MKS Gen L, nozzle not cooling

Ok, so I'm finally making progress with my MKS Gen L! Marlin flashed, display working, steppers all moving in right direction and homing correctly. In order to test the extruder I had to heat the hotend to over 170 degrees. I did this by setting preheat PLA, nozzle. I did not heat the bed. The hotend quickly heated to 200 degrees, and held. I performed my extruder test and selected cool down. Nozzle temperature fell to around 150 degrees, but then did not drop any further. The cooling fan was running, and blowing in the correct direction. I had to turn the printer off to get it to cool down. When heating 2 red led were lit, after I selected cool down 1 led remained lit. Any ideas what is wrong?

The hotend is a V6 clone using the standard glass bulb type thermistor

Tried to change the thermistor from 1 to 5 in marlin but it made no difference. The hotend doesn't cool down, its as if it's stuck at 150 degrees. What am I doing wrong?

It sound like you hot end MOSFET is damaged and not Turning off.

Flashed marlin 1.1.9 in desperation! Hotend heated as soon as power was switched on, I didn't even set to preheat!
Switched off and after letting cool down, changed the heater cartridge and thermistor. Switched on and hotend read around 30 degrees, set to preheat and temperature went up, set to cool down and temperature fell! So it appears that either my thermistor or heater were damaged. Further tests tomorrow, but I think I'm making progress!