2000 Regal LS-P0305 code, SES indicator.

[STONEBOSS1]

Hi to all on this wonderful forum. Been experiencing rough idling with my car lately. Took it to Advanced Auto for a scan of codes and the only code showing up is a P0305 code. The gentleman at AAP explained to me that it points to a misfire in cylinder #5. Possible causes are a faulty spark plug, or a worn plug wire or bad coil. Upon checking the spark plug and plug wire, I found both to be ok. On open road driving the car rides fine. It idles rough only when I'm at a full stop at a red light or in park. My car has 157k miles. It was tuned up 2yrs ago...replaced wires Bosch 7mm, AC delco iridium plugs...what a PITA that was especially the rear plugs and wires. Wondering if its the coils or a faulty fuel injector. Also wondering why at times the SES indicator stays on steadily and at times it flashes. Any suggestions to remedy this issue would be helpfull.

[GTP Dad]

Welcome to AF!!

I wouldn't count on the fact that the car was tuned up two years ago to satisfy the issue. You may have a cracked insulator on the plug in #5 or the wire in fact could be bad. The coils can and often do go bad but when that happens you usually see a misfire on the other plug as well. I know it is difficult but I would pull the plug and check it. This can also tell you if the cylinder is getting fuel. Both too much or not enough given the color of the plug. If everything is alright with the plug then you may indeed have an issue with the injector. You can check it with a noid light. They sell them at Advance or Autozone. This will tell you if the injectors are working right. If you have do work on your car yourself and have basic mechanical knowledge you can also remove the injector and swap it with one of the front injectors. If the misfire moves to that cylinder then replace it with a new one and the issue has been resolved. The reason to move it to the front is because they are much easier to get to than the rear injectors and would be easier to change if you must.

As for the steady or flashing light, on GM cars a misfire is often shown by a flashing light followed by a steady light indicating that there is an issue.