The ignition coil delivers high voltage to the spark plugs so the cylinders can fire. A lot of resistance and heat can build up inside the coil, causing it to short. Many of the older single unit coils were filled with cooling oil to keep their temperature down; but when they became too hot, they could leak or explode. Today's coils come in coil packs that fire spark plugs individually from overhead coil on plug (COP) units or have one-piece designs that still use spark plug wires. When it comes to removing the coil pack, you need to know which model and style are on the vehicle.
Shift the vehicle into park or neutral and set the emergency brake. Raise the hood. Disconnect the negative battery cable from its post with a socket and wrench. Hook up a shop light to an electrical socket. Refer to a shop manual specific to your vehicle's model-year for the location and design of the coil pack. There will either be a COP design or a one-piece coil pack that has multiple plug terminals. Locate all of the parts that need to be removed to access the coil pack.
Remove the plastic engine cowl (plenum cover) on the top of the engine with the socket and wrench. Make note of the cowl bolt locations so you can reinstall them later. If you have a straight four-cylinder engine with four overhead coil packs attached to the top of the spark plugs, you might have to remove the air intake tube on the throttle body (the big black hose that goes to the air cleaner box). Remove the hose by loosening the hose clamps with a flat head screwdriver.
Pull off any braces or brackets that cover the coil pack wiring harness with a screwdriver or socket and wrench. The coil on plug coil packs sit on top of the valve cover, connected to it by a rubber boot. There will be one coil pack per plug. Use the flat head screwdriver to depress the small plastic tang on the coil pack wire connector then pull the small clip up. Pull the wire connector out of its jack connection.
Use the flat head screwdriver to lift the rubber coil pack boot up at its ends, popping it free from the valve cover lip. Use insulated spark plug pliers or your hand to pull the coil pack tube straight up and off of the spark plug. Do not bend it side to side while doing so -- twist gently and pull it straight up. If you need to replace all four coil packs, perform the same removal technique for each one.
Locate the one-piece coil pack in the engine if your vehicle is so equipped. Refer to the repair manual for its location. Most coil packs will be mounted to the intake manifold toward the rear of the engine. If unsure of its location, trace the plug wires from the spark plug location back toward the rear of the engine. Remove any heat cover or plenum that covers the coil pack with the socket and wrench. Remove each plug wire from the coil pack towers manually or with spark plug pliers. Keep the wires in order for proper reinstallation.
Unclasp the small plastic retainer clip from the harness to the coil pack. Push the small plastic tang in with a flat head screwdriver and lift the retainer clip then pull the connector straight out. Note the bolts holding the coil pack to its mount; there might be three or four of these. The bolts could be standard hex head or the Torx design. Use the proper socket or Torx head bit to loosen and remove the coil pack mounting bolts. Pull the coil pack out of the engine compartment.
Most one-piece coil packs have the wire numbers stamped on the top of their face so you can reinstall them if you mix them up. Many coil packs have heat insulators under them. After removing the coil pack, pick up the insulator for reuse if you have to install a new coil pack. Replacing COP coil packs for the V8 or V6 will be the same procedure for the four-cylinder engine although you might not have to remove a plenum on the V8 engine design since the valve covers are out in the open.