How to replace an ignition coil

When to replace an ignition coil

Service schedules do not usually give a change interval for ignition coils. A coil is replaced when it has failed or is damaged, for example when its housing is cracked or shows burn marks. Where the coil's maker supplies the boot that fits onto the spark plug as a separate part, a damaged boot can be replaced on its own, by the maker's procedure.

A failing coil can show itself in these ways:

• The engine misfires, shakes at idle or hesitates when accelerating, sometimes only in damp weather or under load
• The engine warning light comes on or flashes, and a misfire fault or an ignition coil fault is stored
• The engine loses power and uses more fuel
• The engine is hard to start or, where one coil serves every cylinder, does not start at all

Do not keep driving with a misfire. Unburnt fuel can reach the exhaust, where it can overheat and destroy the catalytic converter. If the engine warning light flashes, follow the instructions your vehicle's handbook gives for that warning.

These symptoms have other causes too, such as worn spark plugs, a damaged ignition lead, a fault in the wiring or a connector, a faulty injector, an air leak or low compression. Confirm that the coil is at fault before you buy one.

Before you start

An ignition system produces a very high voltage. Never touch or disconnect a coil, an ignition lead or their connectors while the ignition is on or the engine is running or being cranked. Anyone with a pacemaker or another implanted electronic device should leave work on the ignition system to someone else.

• Switch off the ignition and keep the key or remote away from the vehicle, so that the engine cannot be started. Follow your vehicle manufacturer's instructions for service work on engines that start automatically and on hybrid vehicles, and disconnect the battery if those instructions call for it. On a hybrid vehicle, do not touch the orange high-voltage cables or the components they connect.
• Let the engine cool before you begin: hot parts cause burns.
• If the vehicle stores fault codes, read and record them before you begin the repair or disconnect the battery.
• Find out how the engine cover and the coil come off on your engine. On some engines the intake manifold or other parts are in the way, and the job is considerably longer.
• Have the vehicle manufacturer's tightening specification for the coil's fixings to hand before you undo them and, once the fault is confirmed, the new coil.
• Where several coils, ignition leads or wires have to come off, mark each one with the cylinder, position or terminal it belongs to before you remove it, or work on one at a time. Every lead, wire and connector must go back to its own place.
• Whenever the engine has to run during this job, first make sure that the vehicle stands in the open air and cannot move, with the parking brake applied and the transmission in neutral or in park, and that hands, clothing, tools and cables are clear of moving and hot parts. Never run an engine in a garage or another closed space: exhaust fumes are poisonous.

Confirm that the coil is at fault

1. If the vehicle stores fault codes, read them with a diagnostic tool and record them. A misfire fault usually names the cylinder. A fault in an ignition coil circuit names a circuit, which several coils can share: use the vehicle manufacturer's definition of the code to find which coil and cylinder it concerns. Such a fault can also come from the wiring, a connector or the control unit, so it does not by itself show that the coil has failed.
2. With the ignition off and the engine cool, look at the coil, its connector or terminals and, where there are any, its ignition leads. Cracks, burn marks or fine tracks on the housing, on a boot or on a lead, a swollen or oil-soaked boot, corroded, bent or loose terminals, chafed wiring and, where the coil sits in a plug well, oil or water in the well all point to a cause. A coil that is visibly damaged is replaced, and one that is wet with oil or water must not be run again until the cause has been dealt with; use neither for the test in the next step.
3. If the inspection finds nothing, and the engine has a separate coil on each spark plug, the suspect coil can be exchanged with the coil of another cylinder, provided both are the same part. Remove and fit the coils as described below. Clear the fault codes, start the engine with the precautions given under Before you start, let it run only until the misfire shows again, and read the codes once more. If the misfire has moved with the coil, the coil is faulty. If it has stayed on the same cylinder, or has not shown at all, the test has not cleared the coil: the cause may lie elsewhere, for example in the spark plug, the injector, the wiring or the engine itself, but an intermittent fault or a second fault can also keep the misfire where it was. Have the fault diagnosed further.
4. On other layouts, and whenever the result is unclear, have the coil tested by a workshop: the test needs the equipment and the procedure the vehicle manufacturer specifies.

Never look for the faulty cylinder by pulling a connector, a coil or an ignition lead off a running engine. The exposed high voltage can injure you and can destroy the coil or the engine control unit, and unburnt fuel can reach and damage the catalytic converter.

Choose the right coil

Ignition coils differ in construction. A single-spark coil serves one spark plug and on many engines sits directly on it. Other coils serve several cylinders, through ignition leads or as one unit that sits across the plugs, and some older engines have a single coil and a distributor. Coils also differ in their electrical connector, in the connection to the plug or lead, and in whether the switching electronics are built in. A coil that fits the mounting is not necessarily the right one: fit only a coil listed for your engine.

Select your vehicle with the selector on this page to see the part numbers listed for its engine. Where its supplier states them, each part number is shown with specifications such as the coil type, the number of connector pins, the kind of secondary connection and the rated voltage. Where each coil is a separate part, a single failed coil can be replaced on its own; replacing the other coils of the engine at the same time is a choice, not a requirement, unless the vehicle manufacturer says otherwise. Where several coils are built into one unit, such as a coil strip or a coil pack, the whole unit is replaced.

All ignition coil OE numbers in the catalogue

Remove the old coil

1. Before you remove anything, switch off the ignition, make sure that the engine cannot be started, as described under Before you start, and let the engine cool if it has run. Then remove the engine cover and anything else that blocks access to the coil, as the vehicle manufacturer describes. If the intake manifold or other intake parts have to come off, cover the exposed openings so that nothing can fall into the engine. If access would mean opening the fuel system or disconnecting a component of a hybrid's high-voltage system, that work has its own procedures and precautions: leave it to a workshop.
2. Clear loose dirt from around the coil before you disturb it: wipe it away, or blow or vacuum it out, wearing eye protection when you use compressed air. Where a coil sits on a spark plug, dirt left around it can fall into the plug well when the coil is lifted out.
3. Disconnect the coil's low-voltage side. Where it has a plug, release the plug's catch and unplug it without pulling on the wires; catches can become brittle with age and heat, so release them the way they are designed to be released and do not lever them. Where separate wires sit on push-on or screw terminals, check that every wire is marked with its terminal, then take each one off.
4. On a coil with ignition leads, check that every lead is marked with its position. Release any clip or lock that holds a lead, as the vehicle manufacturer describes, then pull each lead off by its boot, never by the cable.
5. Undo the coil's fixings, if it has any. Keep any earth lead, bracket or spacer with them and note where each belongs.
6. Draw a coil that sits on a spark plug straight off the plug and out of its well, with the puller the vehicle manufacturer specifies where there is one. Do not twist or lever a coil that sticks unless the manufacturer's procedure allows it. A unit that sits across several plugs comes off as one piece: draw it off evenly, by the method the vehicle manufacturer gives. Lift any other coil off its mounting.
7. Where the coil sat on a plug, check that it has come out complete. If a boot, or a part of one, has stayed on a spark plug, it has to come out completely before a new coil goes in, and no piece may be left in the well. Follow the vehicle manufacturer's procedure for this, or leave it to a workshop.
8. Where the coil sat in a plug well, look into the well with a lamp and soak up any oil or water standing in it. Let nothing fall into an open well, and do not leave it open for longer than the job needs.
9. Check the connector or the terminals and the wiring behind them for corrosion, bent or loose contacts and chafed or brittle insulation. Damaged wiring, damaged or corroded contacts and a plug that no longer locks must be repaired as the vehicle manufacturer describes: a new coil does not cure them.

Oil found standing in a plug well may come from a leaking valve cover gasket or plug tube seal, or be left from an earlier spill. Water may have got in past a damaged seal on the coil or when the engine was washed. Find the source and have any leak repaired, since oil or water in the well can damage the boot and make a new coil misfire as well.

Check the spark plug as well

A coil and its spark plug work together. A worn plug whose gap has widened needs a higher voltage to fire, which loads the coil more heavily. A plug with a cracked insulator can let the spark leak away along the crack instead of across the gap. Either can cause a misfire that a new coil does not cure, and can damage the new coil or its boot.

A plug's wear shows at its firing end, which cannot be seen with the plug in place. Unless you know that the plugs served by the suspect coil are within their change interval and sound, remove and inspect those plugs before you fit the new coil. Our guide to replacing spark plugs describes how, with the tools and the precautions that job needs. Replace a plug that is worn, damaged or not the one specified for the engine; if the plugs are due, or their age is not known, it is sensible to replace all the plugs of the engine.

Where the coil sits on the plug, look at the top of the plug in its well too. A cracked insulator, or a burnt track left by a flashover, means that the plug has to be replaced; an even discolouration around the insulator is usually harmless staining. On engines with ignition leads, check the leads and their boots as well, and replace a lead that is cracked, burnt or oil-soaked.

How to replace spark plugs

Fit the new coil

1. Check that the new coil is one listed for your engine. Then compare it with the old coil: construction, electrical connector and number of pins, the connection to the plug or the leads, length and fixing points. Where they differ, do not fit the coil until its supplier or the vehicle manufacturer confirms that it replaces the old part. The coil that was fitted may itself have been the wrong one.
2. Check that the new coil's connections are clean and undamaged. On a coil that sits on a plug, check too that its boot and any seal it has are clean and undamaged and that the contact inside the boot is in place.
3. Use grease or contact spray on the boot, the contacts or the connector only if the coil's maker or the vehicle manufacturer calls for it, and then only the product they name, in the place they name.
4. Push a coil that sits on a spark plug straight down into its well until it is fully seated on the plug and its seal, where it has one, sits correctly. Do not force it: if it will not go down, withdraw it and find out why. With a unit that sits across several plugs, line up every boot with its plug before you press the unit down by the method the vehicle manufacturer gives, and check that it is seated on every plug. Fit any other coil to its mounting with its bracket, spacers and earth lead where they were.
5. Refit the coil's fixings, where it has any, and tighten them to the vehicle manufacturer's tightening specification. Many coil flanges are plastic, and the threads are often in a light-alloy or plastic cover: overtightening can crack the flange or strip the thread.
6. On a coil with ignition leads, push each lead onto the connection it was marked for until it is fully seated, and fasten any clip or lock that holds it. Route the leads as they were, in their holders and separators. Leads on the wrong connections can make the engine misfire or backfire.
7. Reconnect the coil's low-voltage side. Push a plug home until its catch engages. Fit separate wires each to the terminal it was marked for, as the vehicle manufacturer describes: wires on the wrong terminals can weaken the spark or damage the ignition system. Route the wiring as it was, clear of hot and moving parts.
8. Before you close anything, take out every cloth or cover you put over an opening and make sure that none is left inside. Then refit everything that was removed for access as the vehicle manufacturer describes, with new seals or fasteners where the manufacturer requires them, tightened to the manufacturer's values.

Start the engine and check

1. Check that every connector is engaged and that no tool or rag is left in the engine bay.
2. If you disconnected the battery, reconnect it as the vehicle manufacturer describes.
3. Make sure that the vehicle stands in the open air and cannot move, with the parking brake applied and the transmission in neutral or in park; never run an engine in a garage or another closed space. Then start the engine. It should start readily and idle evenly.
4. If it misfires or the engine warning light flashes, switch off and check the coil, the ignition leads, the wires and the connectors disturbed during the repair: each must be fully seated and in its own place. If the fault remains, have it diagnosed before driving. A warning light that stays on steadily while the engine runs evenly may come from the fault stored before the repair, which the next step deals with.
5. After checking the repair, clear the stored faults that relate to it as the vehicle manufacturer's diagnostic procedure describes. A fault that returns, or one unrelated to the coil, needs its own diagnosis.
6. A coil fault can show only under load. Once the engine runs evenly at a standstill, check on a short drive that it pulls evenly and that the engine warning light stays off. If the misfire returns, stop where it is safe and have the fault diagnosed before you drive on. Afterwards, read the fault codes once more if the vehicle stores them.

Mistakes to avoid

• Replacing a coil without confirming that it causes the misfire
• Driving on with a misfire
• Pulling a connector by its wires or an ignition lead by its cable
• Levering out a stuck coil and tearing its boot
• Leaving oil or water in the plug well
• Fitting a new coil onto a worn or damaged spark plug
• Overtightening the coil's fixings
• Mixing up ignition leads, wires or connectors
• Fitting a coil that is not listed for the engine because it happened to fit the mounting