On a 92 Camaro V6, which value would be more likely to be in error relative to coolant temperature. My scan tool, when in the field service mode, shows a coolant temp of 223 F, the instrument panel gauge shows 240F. The single electric fan kicks on at 228F & off at 212F according to the scan tool . This is fine, except the ECM only knows the value the sensor provides. The question is, which sensor/sender has more of a history to be "off". The sensor that feeds the ECM or the sender that feeds the gauge? I'm try to determine if cooling system and the A/C systems are performing normally. I'm coming to the conclusion they are, but these GM systems have alot going on to be normal.

 

They are both going to read different, simply because they are in different spots. The one in the head might run hotter since it might bleed off some heat by the time it gets to the other. Then again, the one at the top is supposed to be the hottest...
I'd trust the one for the ECM first, simply because the gauge is not marked enough to read accurately, not to mention if its like mine the sweep is not even, it varies as to how much the needle moves is how much temperature change. Mine reads 100-220-260, now I'm no math major but 120 does not equal 40.

If you really want to verify it, get a thermometer and stick it in the radiator, or you can use an infared if you have access to one of those.