FIXED: Ford Focus engine won't stop when switched off - and loud clicks from one loudspeaker

In case it helps anyone else... Here's how I fixed two weird faults that had appeared at the same time on my wife's 2013 Ford Focus Mk3, which is a 1.0 EcoBoost UK version.

The faults were:-

1. The engine didn't stop when the ignition key was switched off. It kept on running smoothly (not like the rough "dieseling" you sometimes get when you switch off an old car with a messed up carburettor). The rev counter stopped working, and I could hear a component shut off (probably the fuel pump), but the engine kept running, and responding to the throttle.

2. A loud clicking noise was coming from the driver's door loudspeaker (even though the stereo was switched off).

I managed to stop the engine by turning the key all the way to the starting position, and then switching off - but even then the auto stop/start would restart the engine if the car was in neutral and you pressed the clutch.  Otherwise the car remained drivable, though it sometimes almost stalled when pulling away at junctions.

My local Ford garage couldn't look at it for a week, which gave me some time to ponder the symptoms... 

My first thought was that a relay was sticking closed, keeping the supply going to the ignition. Taking a wild guess, I tried changing relay R16, but that made no difference, so I stuck the original relay back to be on the safe side.

I realised that whilst it might just be coincidence that two faults had occurred at the same time, it was also possible that the two faults were directly related.  I speculated that the car stereo head unit was failing in such a way as to produce lots of electrical noise on the CAN bus - meaning that some of the control messages that normally pass around within the car, would no longer get through: including the message to stop the engine when the key was switched off.

I hooked up an ELM327 OBD2 unit and read the status from the car.  There seemed to be a load of random DTC errors - possibly indicating CAN bus errors, or possibly just indicating that no-one had cleared the DTC's for a couple of years.  I don't know enough about what these diagnostics actually mean.

So, I decided to pull out the stereo head unit and unplug it, and see if that fixed both the faults.  YouTube had several good videos showing how to remove and unplug the central fascia/control panel (this include the two central air vents and all the stereo controls) - this is just two Torx screws.  That done, a further two Torx screws allow the head unit to be removed.  This is a metal box about the size of the car stereos of earlier years, but with no display or controls on it.

I wasn't sure the car would even run with the stereo head unit removed - as I've heard of one car where the stereo performs essential car functions.  But after carefully unplugging the connections on the back of the head unit, I found the car would start - and both the faults were gone - which was great!

At this point I was thinking I'd need to source a second-hand head unit from a car breaker.  So I read the part number off the label on the head unit, and started looking on EBay.  But whilst doing so, I realised there was something metallic rattling around when I turned the head unit over.  I suspected a loose bolt, but after shaking the unit a little, out popped some change: two coins totalling £1.20!  I can only assume that a child stuffed the coins into the CD slot, possibly before we bought the car. My children all deny it (and to be fair I think they would have just kept the money).

After refitting the head unit and the central fascia/control panel, the car now works normally, and the stereo appears none the worse for the incident.

So this was a good outcome!  But in the long term, perhaps the increasingly ubiquitous use of CAN bus technology on modern cars is going to cause some faults that are a real pain to diagnose.