I have a 2006 Volvo V50 2.4 liter with 103,000. I have been driving my normal route and over the past 2 months I have noticed my gas mileage has gone down. A week ago I did an oil change, new air filter and new spark plugs (Volvo OE plugs from the dealer). Drove the car to the grocery store once and when I came back out to the car it threw a code on start up (P0171).