My engine light came on so i hooked up my OBDII scanner & it said cylinder 3 midfire so i changed the spark plugs & now if i gas my car a little or dont let it warm up after starting for 5-10 mins the engine light comes on again. Why?

 

Thats because you did not find what actually caused the DTC and replaced the spark plugs before doing so. A DTC P0303 can be caused from many reasons, actually rarely the spark plugs.

 

whats the worse case scenario for P0303? ie: expense &/or complications of repairing?

 

Worse case scenario you need a new engine.

Could be faulty spark plug, faulty ignition coil, faulty fuel injector, faulty wiring, faulty PCM, carbon build up on valves.