How Preloader get corrupted ?
There could be few reasons but main reason could be that someone or you Flash your complete phone (include Preloader) with wrong files or with wrong ver. Also there could be reason someone Delete Preloader or you Deleted.
How could i know my phone have preloader dead ?
Preloader can be dead if some one Flash it wrong and after flash your mobile will not detect with pc any more. for confirm
open device manager
take battery out from phone
and check is computer detecting any device
If it’s not detect then you have phone which Preloader is dead AKA Boot Dead
How can i repair it ?
you have to disassemble your phone and take pcb out. check there should be some Golden points. check these points there should be written on them Rx,Tx,GND,Vcc,Vcharge etc… check there should be written "COLO" or "KOLO" or "KCOLO" you have to Sold that point with GND After Sold Attach usb now your phone will detect with USB !! and now ready to Flash.
In some phones "COLO" or "KOLO" or "KCOLO" not written for those type of phones you have to test all points one by one Short Golden point with GND and attach usb cable which point short with gnd can able to detect your phone by pc is Force Preloader Mode Test Point !! Keep in mind Don’t Short Vcc,Vcharge or any other pin which have 4.x Volts also i had tested "Colo" or "Kolo" or "KCOLO" don’t have any kind of volts..