Our heat pump has had a leak over the last few years and the past week it hasn't been able to "keep up" with even mild temps. Now the thermostat says "check". What should I do? I turned the unit off.

I do not believe the CHECK was about the heat pump function. It might have to do with the filters or the batterys in the thermostat. It would also be nice for you to post the model number of the thermostat so we can try and look up the CHECK signal.