When Should You Replace Ignition Coils?
Ignition coils play an important role in a gasoline engine’s ignition system. When an ignition coil fails, the engine may misfire, run poorly, lose power, or illuminate the check engine light.
But a misfire doesn’t automatically mean an ignition coil needs replacement.
Spark plugs, fuel injectors, wiring, electrical connections, engine mechanical problems, vacuum or air leaks, and other conditions can produce similar symptoms.
Ignition coils should generally be replaced when testing shows that a coil is actually faulty—not simply because the vehicle has reached a certain mileage.
What Does an Ignition Coil Do?
A gasoline engine needs a properly timed spark to ignite the air-fuel mixture inside the cylinders.
The vehicle’s ignition system uses ignition coils to produce the high voltage necessary for the spark plugs to create that spark.
Modern vehicles use several different ignition-system designs.
Many use an individual coil for each cylinder, commonly referred to as a coil-on-plug system. Other designs may use coil packs or other configurations.
The exact system depends on the vehicle.
What Are the Signs of a Bad Ignition Coil?
A failing ignition coil can produce symptoms such as:
- Engine misfire
- Rough idle
- Hesitation
- Loss of power
- Check engine light
- Difficult starting
- Poor engine performance
- Engine shaking
- Misfire under acceleration or load
However, these symptoms aren’t unique to ignition coils.
They indicate that diagnosis may be necessary.
Does a Misfire Mean the Ignition Coil Is Bad?
No.
An engine misfire means combustion isn’t occurring properly in one or more cylinders.
The ignition coil is only one possible cause.
Depending on the vehicle and symptoms, a misfire can involve:
- Ignition coil
- Spark plug
- Coil connector or wiring
- Fuel injector
- Fuel delivery
- Air or vacuum leak
- Compression problem
- Valve or other mechanical problem
- Sensor or engine-management problem
- Other vehicle-specific conditions
Replacing the ignition coil without determining the cause can result in replacing a perfectly good part.
What Do Misfire Trouble Codes Mean?
A check engine light may be accompanied by diagnostic trouble codes associated with engine misfires.
For example, a code may indicate that the vehicle’s computer detected a misfire affecting a particular cylinder or multiple cylinders.
That information can be extremely useful.
But a cylinder-specific misfire code does not mean:
“Replace the ignition coil on that cylinder.”
The code identifies where the computer detected the misfire. Additional testing may still be necessary to determine why that cylinder is misfiring.
Can a Scan Tool Tell You Which Ignition Coil Is Bad?
A scan tool can provide useful diagnostic information, but it doesn’t necessarily identify a failed ignition coil by itself.
Depending on the vehicle, useful information may include:
- Diagnostic trouble codes
- Misfire data
- Freeze-frame information
- Live engine data
- Other system information
That data can help determine which cylinder or operating condition deserves further investigation.
The next step is determining what is causing the misfire.
How Do You Test an Ignition Coil?
The appropriate testing procedure depends on the vehicle and ignition-system design.
Diagnosis may involve examining:
- Diagnostic trouble codes
- Misfire data
- Spark plug condition
- Coil condition
- Electrical connectors
- Wiring
- Power and ground circuits
- Other ignition-system components
On some vehicles, comparing the behavior of components between cylinders can also provide useful diagnostic information when performed appropriately.
The important point is that diagnosis should establish whether the problem follows the coil or remains with the cylinder.
Could the Spark Plug Cause the Same Symptoms?
Absolutely.
A worn, damaged, fouled, incorrectly gapped, or otherwise faulty spark plug can cause a misfire.
A spark plug problem can sometimes also place additional demands on an ignition coil.
That’s why the spark plug and its condition should be considered when diagnosing an ignition-related misfire.
Spark plugs and ignition coils perform different jobs and shouldn’t automatically be replaced as a set simply because one component has a problem.
Should You Replace All Ignition Coils at Once?
Not necessarily.
There is no universal rule requiring all ignition coils to be replaced because one coil has failed.
If testing identifies one faulty coil and the remaining coils are operating properly, replacing only the failed coil may be appropriate.
However, circumstances vary.
Vehicle age, component condition, accessibility, previous repairs, known failure patterns, and customer preference can all affect the repair decision.
The important distinction is between replacing a failed component and automatically treating every ignition coil as a scheduled maintenance item.
Do Ignition Coils Have a Replacement Interval?
Generally, ignition coils are not treated like routine maintenance items such as engine oil or certain filters.
There isn’t one universal mileage at which every vehicle should receive new ignition coils.
A coil can fail at relatively low mileage or continue operating properly for a very long time.
Vehicle-specific manufacturer recommendations should always take priority if a particular application specifies otherwise.
What Can Cause an Ignition Coil to Fail?
Ignition coils operate in a demanding environment involving heat and high electrical voltage.
Potential factors contributing to coil problems can include:
- Heat
- Age
- Electrical problems
- Internal insulation deterioration
- Moisture or contamination
- Damaged connectors or wiring
- Problems elsewhere in the ignition system
Oil or water entering a spark-plug well can also create ignition problems on some engines.
If contamination is present, simply replacing the coil without correcting the source of the contamination may leave the underlying problem unresolved.
Can Oil in the Spark-Plug Well Damage an Ignition Coil?
It can contribute to ignition problems.
On some engines, a leaking valve-cover gasket or spark-plug tube seal can allow oil to enter the area around the ignition coil and spark plug.
If oil is discovered there, replacing the ignition coil without addressing the leak may not be a complete repair.
The source of the contamination should be identified.
What If the New Ignition Coil Doesn’t Fix the Misfire?
That usually means more diagnosis is required.
If a coil was replaced solely because a cylinder had a misfire code, the original assumption may have been incorrect.
Possible remaining causes can include:
- Spark plug
- Fuel injector
- Wiring
- Electrical connection
- Air leak
- Compression problem
- Valve problem
- Fuel-delivery issue
- Other engine-management or mechanical problem
This is why replacing parts based only on trouble-code descriptions can become expensive.
Can You Drive With a Bad Ignition Coil?
Driving with an active engine misfire isn’t something that should be ignored.
A severe misfire can affect engine performance and may allow unburned fuel to enter the exhaust system.
That can potentially overheat and damage the catalytic converter.
If the check engine light is flashing, the vehicle is running extremely poorly, or there is a substantial loss of power, continued driving may make the situation worse.
Why a Flashing Check Engine Light Matters
A flashing check engine light commonly indicates that the vehicle has detected a condition such as a severe misfire that may damage the catalytic converter.
It should be treated differently from simply ignoring a steadily illuminated check engine light for an extended period.
Reduce unnecessary operation and have the vehicle diagnosed.
Ignition Coil vs. Spark Plug
The two components work together but perform different functions.
Ignition coil: Produces the high electrical voltage needed by the ignition system.
Spark plug: Uses that voltage to create a spark inside the combustion chamber.
A problem with either can cause an ignition-related misfire.
That’s why diagnosing one component based solely on the symptom of a misfire isn’t reliable.
When Should an Ignition Coil Actually Be Replaced?
Replace an ignition coil when diagnosis indicates that it has failed or is no longer operating properly.
Don’t replace coils simply because:
- The check engine light came on
- The engine has a misfire code
- The engine runs rough
- The vehicle reached an arbitrary mileage
- Someone says coils should always be replaced with spark plugs
Determine the cause first.
If the ignition coil is responsible, replace the failed component and address any related condition that may have contributed to the failure.
Mobile Ignition and Misfire Diagnosis in Palm Coast
Michael’s Mobile Care provides mobile engine-performance and ignition-system diagnosis in Palm Coast.
If your engine is misfiring, running rough, losing power, or displaying a check engine light, diagnosis can help determine whether the problem involves an ignition coil, spark plug, fuel system, electrical circuit, mechanical condition, or another component.
When ignition-coil replacement is necessary and appropriate for mobile service, the repair can often be performed where the vehicle is located.
Michael’s Mobile Care has more than 45 years of automotive experience and is licensed in Florida under MV114951.
Call Michael’s Mobile Care at (386) 264-7390 for mobile automotive diagnosis and repair in Palm Coast.