specified, replace it. Disconnect the battery cable and drain the coolant. Remove the engine cooling fan, air cleaner, and air intake duct. Remove the upper radiator hose, unbolt the coolant tube, and loosen the hose clamp. Remove the thermostat housing cover, remove the drivebelts, and the water pump drain plug. Detach the lower radiator hose and the coolant by-pass hose, remove the intake manifold, detach the hose from the water outlet housing, and remove the water outlet cover. Scrape off any old gasket or sealant, clean them with lacquer thinner, and apply a bead of RTV sealant. Installation is the reverse of removal. Bolt the cover in place and wait for the sealant to cure. Refill and bleed the cooling system, run the engine, and check for leaks and proper operation.