heater help please (for a mate).....


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my mate has a micra 1.0l, and we were doing an airlock in the water system of his car. We went to put the heater on full temp being the hottest but when we turned the knob to put the inside heater on full blast it worked but was making funny noises so we decided to turn the heat down to the second notch but it didnt work. It dont work from notches 1-3 but the heater only works when on the 4th notch being full air into the car?

any suggestions on how we can check if anything is faulty, where about is the matrix (if it is the matrix that is)?
I gave up trying to find a good scrap yard one and coughed up my £30 to the Nissan dealer.
New heater resistor (AKA transistor/heater card) card bit was noticably shorter.
Modified design perhaps?
Worked anyway!