Михаил Руденко/Getty Images Ignition issues can be a real pain to both diagnose and deal with. Sure, generally speaking, having a faulty ignition coil is a better problem to have than a worn-out turbo or clogged DPF, but arguably more frustrating. That's because ignition issues can leave you stranded, unable to start the car, whereas other more costly faults might still at least grant the driver permission to limp the car home, or to a garage. Furthermore, something like a faulty turbo or DPF is usually quite easy to diagnose, whereas tracking down the culprit on the ignition side of things could take a while longer, potentially leading to higher diagnosis costs. With that being said, there are a few ways in which you can attempt to pinpoint whether it's a faulty spark plug wire, ignition coil, or something else entirely that's stopping the vehicle from running sweetly. The first thing you should always do is carry out a visual inspection. Do the spark plug wires look damaged, corroded, or scorched? Is it possible to see electrical arcs, or hear snapping noises, when trying to start the vehicle? Is there a check-engine light illuminated on the dash? If so, scanning the car with an OBD-II code reader might reveal all you're looking to find out, but equally, it may just throw up a generic misfire code. This is why it's important to also understand how to thoroughly check the coil or wires for issues and not just rely on a code reader, which is exactly what this article will explain. How to diagnose an ignition coil fault knobsdialsandbuttons / YouTube There are a few tell-tale signs that will immediately inform a driver that there is a fault present with their car. Obviously, a check-engine light is number one, and the other will be rough idling. Neither of these solely indicates that the ignition coil is at fault, but they are both symptoms of it. A little extra homework is required to pinpoint the culprit, though. If a check-engine light is present, use an OBD-II code reader to obtain the diagnostic trouble code; codes such as P0351 through to P0358 indicate circuit problems that directly relate to ignition coils. Most modern engines will use a coil-on-plug system, which means there will be as many ignition coils as there will be cylinders — and no spark plug wires. For those who don't have access to a code reader, taking notice of how the car is running can reveal a lot, too. Trouble starting, misfiring when under load, and engine backfiring are all signs that an ignition coil is playing up. Poor fuel economy is another important sign to watch out for. Sure, these are not conclusive evidence on their own. However, take those symptoms as a sign to visually check the coils – if one or more coils look corroded, cracked, oily, or otherwise damaged, then this is likely the cause of the issue. Equally, a multimeter — one of those simple but life-changing tools – could be used to check resistance across the plug, which would confirm how efficiently it's working still, if at all. How to diagnose faulty spark plug wires Dmytro Stoliarenko/Shutterstock Here's where things get tricky, as many of the same symptoms that suggest an ignition coil is bad, such as misfires and trouble starting, can also imply that a spark plug wire is faulty — as both form part of the ignition system. Of course, if the car in question has a modern coil-on-plug design, then it won't have spark plug wires, which makes things a little easier since you only need to check the coils. If, however, it's an older design with spark plug wires and a single ignition coil, then deciphering between the two can sometimes be tricky. Once again, one of the best ways to check the condition of a spark plug wire — after a visual inspection for anything obvious — is to pull out the multimeter and check the resistance against the manufacturer's spec. If the resistance is too high or too low, the wires can be replaced; if two or more are faulty, it's best to replace the lot. It's also well worth quickly checking if the spark plug wires are routed correctly, as if some are connected to the wrong plug, then the engine isn't going to be firing on all cylinders. This doesn't mean the wires are faulty, so re-routing could be a free fix to your rough-running problem. While here, it's also a good time to check if the spark plugs themselves need replacing. If the spark plug wires are routed correctly, look fine visually, and are testing fine with a multimeter, then it's worth taking a look at the ignition coil to see how that's testing next.
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