Sometime on the way home from the Indy NRA Annual Meet, my car's "Check Engine" light flagged on.
It didn't run badly, so I decided to check it when I arrived home.
The light remained on, and unchecked, until the weekend when the vehicle started running very badly. The engine behaved like it was misfiring on at least one cylinder. When I got the Diagnostic Trouble Codes scanned, they were "P0301: Misfire on Cyl. 1" and "P0420: Catalytic Converter Not Operating Efficiently".
There are lots of things that can cause the second code on my Subaru, but the combination of the two looked like an electrical problem in the ignition. The spark wasn't always happening when needed, so the system was feeding unburnt fuel into the exhaust. This unburnt fuel generated readings consistent with Catalytic Converter trouble.
So I replaced the spark plugs, spark-plug-wires, and ignition-coil. The engine ran fine; until a month later.
This time, the misfire(s) hurt engine performance even more. The new trouble codes read "P0303/P0304: Misfire on Cyl.3/4" and "P0171: Lean fuel/air mix".
Same problem, but different cause. The diagnosis was either a weak fuel pump, a failing sensor in the controls for fuel injection, failing fuel injectors on the cylinders, or a vacuum leak.
One factor pointing towards sensor-trouble is that the engine starts fine, but runs rough after 30 seconds or so. If the problem were injectors or fuel pump, this behavior would not be seen. However, the way that the Engine Control Unit handles engine warm-up would lead to this behavior, if one (or more) sensor were bad.
There may still be a vacuum leak somewhere in the system. However, testing (by spraying starter-fluid onto the hoses in question while the engine was running) failed to produce any sign of that.
Another test was for engine behavior after unplugging the front O2 sensor. This also didn't fix the problem, but narrowed the focus down to the MAP sensor. (Most Subarus have an MAF sensor in the air-filter passage. The model I own uses an MAP sensor on the intake manifold...which is harder to clean, but cheaper to replace.)
Now a replacement MAP sensor is on its way. Hopefully, it will fix the problem.