how to check a temperature gauge
With the ignition on and the engine not running, disconnect the gauge wire (yellow with red stripe) from the sending unit in the coolant outlet on the cylinder head. On 16-valve engines, the sending unit is on the driver's-side end of the cylinder head. Ground the wire. This simulates a high engine temperature signal to the gauge. If the gauge needle moves upward, the sending unit is faulty and should be replaced. If the needle still does not move, either the wire to the gauge is broken (open circuit) or the gauge is faulty.