DS3 Performance 210 intermittent start fault

[Vern]

I have a 2018 Ds3 Performance 210, that has an intermittent staring issue.

To give a bit of history, this might be relevant… last week I changed the coolant and gearbox oil, to do this removed the battery and housing etc for access. On completion the test drive was fine no issues, only odd thing is the start stop actually worked, the first time this has worked in the 12 months I have owned the car! The next day I drove to Leicester and back about 5 hours in total, after 20 mins a fleeting warning to fix engine and the service light illuminated, the car ran fine apart from the warning lights. The next day after using the car it wouldn’t start with the service warning light on, if left long enough the service light would clear and the car will start multiple times.
I have read the codes with diagbox, I have tried clearing the codes and they will not clear, unless the service warning light is clear, which correlates with when car will/ won’t start.the codes are below.
P13C100 Starter control interface
U121881 Communication with BSI

To me this points to the starter control interface, but I have no idea what it or where it is? Is it part of the start/stop module on top of the battery. If it’s not this what else do the codes point to?

[Vern]

Spoke to main dealer, they had no idea what the starter control interface is. Does anybody have a wiring diagram, so can chase through the wiring?

[GiveMeABreak]

It's to do with the Engine START button (assuming you have keyless entry / starting). There's a fault with some part of this system if not the button.

[Vern]

Hi, I don’t have a starter button, just a bog standard key with remote locking.

[GiveMeABreak]

So in that case it must be in the starter circuit / BSI and not something that we can sort on a forum really. It needs some further electrical investigation work by an auto electrician.

[RichardW]

I would check for loose connections around the battery if you have disturbed them.

[Vern]

Thanks for replies
I took all the battery and housing out to investigate anything obvious, I have done this previously with no change to the fault, this time I found that the plug that connects to the starter relay was loose and a previous owner/ mechanic has broken the locking tabs of the plug, so have used a cable tie to prevent movement. After putting it back together all faults have cleared and have been out for 2 runs of 30mins, and no sign of the fault, so .
So it was either a loose plug connector around the battery/ engine ecu area, or it was the plug on the starter that was causing the issue. I will report back in a couple of weeks….