Q: Check engine light is on. Misfiring on Cylinders 3 and 4.

Before replacing all the coils and plugs you may need to have the engine decarbonized. The engine can have excessive carbon build up on the exhaust valves, cylinder head, and exhaust manifold. You should use engine combustion chamber cleaner on a cold running engine.

Let the engine suck in the cleaner or simply spray it into the engine while it is running. Once the whole can is run through the engine, shut off the engine and let it cool for at least 30 minutes. This will allow the cleaner to loosen up the carbon deposits.

Afterwards, start the car and run a second can through the engine. Once this is done, run the engine for about 5 minutes at 2000 RPMs to clear out the cleaner and get rid of the carbon.

Now you should replace the coils and spark plugs as needed. If any oil is on the spark plug wire boots then replace the valve cover gasket.

Once you’ve done all of this I would recommend running fuel injector combustion cleaner in the tank to keep the carbon from returning. However, carbon build up can be extreme and can require additional cleaning or replacement of the parts that are affected.

If you’d like some assistance with this, consider YourMechanic, as a certified professional can come perform this service on your vehicle and properly diagnose your Check Engine Light. Good luck.