ShastaX

I had a contactor that was bad soon after I moved in (maybe). I bought a contactor from the store and installed it. It did not work. I had an A/C guy come out and he said the contactor was bad and replaced it. A/C worked for a while. Then I had someone come back and replace the switch and fill the freon that told me the switch was bad but they can also not work when it is low on freon and mine was. So now it was nearing the end of last summer and everything was fine for those last few weeks. Now I put on my A/C again, my upstairs A/C is fine my downstairs is not on again. I hate to keep paying these people for something that is wrong... can I fill freon myself? Is there a way to check for freon leaks? if you do have an opinion of what it is please let me know? if it isnt freon what else could cause a contactor to repetedly die?