[Closed] Where to start when fixing auto electrics?

Private sale? If it was a trade / dealer, send it back for them to sort.

for these sorts of things nowadays I would find an owners forum where people fix things.  It is unlikely your problem is the first time it has appeared and that can save a lot of debugging

e.g. Volvo Maintenance and Repairs | Volvo Forums

but I'm sure there are others

1. The motor for passenger-side wing mirror works, but it doesn’t actually move the mirror.

Sounds more like an actuator in the mirror has been dislodged.

Might be as simple as popping the glass out and re attaching something. Doesn't sound electrical if you can hear it working.

2. The rear parking sensor worked for a couple of times when I first got the vehicle, but now it just emits a single, prolonged beep when I first go into reverse, then doesn’t actually go on later when I am parking and (ostensibly) need it.

Not something daft like a towbar being fitted? Or if it has a removable one do you have to tell the computer if it's there or not and therefore whether to turn the sensors off?

for these sorts of things nowadays I would find an owners forum where people fix things. It is unlikely your problem is the first time it has appeared and that can save a lot of debugging

+1

Haynes manuals are good, but I've found on my last couple of cars they've had glaring errors or omissions.

Like you can't take the brake pedal out of a UK Berlingo without lifting the engine out first, regardless of what the manual says. It's obviously been written with a LHD car and then someone's added the RHD photos but not checked the content.

YouTube is an absolute gem for so many things like that - along with owners forums and google can save a lot of time out in the cold, research research.
For the general auto electrics the basic code reader , multimeter , and a bit of caution will help , unfortunately the basic code tools can only do so much before more investment is needed.

Ignore the issues until something terminal occurs and feel glad you didn't waste any time fixing the niggles?

Best place to start is always the earth/negative connection at any point. Make sure the connection is good, then look at other stuff.

If the mirror motor works, then it's not electrical.

Parking sensor fault will likely be a faulty sensor, which will really need a computer to tell you which one, unless you want to potentially spend hours removing the bumper, swapping sensors, then refitting to see if you've swapped the faulty one, or there is more than one faulty, or the fault is something totally separate, in which case you've just wasted hours swapping things.

Parking sensors can be checked for operation by listening to them - they emit a rapid clicking sound when working correctly. It's best to use a stick between the sensor and your ear or use a mechanics stethoscope like what I do.

A faulty sensor will be silent or be irregular in its sound.

Wing mirror is just popped off the cup/glass holder normally 3 tabs might just need a push on the glass.
Reverse sensor could just dirty it's caught me out a few time

Which XC90 is it? I presume the older Gothenburg model rather than the newer Geely XC?

If it's the original then you need a Vida/DICE clone - basically a dongle that sits between an XP (VM) laptop and the ODB port of the car for dealer level diagnostics. You can get Volvo specific code readers but they don't go as deep as Vida/Dice.
These cars are gen1 Canbus and really quite complex but to be honest you won't have time to fix the parking sensors and wing mirror once the mechanical issues start!
Change the timing belt/water pump, auxiliary belt and alternator pulley if they haven't been done recently.
Read up on flushing the (presumed) autobox - don't rush it into it needs special oil.
Check right up in the front suspension turrets for rust and get ready to change the wishbones.
Beautiful cars though.

Bring it round here....?

Oh molgrips 🙂 you da man

“ If it was a trade / dealer, send it back for them to sort.”

Waaaaahhhhhh 😰😰😰😰 I bought a second hand car and it SHOULD be the same as a new one sob sob.

The mirror is likely to be an issue with the glass of some sort, come off the mount, glass come away from backing, aftermarket incorrect glass fitted etc. if motor works it's very unlikely to be electrical.
Parking sensors - ignition on, engine off, put the car in reverse, go to rear of car and put your ear very close to each sensor, you should hear a ticking noise, if not that sensor is faulty (check connector first, new/used ebay sensor second, diagnostics 3rd), if no sensor makes a noise you have a greater problem and need diagnostics.

Waaaaahhhhhh 😰😰😰😰 I bought a second hand car and it SHOULD be the same as a new one sob sob.

Not sure how you got there from stuff not being in the described condition (ie not declared as non working) when sold. But crack on...

To add onto what @RustyNissanPrairie said, wishbones are common, dont mess with the bushes. Just do the arms. Rear trailing arm bushes are also common, as are the rear subframe bushes. Like also mentioned above, electrical gremlins on these can be an absolute git with them running gen1 canbus. Mines had a good few, but fortunately no major bust ups. They're great bike lugging cars. Did you opt for a diesel or petrol model?

When you say the motor for the mirror works, do you mean the glass adjuster or the fold in/out function?

I would very much recommend the Volvo Owners Club forum - you don’t have to be an OC member to join. They are very very helpful and knowledgable in the section I use the most.

https://www.volvoforums.org.uk/

An OBD reader, a multimeter and a soldering iron and a logical mind can sort out so many electrical issues.

I've got VIDA if you're anywhere near Fort William (unlikely!)

Rear sensors may just be dirty. You can turn them off - easy fix.

I'd just youtube dismantling the door mirror and take it apart for a look. A bit of courage often pays off.