A misfire that shows up under acceleration, a flashing engine warning light, or a rough idle can quickly turn a normal drive into a costly problem. If a faulty coil is the cause, learning how to replace ignition coils can save labour costs and get your car back on the road sooner. On many modern vehicles, it is a straightforward job - provided you confirm the fault, order the correct part and work carefully.
Ignition coils supply the high voltage that lets each spark plug ignite the air-fuel mixture. When one weakens or fails, that cylinder may misfire. The result can be poor fuel economy, sluggish performance and, if ignored, possible damage to the catalytic converter from unburnt fuel.
Check That the Ignition Coil Is the Problem
Do not replace parts on a guess. A failed ignition coil is common, but worn spark plugs, damaged wiring, a loose connector, fuel issues and air leaks can all cause similar symptoms.
Common signs of a failing coil include a rough or shaking idle, hesitation when accelerating, difficult starting, reduced power, an engine warning light, and a noticeable increase in fuel use. On some vehicles, the warning light flashes during a severe misfire. If that happens, avoid driving the car unless necessary. Continued misfiring can turn a relatively affordable repair into a much larger exhaust-system bill.
An OBD scan tool is the best starting point. Misfire codes often identify the affected cylinder, such as a cylinder 2 or cylinder 4 misfire. A code for an ignition circuit can provide a stronger clue, although it still does not prove the coil itself has failed.
If your engine uses individual coil-on-plug ignition coils, a practical test is to swap the suspected coil with one from another cylinder. Clear the code, run the engine and scan it again. If the misfire moves with the coil, you have found the likely fault. If it stays on the original cylinder, inspect the spark plug, injector, wiring and connector before buying a replacement.
Before You Replace Ignition Coils
First, confirm your vehicle details. Match the replacement coil to the make, model, engine size, year and, where applicable, VIN or engine code. A coil that looks identical may have a different connector, mounting design or electrical specification. This matters across popular models from Toyota, Nissan, Suzuki, Subaru, BMW, Holden and Fiat, where engine variants can differ within the same model range.
You will usually need a socket set with an extension, a ratchet, a small screwdriver or trim tool for connector locks, and a clean rag. A torque wrench is useful if the manufacturer gives a coil retaining-bolt torque figure. Keep a scan tool handy if you need to read or clear fault codes.
Let the engine cool before starting. Ignition components sit on top of the cylinder head and can become very hot. Park on level ground, apply the handbrake and remove the key or keep the smart key well away from the vehicle. Disconnecting the negative battery terminal is a sensible extra precaution, particularly if you are unfamiliar with electrical work. Be aware that some vehicles may lose radio settings or require window reset procedures after battery disconnection.
It is also worth checking the condition and service history of the spark plugs. A badly worn plug makes a coil work harder and can shorten the life of a new replacement. If plugs are due for replacement, doing them at the same time is usually the better value repair.
How to Replace Ignition Coils Step by Step
Most late-model petrol engines use a coil-on-plug design. Each coil sits directly over a spark plug, usually beneath a plastic engine cover. Older engines may use a coil pack with plug leads instead. The principles are similar, but access and removal differ.
1. Find the coils and create access
Open the bonnet and remove any decorative engine cover. These commonly pull upwards from rubber mounts, though some are held by bolts. Take a moment to identify each ignition coil and its electrical connector. On an inline four-cylinder engine, they are often in a row along the top of the engine. V6 and V8 engines may have coils on both cylinder banks, and the rear bank can be tighter to reach.
Remove only what you need for access. Some vehicles require an intake duct, airbox section or brace to come off first. Put bolts and clips somewhere clean and organised so nothing ends up in the engine bay or on the driveway.
2. Disconnect the electrical connector
Each coil has a wiring plug secured by a locking tab. Press or lift the tab as designed, then pull the connector straight back. Do not pull on the wires. If it is stuck, use a small trim tool gently rather than forcing it with a screwdriver.
Check for green corrosion, oil contamination, broken plastic or stretched wiring while the plug is off. A new coil will not fix a poor electrical connection. If oil is sitting in the spark plug well, the rocker cover gasket or plug tube seals may need attention before fitting new parts.
3. Remove the old ignition coil
Most individual coils are held down by one small bolt. Remove it, then grip the coil body and twist it lightly while pulling upwards. The rubber boot may hold firmly to the spark plug, especially on an older vehicle, so patience is better than brute force.
Inspect the removed coil for cracks, burn marks, a swollen boot, oil ingress or carbon tracking. Carbon tracking looks like a thin dark line and can allow the spark to escape before it reaches the plug. It is useful evidence, but not every failed coil will show visible damage.
4. Compare and fit the replacement
Place the old and new coils side by side. Check the connector position, bolt tab, boot length and overall shape before fitting. This quick comparison helps prevent installing the wrong part after the engine is partly reassembled.
Lower the new coil straight onto the spark plug. Push firmly until it is fully seated. Refit the retaining bolt and tighten it to the manufacturer’s specified torque if available. These are typically small bolts threaded into alloy components, so overtightening can strip threads or crack the coil mounting tab.
Reconnect the electrical plug until the lock clicks into place. Give the connector a light pull to make sure it is secured. Repeat the process for any additional coils being replaced.
5. Reassemble and test the repair
Refit any intake pieces, brackets and engine covers you removed. Reconnect the battery if it was disconnected, then start the engine. It should settle into a clean, steady idle without obvious shaking or a flashing warning light.
Use the scan tool to clear stored codes, then take a short test drive. Accelerate smoothly through the rev range and check for hesitation. Scan once more after the drive if possible. A pending or returning misfire code means more diagnosis is needed.
Replace One Coil or the Full Set?
There is no single answer. If testing has identified one failed coil and the others are working properly, replacing that one coil is the lowest-cost repair. This is often the sensible option for a newer vehicle or an engine with relatively low kilometres.
Replacing all coils can make sense when the vehicle has high kilometres, multiple coils are showing weak performance, or you have already had more than one coil failure. Coils often age under the same heat and load conditions, so a second failure may not be far behind. The trade-off is simple: replacing one saves money now, while replacing a set may reduce the chance of repeat repairs and downtime.
Avoid mixing up the fault with the cause. If a failed coil was paired with worn plugs, an oil-filled plug well or a damaged connector, correct that issue too. Otherwise, the new coil may be placed under the same stress as the old one.
Common Mistakes That Cause Repeat Misfires
The most common error is fitting a coil without checking the spark plug. Excessive plug gap, a cracked insulator or the wrong plug type can create a misfire that feels exactly like a coil failure. Another is forcing the coil connector and breaking its lock, which can create an intermittent fault that is difficult to trace.
Do not use excessive grease inside the coil boot. If dielectric grease is recommended for your application, use only a small amount in the correct location. It is an insulator, not a cure for a loose connection or a damaged boot.
Finally, do not ignore a flashing engine warning light after the repair. Stop, scan the vehicle and recheck your work. Confirm every coil connector is fitted, each coil is seated on its spark plug, and no intake hose or sensor plug was disturbed during access.
A correctly matched replacement ignition coil is a practical repair that can restore smooth running without paying workshop labour for a simple job. If you need a replacement for a common NZ vehicle, JBH Auto Parts can help you source the right part at a price that keeps the repair under control. When the diagnosis is sound and the fitment is correct, you can finish the job with confidence and spend less time chasing the next misfire.