iPhone 5c error 21 stuck on recovery mode

Hi everyone I need help. I bought an Iphone 5c second hand , and apparently has hardware problems. It is stuck in DFU mode . If I 'm not mistaken, when it leaves the sign of iTunes and a USB cable . When I connect to itunes throws me an error , error 21 (the iPhone can not be restored there was an unknown error) I tried opening it , I also cleaned the motherboard with alcohol isopropilico and putting a little of cold air with a hair dryer. but neither it seems to work. I have tried everything , even the shift + restore .. try iREB , and TinyUmbrella. I also think it may be the USB cable that is not the original, it is only 8 pins .. please someone who knows the solution !!! (excuse my bad English)