While driving the 96 b23 around last night, my check engine came on (first time I've ever seen it, actually... 215k.) I also noticed that the temperature gauge was as C as it could get (no reading, I assume).

I read around, and got varying reports of what might be causing it. Sounded like it came down to either the coolant temperature gauge, or the coolant temperature sending unit.

From what I understand, the gauge is mounted near the front of the engine, on with the water hoses (dead center of the front, accessible easily with nothing else removed). The sending unit is found a few inches higher than, and back from the oil filter. The gauge has a little wiring harness, while the sending unit has one wire and a little L-shaped boot.

I might have those mixed up. I thought I bought a sending unit, but when I got it home, it matched the gauge (at least as I've described it above.)

Anyway...I replaced the part at the front of the engine bay, cleared the Check Engine code, and drove around for an hour (thank you, Ingress.)

Temp gauge seems to be reading just fine now, and the CE light never came back.

So for what it's worth...there you go. Truck fix0r3d.