S60 Emission system service required and car cuts out
 

I bought a 2001 S60 T S yesterday, drove it home for 95 miles, everything was fine. This morning I got in the car, I parked at the shop, when I drove off from the shop I noticed the emission system service required warning with the engine light on. The car was still working fine but then later when I used the car it was a bit shaky and didn't have as much power in the engine. Got worse when I pulled out at a junction and the car died, the warning triangle in the middle came on and said performance reduced. When I come to a stop the revs drop to around 500 rpm and the engine revs its self to keep its self going but eventually stalls.

I have seen a few people say oxygen sensor but all of the posts I've seen are saying their car is working fine.

Does anyone have an idea of how this has happened and what the likely fix will be? I want to know how it was fine when I took it on a test drive and handed over the cash and got it all the way home no problems but now I have a dead car. Is it something the seller could have temporarily fixed to be able to sell me the car?

Thanks in advance

I saw something about the mass air flow sensor and to just unplug it to diagnose it. I just unplugged it and it improves the situation a great deal, almost wouldn't notice there was a problem but the car idles at just over 1000 rpm and still sounds a bit rough.

What is the mass air flow sensors job? Now I've unplugged it what job is it not doing that it was before? The reason I ask is because I would think I need a new mass air flow sensor but could it be something the sensor was picking up that was wrong with the emmisions system thats the problem?

Also after turning off the engine the fan hasn't turned off, the car was only turned on for 5 minutes.