HOW TO DIAGNOSE BAD IGNITION COIL

There are a couple of ways to check whether or not your car is misfiring (it may sound like it's running on less cylinders or very rough, a tractor or generally a noticable loss in power). While the ignition coil may not be the cause of the problem, it's definitely worth checking as it's fairly straightforward. The first method is simply using a ODB scan tool such as Carly or ODBII and read for fault codes related to misfiring/spark/ignition issues in a specific cylinder. A more traditional approach is to unplug the ignition coil while the car is running, if the car runs the same with the ignition coil unplugged then you know that there may be an issue with that cylinder and you can proceed to check the spark plug, ignition coil and wiring. If unplugging the first ignition coil and the behaviour of the car is worse then plug it back in and move to the next cylinder until you find the coil that is the culprit which means no changes after unplugging. As a test, you could use the ignition coil from a known good cylinder and replace the one in the bad cylinder to see if you can replicate the behaviour (the scan tool may also show the bad cylinder follows the coil). If this is possible then it is highly likely that the ignition coil is bad and requires replacement.