Limp mode on 1.5 mondeo mk5

Hello to all, I am a car audio enthusiastic, I love a good sound sistem but I think this time wasn't the best idea to upgrade it on my 1.5 mondeo as amber spaner came up in dash advising to see manual wich i did, but it just says service soon, i did the service myself but didnt change anythkng. Car Is actually in limp mode and showing P018C:00-27 that says fuel pressure sensor b circuit low. I think it was to much power taken from the battery as you could see the  head lights beaming when the music was loud, I had it installed for about nearly 3 months untill today the 29th of August, and somehow this fried something with in the fuel electric sistem, some times when driving you could feel the car like stopping getting enough fuel or some kind of slightly hesitation. I hope you understand what I'm trying to say there. For what I was reading and researching, this fault Coul be caused by alteration in the voltage that the pump is getting to keep the fuel demand. Car starts and drives no bother but is not going more than 3500 rpm and it doesn't reset the fault when I restart the car. The amber spaner Is always there. Just went away for a while the same day the fault started, after I came from work and restart the car. 5 min latter light came up in the dash and has been there since the 20th of August. Fuel pressure sensor was replace by garage buy nothing changed. Some people also suggest that could be oil pressure sensor or selenoid, wich could kind of make sense as there was some codes in the scanner leading to oil pressure but they were deleted and just the one indicating the fuel problem came up again. 

Car is drinking diesel like crazy to.

I'm starting to get really annoyed about that and very sad as I love this car. I hope somebody has an idea of some sort or a similar fault that I can get an advice for. Thanks