Temperature control

I have this cheap temperature control that was working properly but now I can only set the high and low within 5 degrees of each other.
When the temperature is set to 90c it will come on at 28 and off at 30.
The temperature probe is ok, voltage regulator tests ok, resistors are fine, diodes are ok and I haven't checked the caps.
The only components left are a transistor, microcontroller and display..
My best guess is a faulty controller, so I'm looking for other opinions that might cause this failure.
Thanks.
Kevin.

The one I have (bought on Flea-Bay) looks similar, but is enclosed. It allows me to program the amount of hysteresis from the front panel.

This one has that built in as well.
The default is 2.

30-28=2

The problem is the temperature is set to come on at 85c and below but turn off at 90 not 30.
I swapped out the temperature probe and the results are the same.