That's related to the front O2 sensor. Have you done any mods lately? Pulled the exhaust manifold? I'd check the wiring at both the sensor end and the connector end in the engine bay.
Heard bad thing about cheap ones....don't want to go cheap on the front o2 sensor. That is what the ECU uses to control fueling. WJM seems to think that the aftermarket ones are not calibrated correctly for the Subarus. A OEM one would probably be best.
Don't mod your car, and it won't kill you. Thats my opinion though, and is kind of hard to tell a wrx/sti owner that... :rofl:
did you pull the motor to change the clutch? If so, I bet the sensor or the wiring got smashed when the motor was coming up and out. It looks expensive.... Clickity Clack