I have a 2000 Jeep Wrangler that started making a knocking noise. I have heard this noise on a different vehicle a while back so I assumed oh great a bad spark plug and I was close to work so I let it sit there for the day. A friend had a diagnostic reader in her car so she hooked it up and it gave a reading of P0306 Misfired defected. I thought I would be ok driving it home but I was so wrong. It died on the way home and it seems like the battery is completely drained. Again that is just an assumption. I am lost on where to turn to..any ideas?