MRossmiller

I am posting this for those of you that may be experiencing the P301, P302, P303 and P304 codes - I had the same problem with my 97 Subaru Impreza. After replacing the plugs (needed) and plug wires, knock sensor, and fuel filter yes the car seemed to run fine - but there was a more serious problem. Having recieved all four cylinders showing misfires, I intuitively thought the problem would have to be with a central function of the ignition system - not a single cylinder problem. Turns out that the crankshaft alternator belt pulley bolt had loosened causing the pulley to dislodge from the crankshaft and timing sprocket. This caused the pulley and sprocket to walk - damaging the crankshaft, sprocket and pulley when the shaft key wore away. The choice now is an expensive replacement of the engine crankshaft (hopefully no piston or valve damaged) or replace the engine. My suggestion to all concerned it to have your timing belt, crankshaft pulley and timing sprocket checked ASAP - if this is the probelm it would most certainly result in the four cylinder misfire codes P301, P302, P303 and P304. Good luck.

I am posting this for those of you that may be experiencing the P301, P302, P303 and P304 codes - I had the same problem with my 97 Subaru Impreza. After replacing the plugs (needed) and plug wires, knock sensor, and fuel filter yes the car seemed to run fine - but there was a more serious problem. Having recieved all four cylinders showing misfires, I intuitively thought the problem would have to be with a central function of the ignition system - not a single cylinder problem. Turns out that the crankshaft alternator belt pulley bolt had loosened causing the pulley to dislodge from the crankshaft and timing sprocket. This caused the pulley and sprocket to walk - damaging the crankshaft, sprocket and pulley when the shaft key wore away. The choice now is an expensive replacement of the engine crankshaft (hopefully no piston or valve damaged) or replace the engine. My suggestion to all concerned it to have your timing belt, crankshaft pulley and timing sprocket checked ASAP - if this is the probelm it would most certainly result in the four cylinder misfire codes P301, P302, P303 and P304. Good luck.