In my OBD-II cars, none of which are Volvos, I use a ScanGauge that's semi-permanently mounted and stays plugged into my OBD port full time except for if someone else wants to plug in. I use a product called "Blend Mount" that mounts it cleanly.

I can scan on the fly, get my codes, and clear them. No charge, no trip to the auto parts store, no app required, no phone required, and it adds all kinds of gauges that aren't on my cluster as well as fuel economy, etc.

They're not for everyone, but I love mine.
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A bottle of oil and some basic tools in the trunk. The highway is calling. Always remember the difference between durability and reliability. Embrace the old Volvo in your driveway. She's eager to please and, with a little TLC (and a few headaches), sh

Evan R certainly has the cost-effective suggestion.

I got a scan tool from IPD 2-3 years ago...see here:

http://www.ipdusa.com/products/5742/114877-foxwell-nt-300-diagnostic-scan-tool

It came with very comprehensive documentation and has worked fine on my 1998 S90, on my son's 2001 XC70 (found a couple of codes resulting in a cheap repair), and a friend's Audi TT. About $80 now, $140 when I bought it!

Nice thing is the reasonably large and clear display which is great if you want to glance at operating data while driving.

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Bob: Son's XC70, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles

Honestly, the solution is no longer a dedicated scan tool.

Get any Android device that has Bluetooth. I use an old, deactivated cell phone. Cost = $0.00

Get an ELM827 OBDII -> Bluetooth adapter. Cost = $10-$20

Download and install the Torque app from the Google Play store. The free version works perfectly well, or spring the $4.99 for the paid version.

(note: the free version has ads. Or it should. Since your decommissioned cell phone can't download the ads, you'll never see them!)

For <$25.00 you'll have 98% of the capability of any scan tool you'd need.