I have a 2004 WRX (completely stock) with automatic transmission with around 130k miles on it. A few weeks ago, my MPG dropped by about 20% from around 260 miles per tank to around 200 miles per tank. Driving patterns haven't changed, and I checked all the brakes and caliper pistons and all seem to be retracting just fine. There's no difference in how the car feels when it drives and no CEL has come on.
My next suspicion was that the O2 sensors might be shot, causing a rich fuel mixture to burn gas faster, so I got out my trusty OBD2 scanner software and plotted each of the o2 sensors which generally operated within the following voltage ranges:
O2 B1-S1 (WR V): 3.7 Volts +/- 0.2 Volts
O2 B1-S2: Oscillates between 0.1 - 0.8 Volts
My questions are as follows:
Are these voltages (especially the B1-S1 sensor which didn't seem to oscillate like the B1-S2 sensor) within acceptable limits?
If so, does anyone have any other ideas what could cause such a sudden and continuous drop in fuel efficiency?