I don't know if it would throw a code (it usually doesn't), but possibly the engine temp sensor (not the gage/light sender) could be defective. If the sensor reads as "always cold", the mixture would be rich, hot >> lean.
Scientific Wild-Assed Guess...
I've had that happen to another subie years ago: Not sure if that still holds true with the newer models but certainly the 93 Impreza became very hard to start, when that sensor went bad.
Another WAG, are you losing coolant? I think it can quickly destroy the front O2 sensor.
Carl
Good thought, Carl, but nope, no problems other than the p0420 code.
Load up the tank with known 0% etanol fuel, and 99.9% sure the p0420 will go away. It is a calibration issue. On Hondas it is a problem and there is a TSB that addresses it - a reflash of the PCM to accept E10 without triggering the CEL.
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