Hi, Do you have a security icon on your dash, it may be your alarm system. Do you have the spare key? Try that with nothing else on the ring at the same time. Check all your fuses, the one in the Smart Junction Box may blow. If you’re trying to start, verify you have the steering wheel locked so it doesn’t move first.
Has it rained recently before this occurred? Water makes its way into the smart junction box in 05’-07’ Mustangs. Look for moisture under the dash with a flashlight, check every fuse, check for power at the inertia switch at both sides, check your grounds, and clean your battery terminals
Try the wheel in a few positions. Then pull out the key & try to start again. Be sure the fuel pump is running for about 5 seconds when you turn the key to on, just before “start”. If you have a 5 speed, make sure the neutral safety switch is being hit when you press the clutch in to start it
The 05’07’ Mustangs had an issue with water getting inside the Smart Junction Box, make sure its bone dry inside the fuse blocks ‘ under the dash. Use a good. Flashlight, under hood/underdash.
Check the fuel pump inertia switch if it spins but no start.
The battery must be fully charged for it to start. Pull the terminals, clean the terminals again, check the grounds, battery to block, block to chassis, battery to chassis, battery to CPU. Remove each one, clean to bare metal, use an electrical contact dielectric paste & lock bolts down tightly, these have been known to have alternator grounding problems, jumper negative cable to battery negative to starter chassis.
Once running check for 13.8-14.2 V at battery terminals, should be at minimum, 12.0 volts at battery, just sitting there. Redoing these steps sucks, but it helps to get you reliably running.
Clear every computer code once car charges overnight, tighten clamps (+)/(-) very tight but do not overtighten.
Hitting the brake & holding it will clear codes once each FOB button is held in, each for a few seconds, last is unlock, hold brave in for 30 seconds after starting, use hour old FOB here, use the new one once you clean grounds, ensure all grounds are tight, Neutral safety switch, turn off security Lamp
May things repais that properly wornći works, make sure they are , post what you find, can reset PATS/FOB ‘
Good luck for now!
Starter relay costs 15$, buy a big package of fuses of type, mains for fuse holders & mains for fuse blocks
Talk to you later- John