I have a 2010 Prius and I'm getting a check engine with Cylinder 4 missfire. Head gasket has been replaced and the engine never goes down so don't think it's that. I replaced the EGR (with toyota new) cleaned the intake and all the pipes. Replaced the spark plugs with new one step hotter plug. Swapped coils 3 and 4. Never see a missfire driving but after a couple weeks check engine light comes on. Anything to try next?
Switch the #3 an 4 coils. If the problem stays at 4, switch the coils back, and switch the spark plugs. If it changes, it's the plug. Are the plugs from Toyota? Or ebay/amazon..... cheap chinese copies. Also, check the wiring harness. It's easy to mess something up without realizing it....
Well, that kinda makes it easier. You need to verify: Spark compression fuel in cylinder #4. New spark plugs and moving the coil probably rules out those as a cause, but is that #4 coil getting power or signal? Check the wiring harness for rodent chew marks. Then you need to see if you've got enough compression and fuel in that cylinder. I'd dump some Techron fuel injection cleaner in with your next fill-up and see if there's a compression tester available at your local auto parts store. They'll let you borrow it, if you put down a deposit.
