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PLEASE READ THE FOLLOWING:
I’m a retired master technician and tech instructor with over 30 years experience. However, if you’re thinking on working on the vehicle yourself, all my knowledge and resources won’t help if you…
… don’t have tools or the skills to use them
… don’t know there will probably be some special tools and equipment needed for a proper diagnosis
… don’t realize it may take a few back and forth replies to solve the issue
… don’t understand that I will need you to provide specific information
That engine needs three things to start and run (compression, fuel and spark), which means one or more of those things is missing. If it's cranking but not starting you can eliminate compression and spark as being the culprits just by spraying a little starting fluid down the throttle. If it starts and runs momentarily, then the problem is fuel.
The fuel system in your Focus consists of two branches: fuel delivery (fuel tank, pump, filter, pressure regulator, lines,etc.) and fuel distribution (sensors, on-board computer, injectors, wiring, etc.)
Fuel needs to be delivered to the fuel rail at about 35 to 50 psi - you'll need a fuel pressure tester to check it (these testers are available as loaners from most major auto parts retailers.)
Injectors need to be opened and closed in sequence by the on-board computer - you'll need a Noid light to check (these are available for sale under $20 at the same stores)
However, if the engine doesn't start with starting fluid, then you'll need to check spark (should be strong with 12 KV or more) and compression (minimum of 125 psi on all cylinders).
Please check those things and let me know what you find so we can proceed.