2003 ford mustang gt anti theft light flashing engine will turn over but will not start

The fuel pump or ignition circuit is disabled by the anti-theft system. Beginning with some higher-end models in 1996, Ford embedded an RFID chip in the plastic on the key handle. If that is broken out or damaged, you're locked out, and you'll have to replace the key.

You say 'won't turn over'. Do you means the engine is not cranking (turning over), or do you mean it cranks but doesn't 'start'? Did you disconnect the battery when you installed the fuel pump.

The flashing anti-theft light may be a clue here.

1. Insert the key into the driver's side door lock.
2. Turn the key once, but not all the way to unlock the door. Leave the key in this position for 40-60 seconds. ...
3. Do this again, but turn the key in the opposite direction.
4. Remove the key from the door, get into your Ford, and start the engine. The system should be reset.

How to Reset the Anti-Theft System on Your Ford

Or there are videos. You should also check for DTCs, diagnostic trouble codes.

https://www.google.com/search?q=2004+ford+mustang+reset+security+system

..

The anti theft light is flashing because the key used to start the vehicle is not paired to the vehicle and has activated the immobiliser!

The Anti Theft light is programmed to operate four different ways. A slow flash is visible while the engine is off. This is an added theft deterrent and is by design. With the key in the run position (not running) the Light will function three different ways. A "Fast Flash" will alert you of an active fault and "Solid Light" indicates a stored fault. Both faults will disable the starting system. Lastly a "Slow three Flash, then the lights turns off" indicates the system is operating normal. More than likely the issue will be at the ignition key but diagnostics is needed. Any questions please write back. Good Luck.

99 F-150 ignition/anti-theft question - Ford F150 Forum - FordF150.net

www.fordf150.net > ... > Electrical (Everything except audio systems)

Sep 16, 2009 - I had 2 keys for the truck, one worked perfectly fine, the other wouldunlock the ... over but the anti-theft light flashes and the engine won't actually start. ...Should I just spend the $80 for the switch and hope that is all it needs? ... for Fordkeys (Later 1998-newer Ford, Lincoln, Mercury) ... Mustang 1998 - 2001

electrical - Can I disable antitheft (PATS) on 03 Mustang ...

mechanics.stackexchange.com/questions/.../can-i-disable-antitheft-pats-o...

Jan 4, 2012 - So my question is: Is there a way to just disable the anti-theft system? ... This will make the car always 'see' a key with a chip. ... as they are usually intertwined with the anti-theft system to ensure the engine will not start. ... Where is the Wiper Relay/Control Module located on a 1994-1998 Ford Mustang??? 7.

How to Turn off Theft System on Cars: 7 Steps

www.wikihow.com > ... > Cars > Car Maintenance and Repair

wikiHowJump to Questions and Answers - Be the first to ask a question about this topic: ... Itwill not start and your battery will die. ... Reset a Check Engine Light ... Thanks to allauthors for creating a page that has been ... How to. Make Oat Flour. Cook Bacon in the Oven ... People are incredibly helpful and open to new ideas ...

your anti theft device is activated,you need to lock and unlock your doors a couple times with the key fob,not the key,and then you need to put the key in the ignition and start it,if you dont have your key fob,call your car dealer to get a new one,they are programmed for each car only,you cannot bypass it.

Try the other (spare) key. it could be just the transponder has failed in that key. if not it will be a tow to the dealer problem.

This is from the Ford manual:

When the ignition is turned to RUN or START, the

theft indicator will light for 3 seconds and then go

out. If the theft indicator stays on for an extended

period of time or flashes rapidly, have the system

serviced by your dealership or a qualified technician.

You can view additonal information at this Free Ford website, by viewing the owners manual.

https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

The chip is in the key. Either the key has lost it's program or the transceiver is not recognising the key. Try another key if you have one.