Home › Forums › Stay Dirty Lounge › Service and Repair Questions Answered Here › 2000 Mercury Grand Marquis, check engine code 304. › 2000 Mercury Grand Marquis, check engine code 304. September 24, 2014 at 3:47 am #622890 IngvarParticipant Remove ignition pack off # 4 and swap it out with any other easy access cylinder. If code moved with swap to that cylinder, say, you now have P036, it’s ignition pack. That easy to diagnose.