I posted this in the expedition forum but this appears to have more traffic.

Went to start my 2003 5.4 expedition today, turned key and had full interior power as far as I could tell, but no crank. Cabin lights did not dim when trying to start.

I then went to check pcm relay. I DID NOT UNHOOK THE BATTERY!!! I pulled out the relay, did not look terrible, fumbles around and put it back in. Went to turn key and no dash lights EXCEPT airbag lights and gauges would go all the way up. Also new at this time is the vehicle would not shift out of park which it did prior to me messing with the relay.

I was clearly worried at this point needless to say. Eventually replaced PCM relay with another I had and all acts normal initially. Key in ignition with door open and alarm is dinging, advance key to on and dash lights up normally, than turn to start and it often seems like it just tries to turn over and then instantly power mostly cuts off, like a switch. It won’t act properly again until I remove negative battery terminal and it seemingly resets. The initial relay once I retried it still acts like it did after my initial mistake. Replace in the second relay and it does above sequence in this paragraph.

So is it likely the computer is fried?

I expect some chastisement for my lack of cautiousness with my vehicles electronics. I will now try to always remove key, disconnect battery, and reconnect battery while key is removed!

Thanks for your time!

Good diagnostics! Before I saw this went out and turns out it was the simple fix. I tightened negative and didn’t work. Then jumped it and started right up!