My "engine coolant temp"(2-1 code) sensor light comes on the fault code diagnosis (electric malfunction). The motor runs fine at the right temp. After looking for the sensor to test, I'm wondering is it the sensor on the rear part of the manifold drivers side? There are about four sensors but this one is larger and looks to be the one. I removed it and coolant poured out so I put it back in. Can I test it with a ohm test or voltage checker? Its a 02 Mack. Thanks.

Well, I talked to a Mack service tech before ordering a sensor replacement. Not entirely sure he knew what he was talking about but; He said to put the two probes of a ohm tester on the lobes coming out of the module. If there is no Ohm resistance the sensor is bad. I did and there was no resistance so I ordered a new one.

if by module you mean sensor; that is correct. 

the computer will react according to the resistance of the sensor within a range...like 0.5 ohms is 135 deg, 1.0 ohms is 235 deg, etc.(just an example..dunno what the ranges are)

if there is NO resistance or the signal does not return, it lets the computer know the sensor is bad or disconnected and shits out a code for ya.

Yesterday I got the sensor replaced it and it still fault codes. There are two sensors (coolant temp) on Mack one on the side (it runs the gauge) and one on the rear of the intake manifold, thats the one I should have replaced.. Oh well, I ordered it yesterday. Yeah after the fact everyone is an expert. The Mack guys I mean. Story of my life, once you figure it out " oh yeah I knew that"..

On the intake manifold?  Inlet Air Temp, right?  Did you take an ohm reading on the new sensor?  Good idea to...and write it down in the book in case you need to check it later.

Yea, everybody is an expert when its your money you throw at the problem, right!

Intake air yes, The manifold has circulating coolant. Not sure if its to cool the air or heat it? The temp gauge is for the engine temp but the sensor is mounted in the liquid cooled intake manifold.