The thermistor (R40) reads 50ohms. When I place my industrial worklight over it (that is pretty hot) it slowly, very slowly increases to 52-53 ohms. That seems like a very shallow response, the lamp is much warmer than the heatsink has ever gotten.
Unsure if this thermistor is still doing its work. Unsure whether the ohms reading is good. Nor, if it is indeed broken what replacement is suitable (any PTC thermistor?).