troubleshooting starting issues
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6th Gear
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troubleshooting starting issues
i replaced my ecu and the engine cylindar temp sensor by my mechanics advice and my 2003 f150 5.4 still won’t start. If anyone can identify the issue based off the noise that would be incredibly helpful. The guys at the shop managed to start it with an old snap on code reader by resetting the ecu.
Fleet Mechanic
Joined: Jun 2023
Posts: 1,394
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From: WA
That would be called a crank-no start. Did it do that before you replaced the ECU (PCM) and sensor? Is that why you replaced the ECU and sensor? Need the details.
Your 2003 would have PATS. Is the THEFT light flashing rapidly when you try to start it? PATS will allow the engine to crank but won't allow the injectors to squirt fuel if the wrong key is used or the instrument cluster and PCM are not programmed.
So, give more history and see what the THEFT light is doing. Good luck.
Your 2003 would have PATS. Is the THEFT light flashing rapidly when you try to start it? PATS will allow the engine to crank but won't allow the injectors to squirt fuel if the wrong key is used or the instrument cluster and PCM are not programmed.
So, give more history and see what the THEFT light is doing. Good luck.
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6th Gear
Joined: Aug 2025
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It did that before i replaced the ECU and sensor, yes thats why.
The theft light is on when i put the key into the ignition but it doesn’t blink. I had the PCM programmed by a site who sold it to me because my mechanic said it was a common issue.
The theft light is on when i put the key into the ignition but it doesn’t blink. I had the PCM programmed by a site who sold it to me because my mechanic said it was a common issue.
Fleet Mechanic
Joined: Jun 2023
Posts: 1,394
Likes: 384
From: WA
Has the truck ever started and run right for you? Can't tell if you bought it non-running or if you're fixing a new problem.
So, you bought a PCM (that's what they're called) from a web site, pre-programmed to your truck, and the shop that installed it managed to get the engine started using a Snap-on tool. But they couldn't program the PCM, instrument cluster, and key to work together? That's what it sounds like.
You need to answer the question about if the THEFT light flashes while the key is turned to Start, or right after it won't start. That is the biggest clue, for the moment. The THEFT should blink slowly, like every two seconds, with the key out, then light up solid and go out completely after about a second, when you turn the key on. If it doesn't go out you have a PATS problem.
So, you bought a PCM (that's what they're called) from a web site, pre-programmed to your truck, and the shop that installed it managed to get the engine started using a Snap-on tool. But they couldn't program the PCM, instrument cluster, and key to work together? That's what it sounds like.
You need to answer the question about if the THEFT light flashes while the key is turned to Start, or right after it won't start. That is the biggest clue, for the moment. The THEFT should blink slowly, like every two seconds, with the key out, then light up solid and go out completely after about a second, when you turn the key on. If it doesn't go out you have a PATS problem.
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6th Gear
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So i bought it running but it would only turn over if you leave the key in start for 15-30 seconds then turn the ignition. It wasn’t until this winter during a below 0 day that it wouldn’t turn over at all and i was getting the crank no start.
I bought the PCM preprogrammed and installed it myself and followed the company's instructions to install it.
the theft light is flashing when the key is on pretty rapidly then it slows down to about once every second or so.
I bought the PCM preprogrammed and installed it myself and followed the company's instructions to install it.
the theft light is flashing when the key is on pretty rapidly then it slows down to about once every second or so.
Fleet Mechanic
Joined: Jun 2023
Posts: 1,394
Likes: 384
From: WA
You have a PATS (Passive Anti Theft System) problem. The THEFT light should never flash rapidly.
Either the new PCM is not programmed to the instrument cluster or the keys that you're using are not programmed to the PCM and cluster. Probably the key if it flashes slowly with the key off or out.
A decent shop will know what to do. PATS problems are common. Many mobile locksmiths have the tools to do it also. Might be your best bet.
Here's a video worth watching. Shows what should happen. The symbol is different but the way it flashes is the same.
Either the new PCM is not programmed to the instrument cluster or the keys that you're using are not programmed to the PCM and cluster. Probably the key if it flashes slowly with the key off or out.
A decent shop will know what to do. PATS problems are common. Many mobile locksmiths have the tools to do it also. Might be your best bet.
Here's a video worth watching. Shows what should happen. The symbol is different but the way it flashes is the same.
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6th Gear
Joined: Aug 2025
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Thank you so much man i’ve been trying to figure this out for the past month and you’re my saving grace!
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Fleet Mechanic
Joined: Jun 2023
Posts: 1,394
Likes: 384
From: WA
It's not done yet, but hopefully it's just key programming. If you have a guy come out make sure you have all of your spare keys available so he doesn't have to cut new ones. You'll need at least two chipped keys.
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