1994 Subary Legacy. Non-turbo. 4dr at sedan. 240,000 or so miles (might be 280,000). Burns a bit of oil, but maybe one quart between oil changes at the most.
Today it failed emissions, hydrocarbon too high. It has new plugs, new air filter, new fuel filter. Plug wires look perfect, coil is clean. Runs perfectly, no misfire that I'm aware of. Check engine light does not come on. Fuel mileage is the same as always. I haven't checked compression, but it turns over evenly fwiw.
What else is there that can go wrong to cause this? I hate to start throwing parts at it, but I'm thinking O2 sensor next, and then what? Any kind soul here have any experience with these older cars and emissions and know the things most likely to cause this?