09 dodge nitro a/c or heat knob blows on low and medium shuts off on high

Hello, Most times the issue is exactly reverse of this because most vehicles have a blower motor resistor that runs all but the highest setting. The high setting is normally run straight to the blower motor. In your vehicle, the blower motor resistor runs all settings, so you will need to check for continuity between the blower motor resistor and the HVAC control unit (which is the part with knob on it). If continuity exists, I would either blame the resistor of the switch. The fact that holding the switch is makes the change means I would test for continuity with the knob pressed and not. The switch connected to the knob is a strong culprit, and the resistor should be tested as well.