I have a 2002 impreza 2.5 TS that was purchased at the beginning of this year. Check engine codes have come on and off over the last 9 months and the engine has progressively shown less power over time. I thought it was misfiring, because whenever I put any load on the engine it would chug like it was misfiring. Spark if good, coil pack is good, spark plugs are good. The chugging became worse and worse over time to the point where it would chug even with a small increase in speed or hill, and eventually one day just quit on the road and hasn't been able to start up since, (unless I give it a lot of gas and it was attempt to stay idle erratically). I put injector cleaner in the gas tank while it was still running, no improvement. The fuel pump noise come on when I turn the key to the on position. 

Where should I go from here? Pull the injectors and replace? Might it be something else?

 

Codes:

P0420: catalyst system efficiency below threshold

P1507: idle air control system malfunction

P0456: Evaporative system leak detected (very small leak)

 

 

How many miles? What is the service history?

112k. Recently purchased, don't know much of the history, should probably get the timing belt changed.

if it has never been done, the timing belt system is 5 years overdue. The schedule calls for 105K miles OR 105 months.

 

definitely check for slipped timing. Even if it was done previously, it may not have been done with proper attention to the idlers and tensioner. The toothed idler especially is prone to failure.

 

I wouldn't rule out the possibility of having multiple problems based on those trouble codes either.

I was wondering if it might have slipped timing!

On my 02 legacy, the only time that it started to miss at normal speed, the ignition wires were soaked with oil.  There are probably many things that could cause the problem, but that is one thing to check.  Of course pull the spark plugs and gap them if they look OK, although its an unlikely sourse of the problem. The oil on the ignition wires comes from the tube gaskets which are part of the valve cover gasket pack.