1999 Jeep Laredo '99 Jeep Grand Cherokee Laredo ECT Sensor

Engine Cooling problem
1999 Jeep Laredo 6 cyl Four Wheel Drive Automatic ~160k miles

Hi,

I have a 1999 Jeep Grand Cherokee Laredo with 4.0L 6-cyl engine (and auto trans, 4WD). It has around 160,000 miles on it. This is the car my wife drives on a daily basis, so some of the info here is via her description. I'll try to be as clear as possible explaining the situation.

The amber check engine light came on a few days ago, and according to my wife the temp gauge wandered around between 100 degrees F (at the bottom of the scale) and 260 or so (not maxed out, but near the top). This was after the engine should have been up to regular operating temperature.

I hooked up a code scanner and got 1 code: P0118; which is "Engine Coolant Temperature Circuit High Input". Given the code, plus the funny behavior of the gauge, I figured it was the sensor. I cleared the code to see if it would come back immediately and it didn't. I checked coolant levels and did a visual inspection of hoses, reservoir, radiator, and everything looked OK. (I had also just changed the thermostat a couple weeks ago, and the water pump is only a few months old).

I bought a replacement ECT sensor at the auto parts store, hooked it up and the check engine light came on immediately. I scanned the code and it was the same as before: P0118. I cleared it again and the light came right back on. Perplexed, I put the old ECT sensor back in, cleared the code and the light stayed off. It wasn't until my wife had driven the car again for several miles that the light came on again.

On the off chance that I got a defective sensor, I returned the new sensor to the parts store and got a different sensor at a different store. When I hooked it up, same behavior: clear the check engine light and I immediately get a P0118 code (within a few seconds).

I got out my multimeter and checked the sensor lead wires. There was about 5 volts being supplied to the sensor, so the upstream circuit seems OK. Next, I shorted the connector and scanned the code and got P0117 "Engine Coolant Temperature Circuit Low Input", just as expected.

Finally, as you may have guessed by now, my questions are: Why does the "old" sensor only throw a code after several miles of driving? Why do I get a code immediately after hooking up a new sensor? I know the cooling system isn't a very complex piece of engineering, with all of three moving parts, but is there something I haven't thought of here that could be causing the problem?