blows cold air, no heat, what are the possible problems?

all i can think of is the heater core, im sure theres something else though,,

 

no/low coolent, bad t stat, bad heater control valve, clogged heater core, bad control valve inside the car on the dash heater controls, vacuum leak to heater controls

 

ty, wanted see where all to look before i started poking at it

 

My 87 firebird would only heat after you stepped it down. So I bypassed the heater core and now it heats great. There was also a vacuum leak around the control lever but when I would get that to quit leaking then it wouldnt heat at all.

 

how do you get heat with the heter core bypassed?? without the coolant going through the heter core you get NO heat? can you explain this cause now i'm not getting heat and it only gets hot when i step on the gas.. my heter core is new too i replaced it last winter

 

Sorry about that I ment I bypassed the heater valve.