6 Symptoms Of Bad Ignition Coil ( and How to Diagnose)

Jerry Wilson
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Symptoms of a Bad Ignition Coil

A symptom of a bad ignition coil would be a rough idle, especially at a stop light. The way to diagnose a bad coil is to first remove it and check for spark at the spark plug. If there is no spark at the coil, it is bad and needs to be replaced. If there is no spark at the coil, you test the spark plug wire (small orange wire). Where is connects to the distributor, you should find 12-18 volts at the wire. If this voltage is found, the spark plug wire from the coil to the distributor is faulty and needs to be replaced.

These symptoms are all based on a 2004 Hyundai Tiburon. To diagnose the ignition coil, first check the voltage of the big blue wire on the distributor. This should read 12-15 volts. If this is not the case, check the big red wire going to the ignition coil (this should read 12-15 volts as well). If neither of these wires read 12-15 volts, the wiring harness is faulty. If all the above symptoms show no signs of bad ignition coils, the problem lies within the distributor or coil itself.

How to Diagnose a Bad Ignition Coil

It’s a good thing that we have many different types of sensors in today’s vehicles; it’s kind of like an assembly line for the sensors. The computer in the vehicle checks the values from the sensors and determines when a sensor is seeing abnormal values so that it can alert us with a wrong engine light and tell us what sensor is bad. Fuel level sensor, coolant temperature sensor, you get the idea.

The problem with bad ignition coils is that we don’t always get a check engine light or an abnormal code. Ignition coils are responsible for creating the spark that fires at the right time to ignite the fuel at the right time.

Ignition coils can go bad for many reasons, but these reasons are not always detectable. And if you are not driving your car under load, you may not notice a change in how your vehicle functions.

Maybe the engine light came on, but the engine started running poorly, or maybe the engine light stayed off and you never even noticed a change, but the car’s performance never felt quite right. A bad ignition coil is often a silent killer. If you missed the symptoms of a bad ignition coil, you could waste several hundred dollars replacing sensors that are not bad.

How to Replace the Ignition Coil

The ignition coil is the mini-generator. It converts the 12-volt power coming from the power supply to the voltage necessary to fire the spark plugs properly. It is used in direct ignition systems of 2-stroke engines like those found in motorcycles. Ignition coils also come in 4-stroke engines, but they are more difficult to replace so only official service technicians are allowed to do so.

The coil is located under the fuel tank. When a breaker bar is inserted in the center of a handlebar control joystick, the spark plug wires will be disconnected. So, to replace the coil, you’ll need a socket wrench, a ratchet handle, a spark plug wire connector remover, and a screwdriver.

To begin, you need to remove the battery. After that, undo the screws holding the fuel tank down and replace it with a new one. Now, unscrew the old coil and remove it, but be careful with the electrical wires attached to it, which are also easily damaged and need to be properly connected.

Replace the old coil with a new one by attaching the wires to the new one and making sure that they are pushed in properly.

Now, reconnect the wires to the spark plugs one after the other while making sure that they are in the right position. If the bike won’t start, check the resistance again and make sure that the wires are properly connected.


A weak spark can be caused by many factors.

If you’re experiencing any of these symptoms, you should have your vehicle’s ignition checked by a professional for an accurate diagnosis and repair.

Ignition Wire Resistance: “Test Your Ignition Wire”

Ignition wire resistance should be checked when diagnosing ignition problems. “Testing your ignition wires” is one of the easiest tests you can perform with a multimeter.

To test resistance, remove the wire from the coil terminal or spark plug terminal and attach the positive lead to the distal end (-) and the negative lead to the proximal end (+). Be sure to touch both at the same time and not measure resistance in between!

If the reading is higher than 2 ohms, the wire needs to be replaced.

Distributor Cap Shorted “Testing Your Ignition Kit Components”

The distributor cap acts as an electronic capacitor to supply high-voltage to the spark plug. The distributor cap voltage is a result of the spark inside and is used by the engine’s electronics to control the ignition timing.