The sensor is for the temperature display. If it is unplugged or there's a cut wire, it will say "OC" for "open circuit", but it will still light up. Start by checking for 12 volts on any wires in the unit's connector with the ignition switch off. I seem to recall there should be none, but there might be one. Next, check if there's 12 volts on any other wires with the ignition switch in the "run" position. If there is, you likely have a broken ground wire. Most commonly the feed and ground wires develop corroded terminals in a connector down low. If there's rust holes near the bottom of the cab, that's a good place to look first.
Wednesday, January 9th, 2013 AT 8:06 PM