V90 D4 Engine Management Light.

[Brennain1]

I have a 20 plate V90 D4 24,000 mls with an auto gearbox which i've owned for 3 years. When I first bought the used car I had no issues whatsoever. 6 months down the line the Engine Management Light came on and after taking the vehicle into a Volvo dealership they diagnosed the fault and replaced 2 sensors within the exhaust under warranty. Approx 12 months later the Engine Management Light once again appeared but after a quick blast around the M60 to get the vehicle serviced the light disappeared. As I only do short local trips and low mileage in a year the mechanic told me that I needed to take the car for a blast around the motorway, keeping the revs above 1500 RPM using the manual gearbox.
Now I've done this most months but not all, anyway 10 months later and the Engine Management Light is back on again. I've taken the car for a spin around the M60 doing as the mechanic suggested but the light is still there! I don't have a code reader but suspect the fault is once again to do with one of the sensors within the exhaust.
My questions are;
What is likely to be happening and which sensors are likely to be at fault? What do I need to do to prevent this from happening again? I've read about Regen and DPF sensors and filters but basically have no idea what I'm looking at so if somebody could take the time to explain what Regen is and how it applies to me I'd be most grateful. Lastly it would seem that I need to get a code reader so could somebody please recommend one and perhaps even provide a link, also would I need an app for my phone?
Many Thanks.