I have had 12 good years of this thermostat. Last night I manually raised the temperature setting to shut off the A/C. When I lowered it a few hours later the fan came on but no cooling at all from the A/C.
Help!!
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Well a thermostat is just a switch. It doesn't do any cooling. It only turns things on and off.
the fan came on but no cooling at all from the A/C.

That is an indication that the thermostat is doing its job of turning things on.

Time to walk outside and see if anything (fan and compressor) in the condenser unit is running.