I have changed the spark plugs and used AC Delco as recommended, Changed the Boots, used lucas in the fuel, changed the oil. I have 116K on my 2007 hhr 2.2. Light will not go off. When I had it checked it still has P0303. Help...

 

Switch the injectors around see if that's the problem. Just relieve the fuel pressure at the Schrader valve on the fuel rail, then un bolt the rail. The injectors come right out of their o-ring mounts, be sure to also transfer the wiring (that matters).
Lucas ain't gonna "fix" nothin', it might keep trouble away if used regularly.

 

If that does not work, switch coils and see if the misfire moves... if it does you have a bad coil.

 

I know this is obvious... Have you turned off the light with a scanner? Other wise it will have to go through many drive cycles with out a fault before it will turn itself off.
Good Luck

 

did you check the plug gaps? They aren't always correct

 

now it is flashing.. When I am at an idel it flashes. on the interstate at a constant speed it is a solid light. Really think it is the coil.

 

Did you try any of the suggestions above? If you haven't, continuing to drive it could lead to other problems.Already sounds like the CEL flashing/steady is telling you to do something.

 

P0303 is misfire cyl #3. Focus your attention there (pull the plug check the gap, is it fouled? etc). Then look at the coil pack for cyl #3.