Q: Engine is misfiring and the service engine soon light is on.

Hi there.

If the engine is misfiring you would need to check the spark plugs and fuel injectors to see what is not working. If the spark plug has no spark, then check the spark plug wire for spark by-passing the spark plug with a spark tool. If there is spark, then the spark plugs are not working and need replaced. If the spark plugs are firing, then there could be an issue with the fuel injectors causing the engine to run rough and misfire. Remove one harness to each injector at a time (one at a time) and see if the engine stumbles more. If the engine stumbles more, then the fuel injector is working. If there is no change, then the fuel injector is not working and would need replaced. If the fuel injectors and spark plugs are working properly, then there could be an issue with the cylinder. Then you could take a compression tester and check all of the cylinders and compare the reading. They all should be about the same. If number 6 is lower than the rest, then there could be an issue with the head gasket, a burned or sticking valve, or the piston rings are worn causing a low compression. A leak down tester (requires compressed air) will determine the cause and location of a low cylinder compression. If the mechanic did not perform these tasks to determine the expensive repair, then I recommend seeking out a professional, such as one from Your Mechanic, to help you.