How could any qualified Volvo dealership mechanics inspect a vehicle and pass it off as ok when its returning only 5mpg?
Their diagnostic equipment should/would have to have picked up fault codes.
Maybe they had you marked as a near future revenue source.
It could be a whole raft of things causing it. O2 sensors especially the forward one, spark plugs, leads, injectors,throttle body or sensor and even air filter elements etc.
Suggest you take her in for a fault scan or borrow an Obd11 code reader to narrow things down first, faultwise.
It may save you paying for related parts/labour unnecessarily as many seem to do from reading past posts.
Check for any signs of a fuel line leak or split in your fuel tank seams too.
Does she smell like she is running very rich or is the exhaust pipe outlet very black and sooty looking.

 

QUOTE (volvorobin @ Jul 29 2013, 12:55 AM) <{POST_SNAPBACK}>Hello
I inherited a Volvo S40 2001 with 50,000 miles on it (I don't think it has turbo). The first thing I did when I got the car was bring it to the volvo dealership for an inspection, which it passed with no found problems (I had thought maybe the coolant boiling out when i opened the radiator cap was a problem, but apparently not.) now that I have it back, I just realized that the gas mileage is terrible - around 5 mpg or less in the city!
does anyone know what the problem might be? the air filter looks clean and the tiers are at the correct pressure. I heard that perhaps it could be the spark plugs or O2 sensors, but the check engine light is not on, and could these factors alone account for such a dramatic reduction in MPG?

Thank you for any insights.
as you have been told you need your codes first,as your car will tell you the trouble,so start there first.