I recently bought a 2001 S40 with 88,000 miles.

​​​​​There are some issues I'd like to correct and I'm looking for advice. In the past week I've had a transmission fault. Blinking yellow arrow on the dash. The first time it went away after a reset and it did it again this weekend when my wife was driving it. I've read that this is usually caused by the transmission selector switch. As recommended, I switched from Park to 1st about 20 times and the fault went away. Seems to confirm the switch. I've seen some replace it while others say that a good cleaning is all that's necessary. Any experience with this issue?
The drivers seat foam has become hard as a rock. Any cheaper fixes other than taking it in to an upholstery shop?

My commute has me driving 75 mph back and forth from home to work. I'm feeling a vibration between 65 and 75. It's hard to tell if it's suspension or the wheels. My first thought is to have the tires rotated and balanced. Anyone know of suspension issues with the S40?
The emergency flasher switch is doing the thing where almost daily I have to tap it to get my turn signals back. Any other fixes beyond spending $75 for that switch? The turn signals don't return to the off position once finished with the turn. Looks like I would have to replace that switch if it bothers me enough.
The last issue is an oil leak. As soon as I come to a stop I can smell burning oil I'm pretty sure it's coming from the back of the block. I little research has led me to think I should be looking at the turbo draw tube. I looked on YouTube and I don't find any videos for that repair. Is that a hard to access part? Is there anywhere I can go to for more info on that repair

Thanks for any and all advice.
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my experience with our 2000 S40 - The shifter row is a good idea and tends to work for a while. You can also remove the TCU, clean the contacts and reseat as that is another possible cause with a DIY fix. For the wheel vibration, it can be a tire, wheel or axle so I'd start by doing a front/rear rotation then consider getting the tires balanced. Also inspect the axle boots while the wheels are off. The other thing to inspect is the coil springs - look for the end piece breaking off about 3 inches worth. I've replaced 3 of 4 so far. The flasher is controlled by a relay and you may need to replace that or it could be the multi-function stalk. I'd start by replacing the relay. Shop a Volvo dealer store or a Volvo friendly store like Tasca Parts (dealer), FCP or IPD USA to get OEM part numbers.

I've never had to replace the turbo oil return line seal on the S40 but I have done that on an 850T which is a common failure. Its two 10mm bolts and a $1 gasket but you probably need to reach it from below. If the S40 is like the 850, there's also an O ring seal on the other oil line where it goes to the block - Check Youtube for vids on more specifics and check dealer web stores for diagrams. etc.