Ford Focus

That screams a dodgy sensor or electrical issue to me; once a modern ECU decides it doesn't like a sensor for whatever reason, they seem to have the tendency to switch them off, at least until a re-boot/re-start

 

Quite often, it is an engine restart, yes. Other times, it requires a certain number of 'cycles' (engine off, engine on) before it will restore power or start accepting the information from that sensor again, and in some cases it needs reset by a suitable diagnostic tool before it will play ball.

Try not to think of your car as a car, but a car with a few computers, each of which are in control of their own 'department' (for want of a better way of saying it), and one main one that collects information from those smaller computers to allow it to decide what it needs to do (such as how much fuel or air is needed, and when the spark should happen etc etc) - - if the main computer 'asks' one of these computers for information and then gets an answer that it doesn't understand or an answer that doesn't make sense when compared with the information coming from all the other sensors, it decides that the sensor it has got the duff info from is probably faulty and then either switches it off altogether or excludes its data from the decision making process.

For example, if one of the O2 sensors was reporting that it had a massive amount of oxygen passing through per second, yet every other sensor on the car was saying that there was a shortage of oxygen, the main ECU would exclude the information coming from that sensor as its information was implausible (and this would almost certainly chuck a yellow engine light up to warn you that things weren't quite right); from that point on, the ECU would be trying to give its best guess as to what was going on, however this would almost certainly lead to the car running like a bag of spanners as a considerable amount of the information it needs to run is being based on a 'guess'. It is likely that the ECU would then not bother even asking that sensor anything until the power has been off and then it comes back again (restart of the car)

Many of the sensors rely on remarkably small components, and some of them may work completely differently at different temperatures. A MAF for example is more often than not a thin film sensor (think of it like a thinner version one of the wires in a toaster) - the amount of electricity that flows through it is determined by how hot it gets, and the amount of electricity flowing through is measured, which in turn then tells the car how much air is moving over the sensor (the hotter it gets, the less electricity flows - more air cools the sensor dropping resistance); now, if there was a problem where the sensor had a slightly higher resistance when cold, the info coming from it would be completely wrong, so the car might well disregard it and carry on without - - - yet it is entirely possible that the sensor could begin working normally when it warms up a bit, so switching the car off and back on could get the car to listen to the sensor once more and all would seem absolutely fine until such times as it was started from cold again.

The real problem is finding the guilty sensor, particularly if the fault is intermittent.

 

@longk is bang on @pete - take it in. If Ford is anything like the VAG products, the fault code will be stored along with the conditions that were present at the time of the fault and how many times it has happened.

Whilst this is not the be all and end all of diagnostics, it helps put the technician on the right path. Besides anything else, if (God forbid) problems continue and you have to keep taking it back, you will at the same time be creating a case for continued support once the warranty runs out.

 

It is entirely feasible @pete - many modern cars have wiring looms that disappear right into the engine (in the case of some diesels for example, there is wiring in the top of the cylinder head that basically lives bathed in oil) - sadly, when you run cabling as close or even into an engine, there is always the risk that the wiring can develop a fault due to the stresses of heat and vibration. My Passat had a fault with the engine wiring loom that controlled the injectors, and it ran like a bag of spanners until that section of loom was replaced. Quite a common fault on some Land Rover models too.

See how it goes - with any luck, that will be the end of your woes