Max , the MAF will usually stop the car from starting when its faulty. If so , unplug it ( its the pipe out of the airbox , the unit is just there with a spring clip to release and pull. Let it sit there , tuck it in there somewhere , try starting the car and it should go into default mode. Do this only if it wont start.
As Philia says , its just there from an earlier event most likely.
So , probably easier for you to use this
http://www.ebay.com.au/itm/Small-U480-Memo-Scanner-CAN-OBDII-EOBDII-Car-Diagnostic-Tools-Automotive-WN-/322439532836?hash=item4b12e4d124:g:iTQAAOSwa~BYdoIW
tool to plug in as i gather you aint a techno person with phone apps etc etc.
Its easy to plug into the socket under a cover that hides under the lid of the centre console. Turn key to ACC and get it to scan for codes , clear them and see how it goes before you go to O2 sensors,CTS (coolant temp sensor) etc.
I think you should check the obvious , plugs,( Volvo brand) ,distributor cap and rotor and has it got 'Bougicord' brand ign leads?
Other random faults can be the fuel filter , the vapour hose on top of the fuel tank and some others but sometimes it can be that the PCV system is blocked contributing to the poor running and fuel use. It all ads up and something you could look at are all the little rubber vacuum lines , that they are all connected and in good nick , they are usually degraded by this time so worth replacing anyway. You can get sets specific to these cars in silicone hose.
How does the car warm up? Is it very slow to warm up? This can indicate the thermostat is bung.
Many things at this age , irregardless of its low mileage , they should be replaced or checked.