Heated bed won't heat past 80 degrees

The heated bed on my SD3 won't heat past 80 degrees.

I've had a similar problem before with different temperatures, some as low as 30, and it has gone away by itself. Sometimes by switching the printer off and on again, sometimes by pressing the reset button, sometimes by reconnecting the printer, sometimes by leaving it off over night.

But nothing is working this time.

Any tips?