I had an engine failsafe prog show up a few weeks ago. Like yours it popped up spontaneously. If you try to drive it you will notice that it starts out in 2nd gear. There is no engine power from a standstill start. The vehicle is in a "limp home" mode. The common causes are usually one of 2 things from the research I have done. Its either a bad brake switch module or a bad harness to the MAF sensor under the hood. If the brake module switch is bad then the computer thinks you are engaging the brake and gas at the same time, so it throws the engine into failsafe so you can't burn up your brakes. Not sure what happens with the harness to the MAF sensor.I "limped" mine into the dealer, and they could not figure out what the cause was. My brake pedal module was fine and my MAF harness was ok as well. The diagnostics isolated the cause between two computer modules. It suggested that there was a wiring short somewhere between the 2 modules in question. My tech pulled the bus between the 2 modules and found no shorts in the harness or in the modules themselves. He put it all back together and cleared the codes, and the problem went away. That was nearly 3 weeks ago and the problem has not resurfaced. When the problem arose it was really raining a lot here and it was very humid. We had flash flooding and I drove my X through water halfway up the wheels several times. My tech speculated that some moisture in the form of condensation got between some pins on the bus triggering the fault. Everything is now dry and my X is working fine.Anyway.....hope that sheds a little light on the issue.I have a 2002 4.4iMark