Could I have an injector problem?

2001 MAZDA MILLENIA
15,500 MILES
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POPPABRAND
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The engine codes read P0140 and P0303.
I have already replaced the plugs, wires, distributor and rotor. Additionally all the back plugs are black (1, 3 & 5 with #3 the worst). The engine runs rough and has a definite backfire. Could this possibly be an injector? Or could the o2 sensor cause such an obvious misfire?
Jan 15, 2012 at 8:30 PM
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