Never got an answer to: Did this come about after the plugs and filter, or was it there already?

The P0303 only sets a CEL after it occurs a few times, so you may have had a pending code.

 

Sorry the P0303 and 1174 codes were prior to the new plugs. Swapped coil packs still receiving same error codes. The car runs like normal. If the light was not on I would think everything was fine. Should I still do injectors or would it possibly be something worse such as burnt valve. Finances are tight right now. I also need to replace brakes and rotors and windshield to pass state inspection.

 

Have you looked at the flex pipe? Have you done any troubleshooting or, just parts shooting?

Try switching the injectors around.

Could be a vacuum leak (including the EVAP solenoid and/or valve), could be a busted flex pipe, could be a bad #1 HO2 sensor, could be a bad #2 HO2 sensor, could be a bad MAF sensor, could be a bad MAP sensor.
Probably a burnt valve or bad head gasket. Do compression and leak down tests.