Hello, I have a Ford Fiesta Mk5 2003 and nothing was wrong with it until today. It would start with no issues and seemed fine. However, today I received a new headlight for it and replaced the old one. Very easy job just disconnect the port and reconnect (I had the headlights on for a few minutes after to see if they work) , but after I tried to start the engine and it made this rapid clicking sound (sort of like an electrical arc sound) and then the dash lights turned off and on again and displayed EAC Fail. I tried finding out what this is but not too much luck. The EAC fail doesn't always appear when starting the engine which makes me believe it could just be a dying battery. The car doesn't get driven because I haven't passed my test yet so maybe I need to keep the engine on for a few minutes to keep the battery charged. When I tried to roll my window down it was slower than usual and if I tried two windows at the same time they go even slower (the dash lights also start dimming and flickering when using electric windows.) Could this just be a dying battery? How do I start the engine again? Do I get jump leads? Help is appreciated!

The clicking noise is the starter motor (the solenoid in it if I remember correctly) trying to start the car and not having enough power.  My immediate thought is that the battery is almost flat.

My advice would be to invest in a trickle charger if you're not driving it at all yet as that will keep the battery topped up and save you having to pay the ridiculous cash to actually replace the battery, which is often the case if they get run completely flat.

Pretty obvious that the battery is flat.   Putting the headlights on for a few minutes without the engine running could have flattened it enough for the starter not to work.

I agree the symptoms are typical of an almost flat battery.  You could probably bump start it but unless you use it for many miles to charge it up, it’s not going to help. So battery charger needed