I have a Ford Focus. Several years ago I started to get a flashed-up message saying "engine malfunction" with a big red exclamation mark in a triangle, and the car went into limp mode. The first time it happened I thought the apocalypse had struck. I was very close to a Ford garage but they were very unhelpful. I decided to drive the twenty miles home slowly. Oddly, the car drove normally and the message had disappeared. No problem. But it did it again several times the following week. Each time, I discovered that all I had to do was switch the engine off and switch on again. Eventually, I called out the AA man, who plugged his wotsit into the doodah and told me that there was an error code which meant "turbo underboost." He stripped down the turbo, couldn't find anything wrong and put it back together. I stopped getting the warning. Result!

Except that the issue came back about a year later. Since then, which was four years ago, it might do it once then not again for three months, or twice in a week, or not for a whole year. Just switch off then back on again. Bloody mind of its own. And no manifestation of any turbo issue either. Goes like a rocket!