Sounds like your thermocouple is either out of whack or not
installed in the correct position. Don’t count on the gauge, it’s
likely not accurate.
What you can do to calibrate your setup is to find a ceramics or
pottery supplier and get some firing cones of various temperatures.
I’d get some at 1250, 1300 and 1350. Put one of each in the kiln and
do a burnout cycle. At the end of the cycle, look at the cones. They
will melt at their appropriate temperature. If you had it set for a
max of 1300, the 1250 should be a puddle, the 1300 slumped over but
still recognizable, the 1350 should look like nothing happened.
Check the manual if you have one and see where the recommended place
for the thermocouple is, and make sure that’s where it is and that
it’s not touching anything. If it still isn’t working right, get a
new thermocouple and install it correctly. If it’s still not working
right, it may possibly be the kiln minder itself, but that’s not
likely. They tend to be a go or no-go type of unit, in other words,
they either work right or they don’t work at all. Once again, don’t
count on the gauge on the kiln, if it’s within 100 degrees it’s about
as accurate as it’s ever going to be.
All of this is assuming that you are programming the unit correctly,
and that the kiln is working properly, of course.
Hope this helps.