I found a quick fix for a DTC P0303 (misfire) code on a 2002 Honda Civic LX (D17A1) with 200k miles! :-)

The car had a rough idle, hard starting & poor gas mileage. Then, a DTC P0303 code set (misfire cyl #3) and the MIL light came on. The misfire eventually got so bad the MIL light started FLASHING (cat converter destruction in-process)!
I had recently installed 4 CHEAP "remanufactured" fuel injectors from eBay (GTH Motors).

I first tried moving the ignition coil (per the Honda manual), to see if the DTC code would follow. I cleared the code, ran the engine and got the same code (P0303) set within a few minutes, so I concluded that the ignition coil was not causing the misfire.

I then moved the fuel injector from cylinder 3 position to the cylinder 2 position, cleared the code, and ran the engine and had a code set for misfire on the #2 (P0302)! :-)

I then replaced the bad injector and VICTORY! The engine now purrs like a kitten again, has more power, better starting, gas mileage back up to 34 MPGs, no more codes or MIL lights.
MISSION ACCOMPLISHED. :-)

LESSONS LEARNED: (1) Rough idle, hard starting & poor gas mileage problems can be caused by misfire from worn out/clogged fuel injectors. (2) You must be very careful with quality when buying cheap "remanufactured" fuel injectors on eBay.