my heat pump outside is working but my central air conditioning/heating unit in the garage makes a hissing sound or faint sound of water running. There is air coming from the vents. The very last time I used the heat, about a month ago the same thing happened but when I switched to air conditioning the last month, it worked until now. What are the possible problems?
If all is well with your outside unit, I would double-check your AC coil in the main unit indoors. It could be frozen over and thus not cooling the air. This same exact thing happened to us about 3 weeks ago. We moved into a "new" house (new to us anyways, haha) and not many things were being taken care of well... in simple terms, a freeze-over like this can happen because the coil drain is clogged with dust particles and/or sludge. This means there's no where for the condensate to go and leads to a frozen coil. Hopes this experience can help someone avoid a big maintenance bill in the future! To fix the issue, use a hair dryer or heat gun to melt the ice away. DO NOT CHIP AWAY at the ice since the coils are very delicate and you could puncture one easily.
It sounds like you are low on refrigerant or you have a failed compressor. This is not something you can fix yourself. Call for service.
This might be an easy fix. Go look in your air conditioner, there will be a capacitor which is cylindrical shaped. If it is puffed out or distorted in shape, it needs to be replaced. You can replace this yourself. My suggestion is to first take a picture of how the wires are connected and it's simply plug and play. Take the old capacitor to the supply house so you get the right replacement.