Gearbox faulty, rear suspension faulty, depolution faulty - 2008 GrandPicasso 2.0HDI EGS semi

[ixus400]

My car is 2008 Grand Picasso 2.0HDI semi auto gear (EGS) Exclusive.

I recently have rear suspension faulty and depolution faulty warning happens. I went to Citroen garage for checking, then I got some extra things fixed: front tires changed, battery changed, wiper changed...
But funny things are: the rear suspension faulty is still there, the depolution faulty still there, more than that: I got new issue: 'Gearbox faulty' when I put lever in 'R' position, and gearbox refused to engage. I drove to the garage again, they spent another day, showed me the evidence of some broken rusty wires and say the problem should be solved.
Indeed for the 5 km driving home, all warnings were gone. But it only last for one night...
Now all three issues are still there.

I was so tired on endless money spending on fake repairing, so I now let my car stay home, and decided to learn and try to figure out by myself.
I'm new on car repairing, and I never put my car in non Citroen garage for any kind of things -repairing/maintenance...

I need help for:
the books like service workshop manuals which have detailed electric circuit/diagram/schematics
car diagnostic scanner suggestion (I bought one Bluetooth interface, 14euros, will receive it on next week, not sure it would help)
any suggestions for a newbie like me, ....

[ixus400]

Now after several days learning, I finally solved the two issues: Gearbox faulty and Depolution system faulty. What I did is: take off all the fuses, check them ok, clean them all with contact cleaner and put them back. I hope this solution is not short time, not like the garage, which only last one day. I should drive it for several days and update here again.

But there is one error code: P0561, wchich says the voltage is un-stable.

I read the OBD voltage, it gives 14.4v at idle, 14.8v at 2000rpm. is it normal?
Thank you very much in advance for your helpful replies.