Air conditioner coils | Ask MetaFilter: "We just had our 2-3 year old AC looked at because of ice on coils. There are coils on the outside unit which you can clean yourself with a hose and sprayer. There are also coils in the inside unit. Those would be the ones to form ice. Cleaning those might do some good depending on how old the inside unit is, but you would really want to fix what is causing the ice. In our case the filters (electronic but that does not matter really) were so packed with cat hair, lint, and dirt that there was not enough air flow to the coils. When that happened the coils iced up and the AC barely cooled. Our AC guy told us to keep the filters cleaned and be sure the air intake in the wall was no restricted/blocked. After cleaning the filters and the ice melted it has worked great.
Our AC guy told me that we should see a 15-20 degree drop between the intake and vent temps. Our unit is in a closet with both intake and vent very close to check this.
Not particularly part of every icy coil problem, but we also had trouble with the drip/drain pan's drain pipe on our unit. It was blocked outside which backed up water when the ice melted and tripped the float sensor. The float sensor turns off the outside unit and leaves the inside unit running in 'fan mode', no cooling. For a week we had poor cooling with ice forming and melting and turning off the outside unit which was all remedied by cleaning filters and clearing the drain pipe.
When having the unit looked at get the freon levels checked too."