Replace Spark Plugs and Ignition Coils: interactive 3D repair guide

All 10 steps

1. 1Bonnet up, engine cover off

   Pull the bonnet release inside the car, lift the bonnet’s front edge, free its safety catch and raise it. Swing the stay up out of its clip and put its end into the hole in the bonnet’s underside, then let the bonnet rest on it. The plastic engine cover is held only by four rubber grommets on ball studs: lift it straight up at each corner and set it aside.

2. 2Unlock and unplug the four coil connectors trap

   On each coil’s connector, first slide the red lock back towards the wires, out from under the release clip. Only then press the clip’s back end down and pull the connector straight off the coil by its body, never by its wires. Do all four, cylinder 1 at the timing belt end first, and lay each connector down in front of its coil, clear of it. Avoid this trap: Push hard on the release clip while the red lock is still slid home and the clip has nowhere to go: its thin plastic tab snaps off. The connector still comes off, but it will not latch again; held only by friction, the engine’s shake works it loose and that cylinder misfires. A new connector means cutting into the wiring. Slide the lock back first, then press the clip.

3. 3Coil bolts out, coils twisted out

   Undo each coil’s 10 mm bolt with the ratchet and put it in its own well of the magnetic tray. Then grip each coil by its head, twist it a little to free its rubber boot from the plug, and pull it straight up out of its well. Lay the coils on the cart in order, 1 to 4.

4. 4Blow the wells clean, old plugs out

   Blow each well out with compressed air first, so no grit falls into the cylinder when its plug comes out. Then put the plug socket on the extension down the well onto the plug’s hex, fit the ratchet and turn anticlockwise until the plug cracks loose. Take the ratchet off, spin the plug out by hand on the extension, and lift it out held in the socket’s rubber insert. Lay the old plugs out in order.

5. 5Check the new plugs: type and gap

   Hold a new plug beside an old one: the same thread, the same reach, the same hex. Check the new plug’s gap with a wire-type gauge: here the 1.0 mm (0.040 in) wire should slide between the electrodes with a light drag. Iridium plugs come pre-gapped; never lever against the fine centre electrode, and if a gap is out, use another plug rather than bending it. Put no anti-seize or oil on the thread.

6. 6Start each new plug by hand trap

   Push a new plug into the socket until its rubber insert grips it, lower it straight down the well and turn the extension clockwise by hand, never with the ratchet, for at least two full turns, until the plug runs down freely and its gasket meets the head. If it binds at once, it is crossing its thread: back it out and start again. Do all four. Avoid this trap: Fitted in a plain deep socket with no rubber insert or magnet, the new plug slides out as the socket tips over the well and falls to the bottom, landing on its tip. The ground electrode is knocked in, closing the 1.0 mm gap to about 0.4 mm, and the ceramic nose can crack where you cannot see it. A dropped plug should not be fitted: Ford and Bosch both say to throw it away. Use a socket that holds the plug.

7. 7Torque each plug to 27 Nm trap

   Set the torque wrench to the figure for your plugs and head: here 27 Nm, within the 25–30 Nm NGK gives for a 14 mm spark plug with a gasket seat in an aluminium head. Put the socket down onto the plug’s hex, fit the wrench to the extension and pull evenly until it clicks; stop there. A new gasket plug turns about half a turn as its gasket crushes. Do all four. Avoid this trap: Done up with a long breaker bar “to be sure”, the plug goes far past its 27 Nm. The steel plug does not give; the aluminium thread in the head does. It strips, the plug turns on and on without tightening, and comes out with aluminium in its threads. The repair is a thread insert in the head, a garage job. Use a torque wrench and stop at its click.

8. 8Coils back in, bolts snug

   Lower each coil straight down its own well, connector port to the front, and push it until its boot snaps onto the plug and its ear sits flat. Take each bolt from its tray well, start it by hand and run it down snug with the ratchet: about 10 Nm (Ford gives 10 Nm, Toyota 9 Nm), so a short firm pull, no more.

9. 9Connectors on and locked

   Push each connector straight onto its coil until the clip clicks over its catch, then slide the red lock forward, home under the clip, and give the connector’s body a gentle tug: a locked connector does not move.

10. 10Cover on, bonnet shut, start it

    Lay the engine cover over the coils, its grommets over the ball studs, and press each corner down until it pops on. Lift the bonnet off its stay, clip the stay down and lower the bonnet to about 25 cm, then let it drop to latch. Start the engine: it should idle smoothly, with no engine warning light.

What you need

• Spark plugs, a set of four · the type and gap your handbook gives
• 3/8-inch ratchet and a 10 mm socket · for the coil bolts (check your car’s size)
• Spark plug socket, 5/8 in (16 mm) · with a rubber insert or a magnet; a 14 mm socket for 12 mm plugs
• 6-inch extension · 3/8-inch drive
• Torque wrench, about 10–60 Nm · 3/8-inch drive
• Wire-type gap gauge · not a coin gauge
• Compressed air · a can or a blow gun
• A magnetic parts tray · a well for each bolt
• The car’s handbook · its plug type, gap and torque

Questions

Safety

Work on a cold engine, parked with the handbrake on and the engine off, the key away from the car. Keep fingers clear of the radiator fan, which can start by itself while the engine is warm, and of the belt. Fit only plugs of the type and gap your handbook gives, start each one by hand, and tighten it with a torque wrench.

• Jacks and axle stands: never get under a car, or put a hand or a foot under it, while a jack alone holds it up. Lift it only at its jacking point, on firm, level ground with the wheels chocked, and set it on axle stands rated for its weight for any work under it.
• Car batteries: a lead-acid battery gives off explosive hydrogen. No sparks, flames or smoking near it, eye protection on, and the last jump lead clamped to bare metal away from the battery.
• Hot parts and fumes: brakes, exhausts and engines stay hot long after a drive, and a running engine fills a closed garage with carbon monoxide: let parts cool, and keep the door open.

About this guide

3D Spatial Reconstruction: models are engineered from factual measurements to show the parts, the fixings and the order of the job before you start. Always check the handbook of your own car.

The car is a generic five-door of our own design and its engine a generic transverse four-cylinder of about 2.0 litres, also our own: an aluminium head with a coil sitting on each plug, not any one maker’s engine. The order is the one plug and car makers give: a cold engine; each connector unlocked before it is unplugged (Ford’s procedure: slide the cover over its locking tab back, then press the tab); each coil’s bolt out and the coil twisted out; the wells blown clean before the plugs come out; the new plugs checked, started by hand and torqued; the coils, their bolts and connectors back and locked.

Where the real job differs from a quick brief: a feeler gauge is the brief’s tool for the gap, but NGK says to measure its plugs with a round wire-style or pin gauge, Bosch not to force a feeler gauge between the electrodes, and NGK Australia and Denso say their precious-metal plugs come pre-gapped and are not to be adjusted (Denso: never touch the centre electrode). So the 3D checks the gap with a wire gauge and bends nothing. The brief’s 5/8 in (16 mm) or 14 mm socket: a 14 mm spark plug takes the 16 mm socket, the newer 12 mm plugs a 14 mm socket (NGK’s catalogue). The coil bolts take a 10 mm socket, as the brief says.

The figures are ours and typical, not one car’s. This engine takes 14 mm spark plugs with a gasket seat and a 16 mm hex, gapped at 1.0 mm (0.040 in): Toyota gives 1.0–1.1 mm for its 1.8 litre’s plugs. NGK’s chart gives 25–30 Nm (18.0–21.6 lb-ft) for them in an aluminium head, and, with no torque wrench, about half to two thirds of a turn past finger tight on a new gasket; we use 27 Nm. Car makers’ own figures come first and can be lower: Toyota gives 20 N·m for its 1.8 litre’s 12 mm plugs, Ford 12 Nm for its 2.3 litre’s. The coil bolts take about 10 Nm (Ford 10 Nm, Toyota 9 N·m). No anti-seize: NGK says its plating already stops seizing and that anti-seize changes the torque by up to 20 percent; Bosch says to use no grease, oil or anti-seize.

The price on the card: four plugs at $8.99 each, Summit Racing’s price for NGK’s Iridium IX LFR6AIX-11 (a 14 mm thread, 26.5 mm reach, 5/8 in hex), read on 27 October 2026; check the type your handbook gives. We found no garage price for the whole job that we could read, so the card gives none. The time is our estimate for a four-cylinder like this one. The 3D runs faster than real hands do.

Sources: NGK: 5 things you should know about spark plugs (gap with a wire or pin gauge, move only the ground electrode, no anti-seize, up to 20 percent over-torque), NGK: set the gap when installing new plugs (no pressure on a platinum or iridium centre electrode), NGK: correctly install spark plugs (finger tight until the gasket meets the head; the torque chart), NGK: spark plug installation (change plugs in a cool engine), Niterra (NGK) UK: over-torque sheet (14 mm gasket plugs 25–30 Nm in aluminium; no lubricant), NGK Australia: installation (½–⅔ of a turn past finger tight for a new gasket plug), NGK Australia: FAQ (precious-metal plugs’ gaps not to be adjusted), Denso: spark plug installation (iridium plugs need no gapping in most cases; never touch the centre electrode; torques), Bosch: spark plug change, the most common mistakes (cool engine, no grease or anti-seize, no reuse of a dropped plug; hosted by a distributor), Bosch advice on spark plug fitting (start by hand as far as it will go; a rubberised or magnetic socket), Ford workshop manual, 2.3 litre: ignition coil-on-plug removal (slide the cover over the locking tab back, press the tab; coil bolts 10 Nm), Ford workshop manual, 2.3 litre: spark plugs (12 Nm; a dropped plug is discarded), Toyota repair manual, 2ZR-FXE: spark plugs (gap 1.0–1.1 mm new, 20 N·m), Toyota repair manual, 1NZ-FXE: ignition coils (9.0 N·m; replace a coil that has taken a knock), MotorWeek, Goss’ Garage: spark plug removal (start by hand for two full turns; a piece of hose for deep wells), Summit Racing: NGK Iridium IX LFR6AIX-11, 14 mm thread, 5/8 in hex, $8.99 each (October 2026).

The 3D models, drawings and text are our own. The car shown is a generic one of our own design, not any one maker’s model.