Temp Gauge steady, but way too high???
 

I've got a /94 GMC 350 K2500 and yesterday replaced the left hand temperature sensor. The part No they gave me is a Niehoff DR134AK TS25381. It had two wires instead of the one that the original one had so I ran a ground wire to the other side. After I finished installing it, and went for a drive, the temperature then climbed, and climbed, and climbed. It actually hit "check gauges" at about 125C before it settled back to about 115. The temp on the block measured was at 80C, proper operating temperature.

Did they give me the wrong sensor???

You need the proper sending unit... if it didn't match the original it isn't the right one. Take the original with you and get the right part.