Last year a reputable company replaced my air conditioning evaporator/coil. They struggled to make it work. It took them five visits, three last year and two this year. They kept blaming the parts, returned a few to the manufacturer. They finally got it to work in May 2015. The technician told me the problem was not with defective parts.
There is a valve called TXV (Thermal Expansion Valve) that sits just outside the coil/evaporator box on the furnace where the refrigerant lines go in. TXV meters the refrigerant flow making the A/C more efficient and helps reduce electricity bill. This is a new component which did not exist in older systems.
The valve is shipped with the sensor hole covered with a cap, like the paper seal under the cap on a shampoo bottle.. This cap must be removed before the sensor is attached for the system to work. All previous attempts did not remove this cap. The valve was getting iced up and the system not cooling.
The problem was with technicians out of touch with the systems/equipment they service. Or, they just like visiting your house and try selling more stuff while they are there. I was surprised because this was a big local company that's been in business for many years.
One of my friends had a similar experience this year with another big local HVAC company. He got a new coil installed last year. His valve was icing up with loss of cooling. The repairman was making multiple attempts to make it work. And my friend's unit was running out of warranty. I told him my experience, and he asked his repairman about this. Whola! it was the cap in the hole. Apparently, the repairman asked my friend, "how did you know about this?"