I do not think that it is MOSFET, If you are saying that it is the same way as hooking up SSR. Then maybe config incorrectly?
Here is some of the setting
temperature_control.bed.enable true #
temperature_control.bed.thermistor_pin 0.24 #
temperature_control.bed.heater_pin 2.5 #
temperature_control.bed.thermistor Honeywell100K # see
#temperature_control.bed.beta 3974 # or set the beta value

temperature_control.bed.set_m_code 140 #
temperature_control.bed.set_and_wait_m_code 190 #
temperature_control.bed.designator B #

#temperature_control.bed.bang_bang false # set to true to use bang bang control rather than PID
#temperature_control.bed.hysteresis 2.0 # set to the temperature in degrees C to use as hysteresis
# when using bang bang

## Switch module for fan control
switch.fan.enable true #
switch.fan.input_on_command M106 #
switch.fan.input_off_command M107 #
switch.fan.output_pin 2.6 #
switch.fan.output_type pwm # pwm output settable with S parameter in the input_on_comand
#switch.fan.max_pwm 255 # set max pwm for the pin default is 255

switch.misc.enable true #
switch.misc.input_on_command M42 #
switch.misc.input_off_command M43 #
switch.misc.output_pin 1.3 #
switch.misc.output_type digital # just an on or off pin

Thanks for the help