2000 Mercury Sable heat to cool

I assume this reading was at the low side port with one of those recharge cans? 40-45 is not a good reading! If the compressor is running or not! If the compressor starts, hook up a manifold gauge set that can read both high and low side, high should be 200 psi and low 30 psi. Turn a/c control to coldest setting. There is a low pressure switch that will not allow the compressor to run if the charge gets too low.