The check engine light came on and the code from the ODB port is P0128, which usually means the thermostat stuck open. This is corroborated by the temperature indicator which does not rise quickly and stays below normal. This is an emission related issue because the fuel does not burn completely in sub-normal temperature and more pollutants are released and the gas mileage is reduced.
After draining the coolant and removed the thermostat, the thermostat does not appear open. A new thermostat is installed; the temperature indicator behavior is noticeably different: the initial temperature rises quickly and is regulated to just below midscale. Checking the thermostat operation by placing it in hot water, it opens as expected and closes as the water cools. Why is it stuck open? Close examination finds that the valve is not closed tightly; it is a little crooked, perhaps warped after many years of operation. But it is still fairly remarkable that the thermostat is a simple robust mechanical design. It is based on thermal expansion of material at relatively accurate temperature of 82C; it is submerged in hot liquid, no electronics involved, no energy needed. Sometimes simple mechanical device is a better solution than electronic control.
