My 2005 corolla misfires cyl 3 intermittently, getting code P3503 i took to local repair and they could not figure it out. the dealer swap coil from 3-2 to see if problem moved it stayed with 3, issue only happened when idling. they replaced the plugs and issue cleared for 3 week, now its back and happening more often and at highway speeds. Could it be a wiring or cyl valve issue ? dealer said it could be ignition unit that needs to be replaced or computer!!.
I think you mean code PO353 (ignition coil #3 or circuit fault).
Have the tech thoroughly check the circuit fault. (there may be a corroded or damaged wire or even a poor connection)
Have they done a compression test?
I’m sure there are tests that can determine whether or not a component is no good without throwing parts at it. (Unless Toyota wants to do that at their expense and I doubt that will happen)
This is the circuit to the ignition coil which causes the ignition coil to discharge the spark to the #3 spark plug. Each time the ECM (engine computer) grounds the circuit, it causes the spark (hundreds of times per minuet). That wire, from the ignition coil to the ECM needs to be checked for good connections and wholeness. Check wire connectors, especially.
Here is a little fuller explanation of what P0353 is: http://www.bba-reman.com/content.aspx?content=DTC_P0353
My 2005 Corolla S does the same thing. My check engine always comes on. My Corolla S chugs and misfires. I went into the dealership, they plug it into their machine and said it was misfiring because my coil 1&3 were bad and needed to be replaced. I replaced them both, then about a week later my check engine light came on again, So I took it in and they said my coil 3 was bad. I’ve replaced the coils 3 (3)times and it has not help anything, my car still has the same problems. I also replace the intake manifold. Did you ever figure out what was wrong with your corolla or how to fix this problem. PLEASE HELP!!! Thanks for your time.