V50 intermittent electrical issue
 

Sorry for the long post, you could probably skip to the last sentence if you know about CEM issues but I’ll give some background.

I have a 2006 v50 d5 auto, no sunroof. Recently I had my alternator replaced as it wasn’t charging. To cut a long story short Hutton Brothers diagnosed the faulty alternator but the suppling dealer collected the car and put in a reconditioned alternator at their expense because they hadn’t activated the warranty. Hutton bros lent me a battery to get the car home as mine was almost totally discharged and the car wouldn’t be collected until after they closed. Once I had the car back I returned to Hutton bros to swap batteries, started the car and as soon as the bonnet was closed it came up with “Brake failure stop immediately” and wouldn’t allow me to take it out of park. Also the wipers stared doing their own thing. Being at the garage it was connected to Vida and came up huge list of fault codes. After a play around the diagnosis was that the battery connector hadn’t been seated correctly and that doing this had sorted the issue (it seems the wipers doing their own thing is a good sign as it means the cem hasn’t failed)
That was on Monday, on Thursday we took the car to visit family in Wales, on the M62 transmission service urgent came on the display. As we were near a service station I stopped, turned it off then on again and the issue cleared. Later on the a55 the brake failure warning came on and the car lost power. Not having Any tools, and having my wife and 2 dogs in the car I gave the battery connectors a wiggle, restarted the car with just an anti skid failure warning and managed to move it 200meters to a lay-by to call out the rac. As I stopped here the brake fail warning reoccurred. The RAC technician plugged in his diagnostic machine and got a whole load of fault codes, the wipers were again doing their own thing. He checked the alternator was charging and confirmed it wasn’t over charging, checked the brake lights came on and made sure there were no obviously lose connections to the cem under the glovebox. After disconnecting the battery and reconnecting it the car started normally and all the warnings cleared. We completed the last 30 miles with no issues.
Given that a techie reboot seems to clear the issue the rac suggested get a 10mm socket for the journey home and book it into my garage because there is clearly an underlying electrical issue.

Having spent the evening googling v50 electrical issues everything points to the CEM. As I have a day off I’m going to unplug everything, clean and reconnect on Monday; are there any other connectors to clean or lines of enquiry I should go down while I have a can of contact cleaner out and before resorting to the garage?

Your long list of codes suggest electrical instability.

Try also cleaning the battery terminals and battery lead connections, fine wet and dry paper is OK if you don't have a proper wire brush cleaning tool. If there any sign of rust on the body end of the battery earth leads that may need sorting.

When you've done all your cleaning, if the problem persists I'd get your replacement alternator checked. As all this started when that was fitted it's a candidate for a cause.

Good news, made it from north wales to North Yorkshire without issue. Will be cleaning the cem and breaking out the multi meter tomorrow.