Im driving a stock wrx setup, getting 200 miles per tank. I'm guessing since my CEL light came on that my o2 sensor is faulty. Is there any other possible cause of this sudden drop of gas mileage?

 

When the CEL comes on for many faults, the ECU diverts to a "failsafe" mode and calculates fuel and ignition timing based upon a predetermined volumetric efficiency table that will keep your car running safely as long as it has enough input. This means that the data that is collected from various sensors/meters do not all have to be functioning properly for your car to run. It simply isn't as efficient. It works kinda like algebra. The ECU can take the reading of only a few sensors to make your vehicle run.

Read the code before assuming anything...

 

alright thanks!