Q: Q: Cylinder 3 misfire

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I have a 2003 Pontiac grand am with a little over 152,000 miles. I've recently got a p303 cylinder 3 misfire code as well as a code for insufficient egr flow. We replaced the egr valve, spark plugs and wires. The insufficient egr flow code disappeared but the check engine still read cylinder 3 misfire. So we replaced the coil pack for cylinder 3 and reset the code. The car drove fine/normal for about 20 miles and then started to run rough again. The check engine reappeared with same code cylinder 3 misfire, no other or new codes. I was told I should change all coil packs and not just the one for cylinder 3. I think I'm just being sold unnecessary parts because why would a bad coil pack on cylinder 1 or 5 cause cylinder 3 to misfire? Is the coil pack really to blame or is it something else? I really have little to no knowledge about cars and would just like another opinion. Thanks!

My car has 152500 miles.
My car has an automatic transmission.

The "suspect" coil should have been tested prior to replacement; there might not have been anything wrong with it or the terminals might simply have needed to be cleaned (sometimes the terminals will get rusty or corroded). The other coils are irrelevant to the issue in cylinder No. 3. Among the causes of individual cylinder misfiring are faulty spark plugs, low compression, damage or excessive wear of spark plug wires and/or coils, faulty fuel injector, clogged EGR valves or tubes, burned valves, improper ignition timing, vacuum leaks, leaking head gasket, faulty camshaft sensor, faulty crankshaft sensor, faulty mass air flow sensor, faulty oxygen sensor, faulty throttle position sensor, faulty catalytic converter, and others. If you want the cause in your case pinpointed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a misfiring diagnostic and the responding certified mechanic will get this diagnosed and resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.

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