How do you prevent ignition coils from failing?

Ignition coils are an essential component of your vehicle’s ignition system. They are responsible for providing the high voltage needed to create the spark that ignites the air-fuel mixture in the engine’s cylinders. Over time, ignition coils can fail due to normal wear and tear or issues like oil leaks, causing misfires, poor performance, and difficulty starting the engine. Fortunately, there are steps you can take to extend the life of your ignition coils and prevent premature failure.

What is an Ignition Coil?

The ignition coil is a high voltage transformer that converts the low voltage from the battery (12 volts) to the thousands of volts required to jump the spark plug gap and ignite the air-fuel mixture. Inside the ignition coil are two coils of wire wrapped around an iron core. The primary coil has a few turns of thick wire and is connected to the vehicle’s 12 volt power source. The secondary coil has many more turns of thinner wire and is connected to the spark plug. When power flows through the primary coil, a magnetic field is created in the iron core. As the current is turned on and off by the ignition module, the magnetic field collapses and induces a high voltage in the secondary coil. This voltage jumps the gap at the spark plug and ignites the compressed fuel mixture.

How Do Ignition Coils Fail?

There are several ways that ignition coils can fail over time:

Normal Wear and Tear

The ignition coils are mechanical and electrical components that are subjected to heat, vibration, and high voltages. Over time, the windings, insulation, and connections inside the coil can degrade, resulting in lower voltage output and misfires.


Ignition coils can overheat from extended idling, frequent short trips, or an overloaded electrical system. The heat damages the coil windings and insulation. Overheating accelerates the normal aging process.

Oil Leaks

Engine oil leaks from worn valve cover gaskets can drip onto the ignition coils, causing the high voltage to short out to ground. The resulting carbon tracking erodes the coil casings and allows oil to soak the windings.

Voltage Spikes

Voltage spikes from the charging system can damage the coil’s internal components. Defective alternators and accessories like aftermarket xenon headlights can produce voltage spikes.


Water and corrosion from rain, washing, leaking coolant, or high humidity can short out the windings and terminals.

How Can Ignition Coils Be Tested?

There are a few methods that can be used to diagnose and test bad ignition coils:

Check Engine Light

The onboard computer often detects misfires and ignition coil failures by monitoring changes in rotational speed. This triggers the check engine light with coil related trouble codes.


A multimeter can check the primary and secondary resistance of the coils. Out of spec readings indicate a bad winding or shorted turns.

Output Voltage

A diagnostic tool known as an inductive pickup clamp can measure the coil’s output voltage. Weak voltage indicates internal damage.

Swap Coils

Swapping coils between cylinders can help isolate the bad coil. The misfire should move with the coil.

Spark Tester

A spark tester can check for strong blue spark output from the coil secondary.

Maintenance Tips to Prevent Premature Failure

Here are some ignition coil maintenance tips to help extend their life:

Use Recommended Spark Plugs

Use the correct spark plug type specified in the owner’s manual. Incorrect plugs can lead to voltage spikes and arcing.

Regular Engine Oil Changes

Change engine oil regularly to avoid oil leaks saturating the coils. Consider shorter intervals if the vehicle idles excessively.

Quality Ignition Parts

Buy original equipment or high quality aftermarket coils, wires, and spark plugs.

Routing Wires Properly

Ensure spark plug wires have proper loom routing away from hot or moving components to prevent damage.

Electrical System Maintenance

Fix any electrical issues immediately, like bad alternators, to prevent voltage spikes.

Address Leaks Quickly

Inspect for oil leaks and coolant leaks frequently. Repair as needed to keep coils dry.

Drive Regularly

Take the vehicle on regular drives to allow the engine and electrical system to warm up completely and recharge the battery.

Signs of Failing or Bad Ignition Coils

Watch for these common symptoms of ignition coils going bad:


Weak or missing coil voltage can cause random misfires, especially under load. Misfires feel like hesitation or jerking.

Rough Idling

The engine may idle roughly or stall due to misfires from faulty coils.

Loss of Power

As more coils fail, engine power and acceleration suffers.

Poor Fuel Economy

Misfires waste fuel which hurts MPG performance.

Hard Starting

Weak coil output makes cold starting difficult since more voltage is required.

Check Engine Light

The computer often detects misfires and turns on the check engine light with trouble codes pointing to specific cylinders.

When Should Ignition Coils Be Replaced?

If testing confirms the ignition coil is bad, replacement is required. Follow your vehicle owner’s manual interval for scheduled replacement. Generally, you should replace your ignition coils if:

  • Your vehicle has over 75,000 miles
  • The check engine light is on with misfire codes
  • You notice any symptoms of weak coil performance
  • Oil leaks may have flooded the coils
  • The vehicle fails an emissions test

For scheduled maintenance, ignition coil replacement around 80,000-100,000 miles is recommended for most vehicles. High performance engines often require more frequent coil replacement.

DIY Ignition Coil Replacement

With basic mechanical skills and tools, do-it-yourself coil replacement is easy in many vehicles. Follow these general steps:

Disconnect the Battery

Always disconnect the negative battery cable before servicing ignition components.

Remove Engine Covers

Remove any engine covers or air intakes blocking access to the coils.

Unplug Wires

Label and unplug the electrical connectors for the coil pack.

Remove Mounting Bolts

Remove any mounting bolts or brackets securing the coil pack.

Extract Coil

Carefully pull the coil assembly straight up and out of the engine compartment.

Check Spark Plug Boots

Inspect spark plug wire boots for cracks or damage. Replace if needed.

Install New Coil

Align the replacement coil and gently push straight down until seated.

Reconnect Wires

Reconnect the electrical connectors and any mounting hardware.

Reinstall Engine Covers

Reinstall any engine covers or air ducts.

Reconnect Battery

Reconnect battery negative cable once finished.

Start and Test

Start the engine and test drive. The check engine light should turn off after a few drive cycles.

Cost of Replacement

The parts cost for DIY ignition coil replacement is between $25-$250 depending on the vehicle. Shop labor costs typically range from $150-$400. High end performance vehicles and some luxury brands tend to be the most expensive.


Ignition coil failure is common as engines rack up the miles. Fortunately, there are preventative maintenance steps you can take to maximize the ignition system’s lifespan and avoid premature failure. With regular inspections and scheduled coil replacement at 80,000-100,000 miles, most vehicles can avoid the hassle of unexpected ignition coil failure on the road or having to replace them at high mileages. Careful attention to ignition system maintenance pays dividends in reliable performance and engine longevity.

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