Hello forum,
I want to write here a solution I have to a n electronic parking brake problem, in the hope that it helps someone else in the future.
TLDR: Parking brake stopped working properly, then at all, due to what I found was a broken wire

The Symptoms:
-Parking Brake Service Needed lights on
-Left parking brake tries unsuccesfully to deploy or retract
-left parking brake refuses to disengage
-"Parking brake not fully retracted" light on
-annoying dinging on while driving

The Discovery:
What I tried first was using VDASH and a mongoose cord to manually retract the brake. It worked somewhat but the car inevitably put the brake back into service mode, eventually with it partially diengaged and the warning light on.
Obviously this was unacceptable. I then researched what would cause it, and a reoccuring theme was corroded or broken wires in the part of the wiring harness going to the EPB. Unfrotunately there were very few solutions which had images or a video to go on, and the wiring diagrams for SPA are not helpful at all. The best video I found was for an XC vehicle, but where the right side EPB had this problem:
There was also a relevant reddit thread without images:
https://www.reddit.com/r/Volvo/comme...ue_how_it_was/

This was helpful, but I have an S90 and the left side was being troublesome, not the right like in the video. In the absence of any useful diagrams I started digging on the car itself, trying to trace the harness back to the nearest plug so I could test the wires to make sure that was the problem and then work from there. It turns out the break in the wire was literally right by the plug at the end as I discovered on day 2 but nonetheless I did learn something here that may be useful for others.

In the process of tracing the harness, I discovered that it goes under the car and over a piece of the subframe. At that point, hidden from view, it joins up with the harness coming from the Haldex controller, which is visible in the middle of the underside of the car. These two meet, then turn left and come back up the side of the car behind the shroud in the left rear wheel well, as per image 4/7. You will be able to see in the image where they pass through a grommet and into the interior of the car. In images 2/7 and 3/7 you will be able to see that the grommet requires you to detach the rear seat cushion (easy, google it), fold the seat down, and then use a flathead screwdriver to remove the leather panel and plastic panel in near the door. There's no proper instruction on this but it's not rocket science. Upon doing so you will note that the obvious plug has the wrong wire colours, as it is the Haldex one. The brake one passes in closer to the door, which I didn't realize at first as I hadn't pulled off the plastic cover.
Unrelated to this but noteworthy, if you experience this failure on an S90 but on the right side, your plug is behind the trunk liner.

The solution:
After having ripped everything apart, I used a chunk of wire jammed in the plug and a cheap multimeter to first check that the circuit was indeed broken, then to confirm which wire was broken. In my case it was the brown one, as the purple had a resistance measurement.
The fix was straighforward, I used a bunch of shrink wrap and re-spliced the wire together then taped it real good to prevent stress on it.

The aftermath:
Having fixed the circuit, my problems weren't over. The car refused to acknowledge I had fixed anything until I fooled around in VDASH a whole bunch. I essentially mashed every option available to me until it exited service mode. There's probably a more professional procedure but I know not what. The options in VDASH were as follows:
-engage parking brake
-release parking brake
-enter service mode (release first!)
-exit service mode
-Initialize
After having done this the car finally stopped being in limp mode, and I was able to use the parking brake normally. Allegedly there is a way to do this without VDASH, VIDA or other tools, but someone else will have to chime in.

The lessons learned:
-Check your wire thoroughly before ripping apart the whole underside of the car
-The Haldex and brake join together then separate, and the wire colours are the same from plug to plug!
-Most of the bolts are 10mm or 7mm. There are some stupid plastic tab things to take off the bottom shroud, I just busted them off because I lost my patience. Torx are T20 if I remember correctly.
-The harness is held in place with stupid plastic zip tie holder things. If you cut the zip tie part off you can use another one. If you have to bust the whole thing off the metal then good luck.


1/7 Wire break right beside the rubber hose shield thing.



2/7 plug inside car beside left rear seat

3/7 better image of the plug inside the car

4/7 path of joint harness as it climbs up into the car. note that the larger harness to the right appears to be for the battery.

5/7 image of brake plug disconnected. brake mechanism to left

6/7 close up of the ABS sensor plug from the back

7/7 a view of the harness as it travels back under the car, where it joins the haldex harness and comes back upwards as per image 4/7

I'm going to dump a bunch of keywords here to help future searchers:

Volvo S90 SPA EPB electronic parking brake broken parking brake service parking brake not disengaged warning light brake control module VDASH repair wire splice part number 32214494 brake harness purple wire brown wire abs sensor

 

I have same issue, but the wirer is repaired and I powered the brake to check, unfortunately this is not only problem. Therefore I want to replace the Brake Control Module, but I can't find where it is, I've removed every panel inside and some underneath, can you tell me where it's located please, I've lost the will to live.

 

I don't know where it is but I suspect it's under the hood somewhere. I know it's not anywhere further rear than the rear seats. DId you test your circuit? Please make sure that's done first, it is also possible the wire completes the circuit but there is enough extra resistance from damage that the BCM is angry at it. Also if your battery is low that may cause it. I'm sorry but I have no additional information right now, I can dig around later this week if I have time.