I have a similar problem with my 2004 Wrx. It seems to be firing on only 2 cylinders on the passenger side and I'm getting nothing from the drivers side. I purchased it from the auction and it came with no ecu nor air intake box. I put a used ecu from ebay in the vehicle and an AEM air intake. The car has a full NUR SPEC exhaust and AEM air intake and that's about it.
I had the heads done over by a machine shop as I got the car with a busted timing belt and so this resulted in bent valves. I changed all engine seals, valves, head gaskets, spark plugs, timing belt. crank sensor, map sensor, cam sensor (all brand new parts). The car is dumping a lot of fuel in the oil and so I haven't been driving it, the plugs are filled with gas as well. I also discovered a vacuum leak as the previous owner had a boost gauge, took it out and left a line hanging, I fixed that issue and everything seems to be grounded correctly. What should I do I am sooo fedup and nobody can seem to figure out what the problem is. I did a diagnostic, at first it told me i needed to change cam sensor, crank sensor and map sensor. I changed all of them, now I get a code P0171B-(bank 1 system too lean) and also P0341(camshaft position sensor "A" Circuit Range Performance B1) PLEEAASEE PEOPLE I NEED YOUR HELP BADLY I"M ABOUT TO TOSS THIS CAR. I just really want this problem resolved.