Repair guide

How to replace spark plugs

Spark plugs wear a little with every spark. Replacing them is a job many owners can do with hand tools, provided the engine is cold, the plugs are the ones specified for the engine, and each plug is tightened to its specification. This guide describes the job on petrol engines in general; where your vehicle manufacturer's instructions differ, follow them.

Published

Safety

Secure the vehicle before you start: on level ground, with the parking brake applied and the ignition off. If the vehicle has to be raised, support it on axle stands, never on a jack alone. Where this guide and your vehicle manufacturer’s instructions differ, follow the manufacturer. If a step is unclear, leave the job to a workshop.

Find the parts for your vehicle

Choose your vehicle to see what is listed for it as Spark Plug: OE numbers, aftermarket part numbers and each part’s specifications as its supplier states them.

Tools

  • Spark plug socket that fits the plugs and the plug wells, with a rubber insert or magnet to hold the plug
  • Ratchet with an extension bar
  • Torque wrench or wrenches that cover the tightening values of the plugs, the coil fixings and the parts removed for access
  • Angle gauge, where the fitting procedure prescribes a tightening angle
  • Gap gauge, feeler or wire type
  • Compressed air or a vacuum cleaner, and lint-free cloths to soak up liquid and to cover openings
  • Eye protection
  • Sockets, screwdrivers or bits for the coil fixings, for the parts removed for access and, if the battery has to be disconnected, for its terminals; the vehicle manufacturer's procedure says which
  • Ignition coil puller, on engines whose manufacturer specifies one
  • Diagnostic tool to read and clear fault codes, if a fault is stored

When to replace spark plugs

Replace spark plugs at the interval in your vehicle's service schedule. The interval depends on the engine and on the electrode material of the plug, so no single figure holds for every car.

As a rough guide: where a supplier in our catalogue states a change interval for its spark plugs, it is 30,000, 60,000 or 90,000 km, depending on the plug. The vehicle's page shows the interval beside the part number where its supplier states one. Your vehicle's service schedule takes precedence.

Between services, worn plugs can show themselves in these ways:

  • The engine misfires or idles unevenly
  • It is slow to start, especially when cold or damp
  • Acceleration is hesitant and fuel consumption rises
  • The engine warning light comes on and a misfire fault is stored

These symptoms have other causes too, such as a failing ignition coil, a damaged plug lead or an air leak. If only one cylinder misfires, check its coil and connector as well before blaming the plugs.

Choose the right plugs

A spark plug must match the engine in thread, reach, seat type, heat range and electrode gap. A plug that screws in is not necessarily the right one: with the wrong reach, a piston or valve can strike the plug, and the wrong heat range can damage the engine.

Select your vehicle with the selector on this page to see the part numbers listed for its engine, each with the specifications its supplier states, such as electrode gap, tightening torque, thread and hex size. When the plugs are due or worn, replace all the plugs of the engine, each with the plug specified for its position; a single damaged plug can be replaced on its own. Most engines take the same plug in every cylinder, but some do not.

All spark plug OE numbers in the catalogue

Before you start

Work on a cold engine unless the vehicle manufacturer's procedure says otherwise. A plug removed from a hot aluminium cylinder head can tear out the thread, and hot parts cause burns.

  • 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 disconnect the battery if those instructions call for it.
  • If a fault is stored, read and record the fault codes before you begin the repair or disconnect the battery.
  • Find out how the engine cover and the ignition coils or plug leads come off on your engine. On some engines the intake manifold or other parts are in the way, and the job is considerably longer.
  • Before you remove anything, have the new plugs, a torque wrench and the applicable tightening specification to hand. The vehicle's page lists the specifications suppliers state beside each part number; where the vehicle manufacturer gives a value, that one takes precedence.
  • Work on one cylinder at a time, so that each coil or lead goes back to its own cylinder and only one plug hole is open at once. If several plugs have to be out at the same time, label the coils and leads and cover every open plug hole.

Remove the old plugs

  1. Remove the engine cover and anything else that blocks access to the ignition coils or plug leads. If the intake manifold or other intake parts have to come off, cover the exposed openings so that nothing can fall into the engine.
  2. On engines with a coil on each plug, release the catch of the coil's electrical connector and unplug it; do not pull on the wires. On engines with plug leads, pull the lead off by its boot, never by the cable.
  3. On engines with a coil on each plug, undo the coil's fixing, if it has one, and draw the coil straight out of the plug 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. On other ignition layouts, such as a shared coil pack, follow the manufacturer's procedure for reaching the plugs.
  4. Before loosening the plug, clear its well completely: blow or vacuum out all loose dirt, wearing eye protection when you use compressed air, and soak up any oil or water standing in the well. No other plug hole may be open or uncovered while you do this: nothing must be able to fall into a cylinder.
  5. Seat the spark plug socket squarely on the plug and loosen it anticlockwise with steady force. Keep the extension in line with the plug: a tilted socket can crack the insulator.
  6. If a plug is very tight, stop and do not force it. Look up the vehicle manufacturer's procedure for a seized plug before you try again: some plug designs break when they are worked loose in the usual way, and a plug broken off in the cylinder head is a workshop repair.
  7. Lift the plug out with the socket and lay the old plugs out in cylinder order.

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. Find its source and have any leak repaired, since oil can damage the coil's seal and cause misfires.

Read the old plugs

The firing end of a plug can show how its cylinder has been running. The appearances below are pointers, not a diagnosis. Compare the plugs with one another: an unexpected difference may point to a fault in that cylinder. Allow for engines whose plugs differ by position.

  • Light grey or tan deposits and evenly worn electrodes: usually normal wear
  • Dry black soot: can point to a rich mixture, many short journeys or a weak spark
  • Wet, oily deposits: can point to oil entering the combustion chamber, for example past valve stem seals or piston rings
  • A white, blistered insulator or melted electrodes: signs of overheating, for example from the wrong heat range, a lean mixture or an ignition fault. Have the cause found before driving on.
  • A cracked or chipped insulator: can point to mechanical damage or engine knock

If part of an insulator or an electrode is missing, do not start the engine. The fragment may be in the cylinder: have the cylinder inspected and the cause found first.

Fit the new plugs

  1. Check each new plug against the specification for your engine: thread, reach, seat and heat range. Comparing it with the old plug is a useful second check, but the plug that was fitted may itself have been the wrong one. A plug with a sealing ring must have it in place and undamaged.
  2. Check the electrode gap against the value specified for the plug in your engine, with a gap gauge and only in a way the plug's maker permits. Never touch or lever against a fine precious-metal electrode. Adjust a gap only if the plug's maker permits it for that plug, and by the method the maker gives; where it does not, exchange a plug whose gap is wrong.
  3. Inspect the thread and the sealing seat in the cylinder head. They must be clean and undamaged, with no remains of an old sealing ring. Clean or repair them only by a method the vehicle manufacturer approves, keeping debris out of the cylinder; if the thread is damaged, stop and have it repaired.
  4. Fit the thread in the condition the tightening specification assumes, dry or lubricated: a torque value given for a dry thread over-tightens a lubricated one. Do not add grease or anti-seize paste unless the plug's maker or the vehicle manufacturer calls for it.
  5. Start each plug by hand, using the socket and extension without the ratchet, and turn it until it seats. It must turn freely. If it binds, withdraw it and check its alignment. If it still will not turn freely, stop and have the plug and the thread in the cylinder head checked.
  6. Before tightening, have the tightening specification that applies to the plug's seat type and your engine. Follow the vehicle manufacturer's instructions; where they give no specification, follow the plug maker's complete fitting procedure for that plug. Do not guess a value.
  7. Tighten the plug as that specification prescribes: to a torque with the torque wrench, through a tightening angle, or both. A plug that is too loose can overheat, and one that is too tight can break or damage the cylinder head.
  8. Check the boot of the coil or plug lead for cracks, burn marks and oil, and replace a damaged one. Seat the coil or plug-lead boot fully as the vehicle manufacturer describes. Where a coil fixing was removed, refit it to the manufacturer's tightening specification, and reconnect the electrical connector until its catch engages.
  9. Repeat for the remaining cylinders. 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. Start the engine. It should start readily and idle evenly.
  4. If it misfires or the engine warning light comes on, switch off and check the coils, plug-lead boots and connectors disturbed during the repair: each must be fully seated and routed to its own cylinder. If the fault remains, have it diagnosed before driving.
  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 plugs, needs its own diagnosis.

Mistakes to avoid

  • Working on a hot engine
  • Letting dirt fall into an open plug hole
  • Starting a plug with the ratchet and cross-threading it
  • Tightening by feel instead of to the specification
  • Lubricating a thread and then using a torque value meant for a dry thread
  • Fitting a plug with the wrong reach, seat or heat range because it happened to screw in
  • Mixing up coils or leads between cylinders
  • Fitting a plug that has been dropped: its insulator may be cracked or its gap closed

What the suppliers state

From the 904 part numbers listed as Spark Plug on our vehicle pages (1901 OE numbers, 10224 vehicles). The values differ from part to part: use the one stated for the part you fit, and your vehicle manufacturer’s value where it gives one.

SpecificationParts stating itValues, with the number of parts
Electrode gap637
  • 0.8 mm × 106
  • 1 mm × 95
  • 0.9 mm × 91
  • 0.7 mm × 58
  • 0,7 mm × 50
  • 0,8 mm × 40
  • 0,9 mm × 31
  • 1.1 mm × 26
  • 0.75 mm × 23
  • 1,1 mm × 16
  • 0.65 mm × 12
  • 0.6 mm × 12
  • +24
Tightening torque326
  • 25 Nm × 124
  • 28 Nm × 80
  • 23 Nm × 53
  • 20 Nm × 42
  • 15 Nm × 14
  • 17 Nm × 5
  • 30 Nm × 5
  • 22 Nm × 2
  • 18 Nm × 1
Width across flats132
  • 16 mm × 68
  • 14 mm × 37
  • 20,8 mm × 26
  • 17,5 mm × 1
Wrench Size (Metric)33
  • 16 mm × 21
  • 21 mm × 12
Thread124
  • M 14 x 1,25 × 81
  • M 12 x 1,25 × 43
Outside Thread35
  • M14 x 1,25 × 34
  • M12 x 1,25 × 1
Sealing seat132
  • flat-mouldable × 118
  • flat and solid × 12
  • conical × 2
Recommended change interval36
  • 30000 km × 21
  • 60000 km × 12
  • 90000 km × 2
  • 60,000 km × 1

Questions and answers

How often should spark plugs be replaced?

At the interval your vehicle manufacturer gives for the engine. It differs widely with the electrode material and the engine, so follow the service schedule rather than a general rule. As a rough guide, the change intervals that suppliers state for spark plugs in our catalogue are 30,000, 60,000 and 90,000 km, depending on the plug.

Can I replace only one spark plug?

A single damaged plug can be exchanged, but the plugs of an engine usually wear at a similar rate. At a service, replace all of them, each with the plug specified for its position.

Do new spark plugs need to be gapped?

Most are supplied with the gap already set. Check it against the stated value all the same: a plug can suit several engines with different gaps, and it may have been knocked in transit. Adjust a gap only if the plug's maker permits it for that plug, and never lever against a fine precious-metal electrode.

Should I put anti-seize paste on the thread?

Only if the plug's maker or the vehicle manufacturer calls for it. A torque value given for a dry thread over-tightens a lubricated one, so the thread must be in the condition the tightening specification assumes.

What happens if a spark plug is too tight or too loose?

A loose plug passes its heat poorly to the cylinder head, can overheat and can work free. An over-tightened plug can stretch or break, crack its insulator, or strip the thread in the cylinder head.

Do diesel engines have spark plugs?

No. A diesel engine ignites its fuel by compression. Most have glow plugs to help cold starting, which are a different part with a different procedure.

Which spark plugs fit my car?

Select the make, model and engine on this page. The vehicle's page lists the part numbers that suppliers state for that engine, each with the specifications its supplier gives.