Max temperature for ABS not reached

The temperature for the right hot-end never reach the 240C that I have set during printing. To be more correct, it is reached at start and during the first layers and then falls down 10-20C when the fan is turned on (100 of 256 in fan speed). I measured the resistance for the heater cartridge and it is only 7,1 Ohm.

I checked the temperature with an instrument and a K-therocouple and it is correct.

Anyone got any idea? What is the max current the MOSFET can handle? Can I change the heater for more power without damaging the MOSFET?

Actually 225° is enough to print with ABS. Higher temperatures are hard to reach because the fan is blowing on the heater.Set the fan to max 50% in Cura.