Sensor

There is an engine coolant temperature sensor at the front of the head. To remove it you need a special tool, "J 45861", basically just a plastic sleeve that fits over it to turn it without damaging it. You also have to drain coolant to below the sensor. BTW, there are a couple TSBs on non-working temp gauges for Chevy Colorados. You may want to check on these before going to the trouble of replacing a sensor. One of them has to do with the lousy thermostats in these engines. TSB: 1461920 (Engine Coolant Gauge Drop on Acceleration) and TSB: 1471209 (Temperature Coolant Gauge Low or Inoperative), which reads: "The temperature gauge will not move until the engine temperature is over 160 degrees F. Warm engine to operating temperature. Monitor ECT sensor with the Tech 2. If the ECT reads 160 degrees or lower, the thermostat may be stuck open. Replace thermostat." - so, try replacing the thermostat first before spending money on an ECT sensor.