Warning: boiling point!
I own an American classic car and it comes with the usual 1960s instrumentation—speedo, fuel gauge but only warning lights for oil, alternator, and water temperature. The alternator light comes on when you turn on the ignition, but the water temperature light is designed to come on only when the water temperature reaches 120-125 °C which is usually too late.
To fix that, I have devised this program powered by an Arduino micro-controller that will operate the temperature light when the engine powers up and light it up again to warn the driver as the temperature approaches 100 °C. It uses a readily available sender that will fit most vehicles.