I had that on my '97 406 1.9TD. My mate had power washed it & water was sitting in the big 'scart like' plug on the ECU (under a plastic cover on left side of engine compartment, as you look into it, above all the belts). I disconnected the plug, took out the ECU, blew out the water that he'd forced in, used GT85 water displacement spray & I have no more problems. I hope yours is as simple.
Mark, Grimsby UK
January 2010