97 150 blows cold-(no heat)

[idifamilycab]

helloi there i have a buddy with a 97 f-150 5.4l . He says the heater only blows cold. He also said that it periodically blows warm, and then it blows cold most the other time. I went to look at it and found that his overflow tank was empty. Would not having enough coolant in the rad cause this. Or do you think its a blend door problem. or can it be something else?

[ynevada]

low coolant level will indeed cause the heater to not work properly