Limp mode is what it's called. It is the computer's attempt to keep you from damaging high pressure fuel system components, and occurs most often when your fuel filter is clogging.

It is reset to normal by simply switching off the engine and restarting. No need to wait for anything to cool down. In fact, if you are moving slowly enough and in a straight line, it is possible to do it without even coming to a halt, just switch off, and restart.

Clearly the designers of this vehicle never considered the possibility that saving the high pressure fuel pump could cost you your life as the speeding truck coming 'round the blind corner squashed you flat 'cause Limp Mode prevented your getting on with getting on down the road.

Fuel filters for this vehicle are expensive, as fuel filters go, but cheap, compared to many other components that can cause this problem. And face it, they need to be replaced from time to time, anyway. Since a single tankful of really crummy diesel can cause the demise of a brand new fuel filter, and since if the filter clogs, your only option is to replace it -- you can't "adjust" your way out of that one -- when you start getting limp modes, save yourself some hassle and just replace the f'ing filter.

And for what it is worth, for some reason the reduced fuel flow due to a clogging filter can sometimes create a P0093 "large fuel leak" error.