What causes Limp Mode

I have a 2008 Ford Explorer, with 120K the heads were already redone when I broke the chain, the radiator has been replaced the heat sensor replaced, the fuel filters replaced , ETB replaced other then the transmission I do not know what is causing it to go into limp mode constantly, I have taken the battery out tapped the wires together, turn the key while holding the gas pedal down . My husband was a Ford mechanic and always took care of the vehicle, since his stoke I have had to do all the above. I need to get the vehicle on the road and can not afford to take it in,