I bought my 2004 (early) S40 in January of this year (2014). The average temps have been cold with many nights below zero F.
My gas gauge 'froze' one time and as it worked fine, I figured it was some water in the gas. Mind you the car runs good but I was in for a surprise!
I added Valvoline "Nitro Shot" mainly for H2O removal and it touts some other benefits but this is not an endorsement for Valvoline. Next day the fuel gauge worked! But.... later in the day I tried to start the car and straight away it failed to 'catch'. Second try it cranked many times then started but kinda rough, that cleared up in short order.
So on my way to the store the check engine light came on.... Now what? I have a friend that works at O'Reily Auto and for free they have an OBDII tool. Connecting it, the tool showed three faults, two for engine misfire and one for a temp sensor. After telling the water in gas story we both come to figure the Check light was on because of these, so he cleared the codes.
Well! Took a ride with the light out and the little 1.9 became a little beast and it has never ran so well! We both figure the codes set the computer into a 'limp' mode.... clearing them unleashed the beast in my little Volvo!

poikaa