I have a 1999 2500 that is generating code P0118 (High input - engine coolant temp). The truck is idling a little rough when it is cold, so it was sent in for a regular service (oil, oil filter, and fuel filter), but that had no effect on the idle. I changed the thermostat even though the gauge always seems to read in the normal range. I can clear the code, but it returns within a few minutes. I even disconnected the battery for a couple of minutes to let OBDII reset. The engine light will go off by itself at times also. Even thought the gauge reads as it should, and I think that the coolant temp is within its normal range (heater blows warm as always), I was considering changing the coolant temp sender next. Anybody had this problem?

 

P0118 - ECT Sensor Voltage Too High

It could be a skewed sensor that is worn and operating in a lower resistance range and causing the voltage to be too high. I'm not sure what the Ohm rating is supposed to be but if you could find that out it could be measured at the sensor to verify it is the problem. It is highly probable that the ECT is smoked.....replacing it would most likely solve the problem.