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My MES bounces the temperature up and down. I think I saw 291 when set to 275, as it was settlling in.
But running hotter than setpoint is good, you can always set it to 265 if you need it to be a little cooler. But you can't set it to 285 if it's not even reaching the requested temperature. So better to have it run a little hot.
Sometimes that high temp is coasting after the heating element turns off.
Sometimes it can be the difference between the MES Temp sensor position & where you put your second Therm.
Other than that, if you put your second Therm near the MES sensor, you should be able to compare the difference.
Mine is usually about 15° higher than the MES says. I like that (Like Red Octobyr said).
I believe Masterbuilt guarantees their temp to be within 15°, but the only way I would complain or do anything at all is if mine was actually more than 15° LOWER than the set point, because like Red said that would keep you from being able to get to the 275° that you paid for.