The device PTC1 is probably a thermistor (Positive Thermal co-efficient) these work opposite to the usual NTC (negative thermal co-efficient) thermistors like the ones that you replaced on the evaporator.
As the temperature of the PTC increases so does its resistance, thus limiting the current that flows to the fan. As they often exhibit a steep rise in resistance the current to the fan will be negligable above a certain temperature. If the current increases above normal (as in a stalled fan due to ice buildup) the PTC heats up and shuts down the fan.
Your PTC was probably faulty shutting down the fan too early.
This type of thermistor is used in place of compressor starter relays to shut down the current to the start coil after the initial surge has started the motor thus preventing the start coil from burning out.