I made a seperate post about a rough idle. This should definitely fall under a new thread because some of the answers led me to suspect that while the car is most definitely a 2002, the ECU may not be. A friend told me the engine in my car is a 2003... It threw a P0171 code, and all my research says this shouldn't be possible on a 2002 forester. My wonder stems from the fact that I just replaced the O2 sensor, but with a 2002 sensor which could be causing the misfires in 3 and 4, the carbon on the plugs, and the terrible mileage. Here is a picture of the ECU. Before I replace the sensor again.. can someone tell me if this is an 03 or an 02 sensor?