What is an Engine Misfire? Welcome to Mercie J auto care, where compassion and craftsmanship come together. I’m Rick Smith, master certified technician. So when your check engine light is on and you have a P03 series code so like a 301, 302, 306. That’s a misfire code and the 301 would be be cylinder, 306 would be cylinder 6, etc. So what does it take for a cylinder to fire? Well, you’re piston starts out of the top, your intake valve opens and like a syringe it sucks in air and fuel? Then the valves close, the piston comes up, compresses all that, when it gets to the top and it’s all nice and compressed, the spark plugs fires. This shoves the piston down just like if you’re pushing your leg down on a bicycle crank shaft and then after that, completes the firing process then the exhaust valve open, piston comes up and pushes it out of the tailpipe. So three things we have to have, one is that spark plug has to fire. So ignition coils, spark plugs and things like that need to make that happen. Two is we have to have fuel, so the fuel pump has to pump pressure, the injector has to squirt fuel into the cylinder. Third thing we have to have is that compression. So we could have anything like this 2012 Caravan that had a P0302. The exhaust valve is leaking, so we had low compression. We had to do a cylinder head. So you could need anything from the cylinder head to a major engine problem to a simple, spark plug, injector or even a vacuum leak. I hope this helps you understand a little bit more about what a misfire is with those P0 three… codes. Until next time so travels and blessings to you.