Anyway, as per the subject, I have a 2007 C4 1.6hdi SX which was left for 2wks in May when we went to Croatia. Came back, started the car and all bells & bongs going doolally.
I eventually traced it to a damp fuse box (I presume it is also the BSi). The tree's had infiltrated and there was a mess of tree junk and every fuse was covered in furry corrosion. Took me an hour to remove it, clean the blades, spray switch cleaner into the fuse holders and poke each fuse in/out a dozen times to clean up the slots. Then I blew it dry with an air nozzle.
Reinstalled it and all the problems went away
Fast forward to mid July, wife went to visit her family for 2.4 weeks and when she returned, same exact thing! I never used her car as I had my own and/or the motorbike to use. Although I have to admit, her car is great to drive and I have always loved Citroen. Not to mention I advised her to get this car and in the almost 7 years of ownership it hasn't put a foot wrong until this year. It gets regular services and never pushed hard....unless I'm driving it
So I was advised by Citroen that the BSi got wet so it must need a new one, we got a used one that was previously working. And yes, all the barcodes & numbers match.
The issue seems to be no comms with the ABS module. Funny how it was perfect until both times it sat for 2wks before going faulty. The rest of the time it is used for a 4 mile round trip each day and every other Saturday. It does a couple of longer trips to the shops twice weekly or so.
It sits at the end of a building and is reasonably protected from the weather except when at her work when it sits under tree's (although it was drummed into her to cease and desist or park it 180 degree's as in reverse in).
So, I stripped the whole wiring under the bonnet, removed ECU's, relay banks and fuse boards. Nothing was left unchecked and everything came back 100% perfect. No lumps, bumps, burns, breaks or chafed.
There is continuity on every circuit I tested and 12v going to/from each pin on the ABS pump plug. It had been greased at some point in its life so the pins were all clean and not green. An ex Citroen mechanic told me this model had been subject to a recall not long after launch to address corroded pins at the module, so I assume the fix was a dollop grease.
The warnings are no ABS, ESP, Cruise, Gearbox Failure, etc. It's the auto clutch model so it doesn't change automatically, but manually only goes up to 3rd gear. It will change down reluctantly while braking from 3rd, but will not change up unless using the paddle or gearstick.
The lights pertain to ABS ESP and SERVICE.
Battery was checked and holds volts around 12.6v at rest and engine running 14.8v average under charge conditions. No fluctuations.
The only other issue apart from no comms is a fault code for emissions which would appear to be one or more glow(s) need changed. The only issue is slightly lumpy idle and smelly exhaust. I would fix it, but we need the ABS issue addressed first.
However, knowing can bus systems are voltage critical, is it at all possible that either the glow plug issue has somehow affected the operation of the ABS circuit (seems unlikely, but....) or could it be something as simple as a faulty brake switch preventing communication with the module?
The brake pedal must be pressed before the starter will engage and this operates perfectly as do the brake lights. I realise they operate using resistance so am open to this suggestion, but without a fault code we are stumbling in the dark. All other fault codes forced or otherwise were able to be erased.
I beg your wisdom people, wife going nuts as she's driving a faulty Smart For Four (it conks out now and again, but at least it runs) and wants her beloved Citroen back
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Evil dad 
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