Hello XXXXXXXX- Any time you are dealing with a fuel economy issue, and the check engine light is on, you need to address the light first as the check engine light indicates a problem with the on board computer system-which controls the fuel delivery.
Most shops should be able to retrieve the codes it is fairly simple. Her are instructions. By the battery there is a duty check data link connector.
Connect a jumper wire between terminals 2 and 3 (throttle body injected vehicles) or between 2 and 4 (mulitport injected vehicles). Then turn the key on and count the flashes of the check engine light. This is the code number.
The oxygen sensor is a common failure that would effect fuel economy.
Technician
23 years import and domestic auto repair. Specialize in no-start, check engine light, driveability.