Very poor fuel efficiency

Your car may stil be under a powertrain or emissions warrantee. Check that out, call the dealer or a Mazda specialist to find out if you car is still covered. If your check engine light comes on, then there's trouble code data stored in the control module which could be accessed by a mechanic with a scanner. You need to find out what codes are stored. I'd suspect that an oxygen sensor is failing but you need to know for sure before you try to fix anything. Here is a directory link for you: http://repairpal.com/mazda-repair-in-san-francisco-california