I had the F11 code, replaced certain parts, then it worked for a few weeks as well, before giving me the F21 code. Turned into a nightmare. Luckily, these guys helped me out the same day after I gave them a call and they were able to come back the next day with the part, replace it, and I haven't had an issue since. It has been about 5 months now with zero problems. I don't know exactly what part they replaced but I am a happy customer now.