How to Test Ignition Coils (2024)

Types of Ignition Coils

Ignition coils come in various types of ignition systems, but all of them take energy from the battery (usually 12 volts) and convert it to a high enough voltage (50,000 volts or more) to create a spark. Depending on the type of coil you have, this voltage either travels from the single ignition coil into a distributor, from the coil itself into a spark plug wire, or in the case of Coil-On-Plug (COP) systems, the coil sits directly on top of the spark plug. Older vehicles with a distributor have a single coil which provides voltage to the distributor. DIS, or wasted spark systems, use what is commonly referred to as a “coil pack” to send voltage to each spark plug wire, where other vehicles use a single coil for each spark plug wire. Most newer vehicles now incorporate COP.

Regardless of how many coils your vehicle has, you’ll need some basic tools to properly test a coil:

A basic tool kit to remove coils
A multimeter or an ohmmeter
Spark plug or ignition tester

Ignition coils fail in 2 ways – either complete failure and lack of output, or intermittently, meaning, the coil gets weak, output gets low, or the coil sometimes works and sometimes doesn’t. Because of this, intermittent problems are more difficult to diagnose. You’re going to want to do your initial diagnosis by looking for common bad ignition coil symptoms, because you need to remove the coils before testing them. That way you can save yourself some time if you find that the symptoms don’t match up. These are the symptoms of a bad ignition coil:

Misfires – Check Engine Light: Misfiring is usually the first and most common symptom, and in most vehicles today, this will appear as a Check Engine Light. The misfire will either be on a particular cylinder, or random. Most of the time, specific-cylinder misfires become easier to diagnose than the random-cylinder. If the car is running rough, stumbling, or lacking acceleration, and a check engine light code is revealing a misfire, an ignition coil or coils may be the culprit.
Rough Idling or Hesitation Under Load: As stated before, most ignition problems that are misfires also cause a rough or bad idle, but in cases where a vehicle (mostly older vehicles) does not have a Check Engine light or ability to detect a misfire, a rough idle or constant stalling of the vehicle may indicate a bad ignition coil, or a coil that is over-heating, whereas hesitation under load could also be a sign of a weak coil.
No-Start Situation: When a vehicle has a no-start situation, especially on older vehicles, the ignition coil is the first thing some people turn to. This often is not the culprit, as there are many other pieces of the ignition system, but even on modern vehicles, there are cases where a coil can fail between the last run cycle and the next start-up. In these cases, the vehicle may have ran fine, then on the next start, suddenly either won’t start, or runs horribly. Either way, the ignition coil, or coils could be suspect.

Testing Ignition Coils

There are several key things to know first about testing ignition coils. For one, there are many different methods for testing them, some low-tech and others high-tech, and we will cover them all.

It’s important again to remember that most coil failures are intermittent, which make them more difficult to diagnose. It’s also important to note that many ignition systems, most notably DIS or “wasted spark” coil pack systems, and several COP systems use a processor, or “Ignition Module” to properly power and send signals to the ignition coils. While these issues may appear that the coil isn’t working, it’s actually the Ignition Module causing the problem. Below we will highlight several different methods for diagnosing coil issues.

How to Test Ignition Coils

1. Swapping Coils

Swapping the ignition coil with another known, good unit on the car is the easiest and simplest way to begin diagnosis and works extremely well on any vehicle with single coils, multi-packs, or Coil-on-Plug.

As mentioned before, in these cases, you will usually have a misfire on a particular cylinder. In this case, let’s say you have a cylinder 3 misfire on a vehicle with Coil-on-Plug packs. Simply remove another coil – say – the coil from cylinder 6, and swap with the coil from cylinder 3. Now, if you have a code reader, you can clear your engine codes. If not, take note of which engine code you had.

Now, start the vehicle and run for several minutes. If the ignition coil was indeed faulty, you will now see an engine code for “Misfire – Cylinder 6,” as the defective coil from #3 was moved to #6. Your next step is to pull the #6 coil you just moved and replace.

If you swap these coils and still have a misfire on cylinder 3 and it doesn’t move, you know the coil is not the suspect. You either have an Ignition Module issue (some COP units use a module, some don’t), an issue with the coil’s connector or lead, you have a spark plug issue, a fuel injector issue, or mechanical internal engine issue with that cylinder.

2. General Inspection

On any coil – look it over carefully for signs of cracks, burns, melting, or leakage in older canister coils. This often points to a problem on its own, so check carefully.

3. Test the Windings with a Multimeter

Testing coils with a multimeter for Ohm’s resistance has been around for a long time. While it works to some degree, a major issue is that you are not able to simulate load, or measure the coil during operation, so weak coils that are not performing may still pass this test, but still be bad.

Nonetheless, testing the coil’s windings for proper resistance can help you find potential problems. Each ignition coil consists of two separate coils wrapped around each other: the primary winding and the secondary winding.

Check your vehicle’s specific repair manual, or online to find out what the correct resistance reading should be for both the primary and secondary windings. Both must be tested.
Connect the multimeter’s positive and negative leads to the corresponding terminals as instructed for testing Primary and Secondary windings. Different coils will have different touch-points depending on how the coil is designed. Your repair manual, or online information should tell you exactly where and which pins to touch off to.
Check the reading on the multimeter and compare it to the resistance specifications for both the primary and secondary windings.

If the reading falls outside the normal resistance range, you’ll need to replace the ignition coil. A zero means the coil shorted internally. An excessively high reading means the coil is open. Double check the placement of your leads to be sure they are correct.

4. Spark Test or Active Analysis

It’s important to remember that many times when a coil fails, it becomes weak. It still produces spark, and may still run the vehicle fine at times, but often under load, or acceleration, it can produce a misfire. Other times, the coil completely fails and stops working altogether. Doing an actual, on-vehicle spark test or output test is sometimes the only way to validate what is going on without simply replacing parts and hoping. It’s worth noting, that with any Coil-on-Plug vehicle (COP), that the ignition coil swap highlighted in step #1 is usually the quickest and easiest way to determine you have a coil problem.

A spark test or active analysis of the coil’s operation while on the vehicle can be performed with a number of different tools or procedures. Keep in mind, with any of these, you are often going to be testing the coil’s output while the vehicle is running, so review any spark test procedures either in a repair manual, or online tutorial. Simply yanking off coil wires while the vehicle is running can result in you getting shocked or seriously injured. You can also research a method known as the “power balance test” to help isolate where you have a cylinder issue. We will highlight several different style coils and basic methods of how you can test those, along with the tools for doing so:

An inline ignition tester like OEM 25227 can be helpful. This unit runs inline with many spark plug wires and/or coils and gives you a window to see active spark.
An ignition spark tester like OEM 25069 takes the place of a spark plug – is grounded to the engine and can easily show you whether a particular wire or coil is providing spark.
There are several new technology items, like a COP or coil on plug testers, which use inductive technology to tell that a coil is firing properly without even removing the coil. There are also several small, affordable oscilloscopes, along with tutorials online and on youtube on how to use them, that show you how you can pinpoint bad coil output without even removing the coil or wires.

Now, let’s cover different types of coils, and diagnostic methods to pinpoint an issue.

Coil-on-Plug (COP): You will recognize these coils immediately as they sit directly on top of the engine’s valve or cam cover, are bolted down, and attach directly to the spark plug via a rubber boot. The easiest and best method to pinpoint a problem on these coils is to do the #1 method of swapping the coil with a neighboring coil to see if the misfire moves to a different cylinder. If you suspect the coil is dead, you can also remove the coil, plug it back in, and use an ignition spark tester like OEM 25069 to determine if the coil is firing. You can also use a multimeter to test resistance. Individual coils for each cylinder that attach with a spark plug wire, similar to the LS motors used on Chevy and GMC trucks, can also follow this method.
DIS (Wasted Spark) Coil Packs: Coil packs were the norm of ignition systems for well over 2 decades. These units use either a single pack or series of packs that have tower terminals that spark plug wires hook to. They often have pairs of the terminals that fire in unison, where one cylinder fires on the compression stroke and one is wasted on the exhaust stroke of another cylinder, giving it the name “Wasted Spark” ignition. They are usually powered and controlled by an Ignition Module, which can also fail. Sometimes, a pair of terminals can go out on the pack, and other times, just a single terminal running a single cylinder can fail. If your system is composed of several of the same packs (like a V6 engine with 3 packs, each having 2 terminals) you can also do the #1 “swap” method and move the coil pack to a neighboring position, and see if the misfire moves to those cylinders. You can also test with an ignition tester like OEM 25227. In this case, it’s good to compare the intensity of the spark with a cylinder you know is good and working properly, with one that’s suspected of being bad. If you notice the spark is considerably weaker, the chances are the coil is bad and needs to be replaced. There are also methods of testing the windings on these coils with a multimeter, but they can get complex as you have multiple terminals and towers to test.
Traditional Single Coil: The single-terminal “canister” coil has been around in one form or another since the 1920’s. While they are not used on vehicles today, there are millions still on the road in vintage vehicles which use a distributor. Testing these coils for resistance via a multimeter is a good start, along with doing an inline spark test to be sure the coil is firing properly. Keep in mind, many of these coils can suffer intermittent problems, especially from overheating or vibration and your resistance test will not necessarily lead to a proper conclusion. Luckily, most of these coils are rather affordable, and often times if you are dealing with a pesky intermittent problem and all testing hasn’t led to a conclusion, swapping the coil with a known good unit may be the best course of action.

Diagnosing coils or the ignition system can sometimes be difficult. Simply swapping parts to diagnose the problem can sometimes be costly and ineffective. Professional shops often have a series of expensive, state-of-the-art equipment to help with these processes, and often giving them an opportunity to diagnose the problem ends up being more cost-effective and less time consuming. Find one of our Preferred Shops in your area that can help with this.

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