I have a 2006 Jeep Liberty and I noticed recently that my heater don't work all the time. It is currently working, but I don't know for how long. What does that mean? Can I fix it myself to save on cost incase it needs to be replaced?

What do you mean by "heater don't work"? Do you mean that the blower motor or fan is stopping, or that sometimes, you just have no heat? (blower on, but blowing cold air)?

Sometimes I have no heat where the blower is on, but it's blowing cold air instead of hot.

Gas, and what happens is that it starts blowing cold air and then gets hot and then just stops hot air and blows cold again and repeats this.

the heat is controlled by a vacuum driven blower door that swings back and forth when you move your heat control knob...those doors are known to hang up and drag, or even freeze in the closed position. One trick you can try is to move the knob to the cold position, then slowly bring it towards hot. If you occasionally get heat doing this, it is the blower door hanging up. They are a stinker to fix, involve removing a lot of the blower housings under the dash. Usually a shop job unless you have a lot of patience dismantling the heater...they make blower door repair kits. Another item to check is, when the engine is hot, touch the heater hoses near where they enter the firewall. Have the temp control set to hot and blower on..hoses should be quite hot to the touch. If not, can be a plugged heater core or air bubble in the heater line. Is yours a gas motor or a diesel?

Gas where is the heater door located