CLT & MAT not the same before cold start
 

CLT =24C
MAT = 18C

I believe calibration is in order, what values should I use ?
If all is right I should see the same temp when cold before start - right ?

Thanks

I think better than 6C, yes. But, realize that it takes a long, long time for CLT to actually get to ambient temp, so that fact that it is higher than AIT does not guarantee that one is wrong. If, however, the AIT was higher than CLT, something would definitely be wrong.

The way to calibrate is to remove the sensor, put it into a temperature chamber and get resistance readings at, say -40, 20, and +140C (not needed to go that high on AIT). Then put in the actual, measured values under Tools in TS, and write to MS.

The trick, of course, is access to the equipment.

Not necessarily, since the coolant has a high thermal capacity it will change its temperature more slowly than the general ambient air temp affecting it.
It should be slightly warmer in the morning, but colder than ambient later in the day.

Edit: Too slow...

which one is closer? alter the bias of the other to move it in line.