Reading time: 5 minutes
Immediate response
A faulty ignition coil can cause a rough engine, jerky acceleration, loss of power, rough idling, and the engine warning light coming on.
The problem may seem almost identical to that caused by a worn spark plug. For this reason, it's not a good idea to replace the coil immediately: OBD2 diagnostics can pinpoint the affected cylinder, while some targeted checks can help determine whether the problem actually stems from the coil.
If the engine light flashes If the engine runs very poorly, it is prudent to avoid continuing to drive unnecessarily: a significant lack of combustion can also put the catalytic converter at risk.
What you will learn:
- What are the symptoms of a bad ignition coil.
- How to understand which coil is not working.
- How to distinguish a bad coil from a worn spark plug.
- What OBD2 codes can indicate.
- How to check if the defect follows the coil.
- If you can continue driving.
- When to replace one or more coils.
Index
When a petrol engine starts to jerking, losing power, or turning erratically, the ignition coil is one of the components to check.
But he's not the only one.
A worn spark plug, an injector, or other problems can cause very similar symptoms. Before purchasing a new coil, it's important to identify the affected cylinder and understand what's actually causing the misfire.
What is the ignition coil for?
The voltage supplied by the battery is not sufficient to directly create the spark needed by the spark plug.
The ignition coil provides the high voltage needed for the spark plug to generate the spark.
On many modern engines we find one coil for each cylinder, often mounted directly above the spark plug. However, there are also ignition systems with different configurations.
When a coil is not working properly, the spark may become weak, erratic, or missing.
Symptoms of a bad ignition coil
The most common symptoms are:
- engine running rough;
- jerks during acceleration;
- loss of power;
- unstable minimum;
- irregular throttle response;
- possible increase in consumption;
- engine light on or flashing;
- misfire codes stored in the control unit.
If a coil stops working properly, the affected cylinder may contribute little or nothing to the engine's operation.
On a four-cylinder engine, for example, a major misfire can produce very noticeable vibrations.
The symptoms, however, are not enough to establish that the coil is faulty.
Why a coil fails
The coil works continuously in the presence of heat and vibration.
Over time, insulation, connections, or internal components may deteriorate.
Possible causes include:
- normal aging;
- high temperatures;
- water or oil infiltration into the spark plug well;
- damaged connectors or wiring;
- problems in the ignition system.
A badly worn spark plug may also require more voltage to properly fire a spark, placing increased stress on the ignition system.
Therefore, when you find a faulty coil, it is worth checking also the state of the spark plug of the same cylinder.
How to understand which coil is faulty
The first goal is to understand which cylinder is misfiring.
Starting from OBD2 diagnosis
If the ECU detects a misfire, it can store DTC codes that help pinpoint the affected cylinder.
A code referring to a specific cylinder, however, does not automatically mean “bad coil”.
Indicates where the misfire was detected.
The cause may be:
- coil;
- candle;
- injector;
- connector or wiring;
- diet;
- mechanical problem.
To understand how to read the stored errors you can consult our OBD2 car guide: what it is and how to read errors, where we explain how to interpret codes without having to replace components by trial and error.
Check coil, connector and well
Once you have identified the affected cylinder, also check:
- coil connector;
- wiring;
- possible presence of oil or water;
- visible cracks or damage on the coil;
- spark plug condition.
A visual inspection does not always confirm the fault, but it can immediately reveal an obvious problem.
Ignition coil or spark plug? How to tell the difference
It is one of the most common doubts because A faulty coil and spark plug can cause very similar symptoms.
If the diagnosis indicates a misfire on one cylinder, both components must be checked.
When the engine uses single compatible coils between cylinders, a useful diagnostic check is to check if the fault it moves along with the coil.
Example: The diagnosis reports a misfire on cylinder 2. After ruling out obvious problems, coil 2 is swapped with that of another compatible cylinder.
If after the check the misfire occurs on the cylinder where the coil was moved, the suspicion on the coil becomes much stronger.
If the problem remains with the original cylinder, you need to continue the diagnosis without trying to change the coil.
Those who want to learn more about the other part of the ignition system can also read our guide Worn spark plugs: symptoms and when to replace them.
What does OBD2 diagnosis indicate?
In the event of misfires, family codes may appear P0300.
Depending on the problem, the ECU may report a random/multiple misfire or a misfire related to a specific cylinder.
But the important principle is this:
The misfire code identifies the problem detected, not automatically the component to be replaced.
For example, a cylinder 3 error does not necessarily mean that the cylinder 3 coil is bad.
The code should be compared with symptoms, spark plug condition, and checks performed on the vehicle.
Can you drive with a faulty coil?
It depends on the severity of the defect.
If a coil is malfunctioning, the engine may continue to run, but the car may be used for a long time with significant misfires. can create further problems.
Improperly burned fuel can reach the exhaust system and increase the risk of overheating and catalytic converter damage.
Particular attention should be paid to the engine light flashing.
If the engine is jerking strongly, losing a lot of power, or the warning light is flashing, it is best to reduce vehicle usage and diagnose the fault before continuing as normal.
When to change the ignition coil
There is no universal interval after which a coil should be replaced.
It is normally changed when checks indicate that it is not working properly.
Before replacing it is advisable to check:
- stored errors;
- affected cylinder;
- spark plug condition;
- connector and wiring;
- possible presence of oil or water;
- result of diagnostic checks.
Replacing a coil just because the car has a lot of mileage does not guarantee that the problem will be solved.
Do all the coils need to be replaced?
Not necessarily.
If only one faulty coil has been identified, in many cases it is possible to replace just that one.
If the others are the same age and have many kilometers, it may be useful to check their conditions, but they do not have to be changed automatically.
Before ordering the spare part it is also important to check the compatibility with engine, spare part code and vehicle, because apparently similar coils can have different characteristics.
In summary
A faulty ignition coil can cause jerking, loss of power, rough idling and engine warning light.
When these symptoms appear:
- read the errors with the diagnosis;
- locate the affected cylinder;
- check coil, spark plug and connections;
- check for oil or water in the sump;
- when applicable, check if the defect follows the coil;
- do not automatically interpret a misfire code as a bad coil;
- Avoid continuing to drive if the engine runs very poorly or the check engine light is flashing.
A correct diagnosis avoids replacing perfectly functioning components.
FAQ - Faulty ignition coil
How to tell if an ignition coil is bad?
Jerking, rough idling, loss of power, and a check engine light are possible symptoms. To confirm the fault, however, you need to identify the affected cylinder and check the coil, spark plug, and connections.
Does a bad coil cause the engine light to come on?
It can make it turn on. If the control unit detects misfires, it can store an error code and activate the engine warning light.
How to tell if it's the coil or the spark plug?
The symptoms can be very similar. Both components should be checked. When the system allows it, checking whether the misfire follows the coil's movement can provide a very useful indication.
What does the P0300 code mean?
Generally speaking, it indicates that the ECU has detected multiple or random misfires. It does not, by itself, identify a specific coil as the culprit.
Can I drive with a bad coil?
If the engine suffers from significant misfires, continuing to drive can cause further problems. If the engine light flashes or the engine thumps heavily, it's best to avoid continuing unnecessarily.
If one coil is faulty, do I have to replace them all?
No. If only one faulty coil has been identified with sufficient certainty, it is not automatically necessary to replace all the others as well.
Conclusion
When the petrol engine is jerking and a misfire error appears, the ignition coil is one of the first components that comes under suspicion.
But misfire does not automatically mean a bad coil.
Identifying the affected cylinder, checking the spark plug and connections and, where possible, checking whether the fault follows the coil allows us to get to the cause with much more precision.
And avoid the most expensive method: change parts until the problem goes away.
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