Ok, so back in early August the CEL came on in my '07 XK. OBD code scanner showed a P0128 error code (thermostat under temperature). From info on this and other forums, I determined it was a stuck open thermostat, so I order a new thermostat and housing, figuring to change it out myself.
Due to other stuff, I didn't get around to finally starting the work until the last Sunday in September. What a PITA to do it! (I've got small hands, so I don't know how guys with bear-paws can get in there to do this sort of thing!)

Anyway, I finished it up this past weekend, and got ready to take 'er out for a test run. Cleared the codes with my scanner, and almost immediately I got a warning message pop-up in the middle of the instrument cluster about 'engine temperature monitoring' (sorry I don't recall the exact words, and didn't take a picture of it). The CEL wasn't on, but I reread the engine codes, and saw a pending P0116 code (Engine Coolant Temperature Circuit Range/Performance).

WTF? I just replaced the thermostat and housing, which included a new ECT (Engine Coolant Temperature) sensor.

Googling the P0116 code, pointed at the sensor, or wiring as a problem.
I didn't like the thought of taking the whole thing back apart to get at the sensor, and maybe replace it with the old one from the old housing, but what were my other options? So either the sensor was bad, or maybe I pulled a wire loose in a connector when trying to take the old housing off.

One web site suggested testing the sensor's electrical resistance, to see if it varied with temperature. I couldn't get to the 'lock' on the sensor connector, but found that the wires from the sensor actually went to a conveniently located electrical connector on top the LH valve cover.
After futzing with that connector a while to get it apart (why are all the electrical connectors on this car different? I've had to struggle to figure out each one I encounter), I put an ohm-meter on it and read about 0.6K ohms. I compared it to the old sensor, which read about 2.6K ohms. The engine was still warm, so I waited to the next morning, and the reading was about 2.7K ohms, close to the old sensor. So it was varying with temperature, and was probably ok.

I then recalled reading how it was necessary to disconnect the battery after changing the rear brake pads, so the computer would relearn the parking brake position and not wear out the rear pads prematurely. Could the computer be remembering some electrical characteristics about the ECT sensor?

The workshop manual states to disconnect the battery when starting to replace the thermostat, but that's just a standard 'safety warning' to prevent the fan from coming on, and since I was working on a dead cold car, there was no reason for that fan to come on, and I wasn't going to stick my hand in its path anyway, so I had not disconnected the battery while I worked on it. But could it be part of an unwritten reset procedure?

So I disconnected the battery, touched the cables together to discharge any residual charge, waited a couple of minutes, and reconnected it.
Started up the car, and damn! The warning was gone!

So just a note to all you DIY'ers out there, if you change your ECT sensor, and get a P0116 error code (or an annoying message on your dash), try disconnecting and reconnecting your battery.

 

Thanks for the info. Hmm.. I wonder if the same "disconnect battery rule" would apply for changing a headlight assembly? I had posted on the forum about an error after replacing the driver side assembly, and thought the car needed to have the computer reset by Jag. However, the jag tech said they believed the connection was faulty. So they put my old headlight back in place to test that the problem was with the new light and not with the car. I wonder if a good old battery disconnect would solve the issue?? May be worth a try.

 

Just had the P0116 fault after changing the throttle body hose, completely un related but possible disturbed something
Disconnected battery and shorted wiring
Fault cleared, pleasantly surprised
Thanks