no heat

first make sure coolant level is up if that is not the problem next remove both heater hoses and attach a garden hose to one hose and force water through the heater core then reverse the water to the other hose. if the water comes out clean both sides try changing the thermostat. also check the heater door is working properly from cold to hot. 3 of the 4 things will not cost you any thing to check.