Heaters only blowing cold air!

Firstly your probably wondering why I was using them in the summer ish!, well when I left for work today it was chilly haha! so went to use the heaters, yet they only seem to be blowing cold air!



would I be right in saying it could be an airlock, heater resistor or the heater matrixs?



Btw there is no sign of hgf! so its not that

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