Not my printer I have minimal experience. Shared maker space, usual repair guy away. As I understand the situation.
Original head was swapped for an Aerostruder. Still have original head and a Double head lying around, Someone thinks wrong parts were used for last attempt to repair. The leads from the board are different to the original head, but the lead for the dual head is there. What is the SIMPLEST solution? Is thee a way to fix “thermal runaway” without changing heads?
Does the “thermal runaway” happen immediately when you turn the printer on? This error normally triggered when the thermistor is reading a temperature higher than selected. If the wiring is bad, you usually get a “mintemp error”. The thermistor wires are at the end of the connector with two wires of the same color (see http://download.lulzbot.com/TAZ/6.03/production_parts/electronics/Wire_Harnesses/Extruder_Wiring/).
At room temperature, the thermistor should be ~100Kohms.