Dell XPS L502X fan won't turn on even when using intensive programs causing overheat and shutdown.

Hello, i have had this dell xps l502x laptop for two years and since about 6 months ago i been experiencing this problem with the fan. Roughly only every one in six times will the fan actually ever turn on when the computer is running. I understand that the fan does not need to be on the whole time but when it comes to the stage where the system is running hot and requires cooling the fan still won't turn on. e.g. when playing games or running stress tests. I have ran a stress test watching my  GPU and CPU reach temperatures over 100 degrees Celsius and the fan still doing nothing. Before i had this problem when ever i ran this test the fan would turn on a few seconds after the test had started.

After my system would shutdown due to overheating i would then have to restart it about 5/6 times before the fan would kick in and cool the system. The same thing would happen when trying to play games, fan wouldn't start, shut down due to overheating and i would have to restart many times hoping fan would start. There are times that the fan would turn on first time round when needed and i could play happily away for hours and not experience any problems with it but more often than not it won't start up no matter what. I have tried updating the bios, i have ran the diagnostic test by pressing f12 on boot up and i have tried the diagnostic tests on Dells website but still no luck its not picking up any problems. Is it possible that my fan has gone faulty? is it a hardware or software problem? Its sort of like my computer does not recognise that its running really hot and that the fan should be enabled.

So quick summary- Fan will only ever turn on in roughly 1 in every six times when necessary and in the common event that it doesn't i have to restart my computer like 5 times before it will start up again. Any suggestions will be greatly appreciated thanks.