Ignition and tune-up
On most engines built since the late 1990s, each cylinder has its own coil sitting on top of the spark plug, and the engine computer decides when each one fires. Older engines use a coil pack with plug wires, or a single coil feeding a distributor. A tune-up used to mean points, timing, and a carburetor adjustment. Today it usually means new spark plugs, a look at the coils and boots, and the filters and fluids that are due at the same mileage.
Common parts
| Ref | Part group |
|---|---|
| IG-01 | Spark plugs in copper, platinum, double platinum, and iridium designs |
| IG-02 | Coil-on-plug ignition coils and coil boots |
| IG-03 | Coil packs and spark plug wire sets |
| IG-04 | Distributor caps, rotors, and complete distributors |
| IG-05 | Ignition control modules and ignition switches |
| IG-06 | Glow plugs and glow plug controllers for diesel engines |
Finding the right fit
Use the plug the engine maker lists, or a direct cross-reference to it. Heat range, reach, thread size, seat type, and gap all matter, and a plug that fits the hole can still be wrong for the engine. Some iridium and platinum plugs come pre-gapped and can be damaged by a gap tool. Coils differ between engines that look alike, and some engines use different coils for different cylinder banks. A plug or coil code stored in the computer tells you which cylinder to look at. Swapping the coil to another cylinder and seeing whether the misfire follows it is a simple and widely used test.
Symptom to part
| What you notice | Usually points at | Check first |
|---|---|---|
| Flashing check-engine light | Active misfire | Reduce load and have codes read soon |
| Misfire only in rain | Cracked coil boot or plug wire | Look for carbon tracks with a light in the dark |
| Misfire that moves with the coil | Ignition coil | Swap coils between cylinders and re-read the code |
| Long cold cranks on a diesel | Glow plugs or their controller | Resistance test on each glow plug |
Spark plug electrode types
Copper core
Short service life; still specified for some older and performance engines
Platinum
Longer life; common on engines from the 1990s and 2000s
Double platinum
Made for waste-spark systems where current flows both ways
Iridium
Fine center electrode and the longest service intervals on many newer engines
Before you order
Start with the year, make, model, and engine, then narrow it down with the trim, drive type, and any option the part depends on. The VIN is the most reliable way to do that, because many parts catalogs and dealer counters decode it to the exact build. If the old part is in hand, its original-equipment number is the best cross-reference. Aftermarket brands publish their own numbers and usually list the original number they replace. Two listings that both say they fit can still differ, so compare the photos, connectors, mounting points, and the notes in the listing. The find the right part guide covers this step, and the all makes reference lists each maker’s official parts catalog.
Once you know the part number, compare the same number across stores. Look past the first price at the core charge, shipping, the return window, and what the warranty covers and who honors it. A remanufactured part, a new aftermarket part, and an original-equipment part can each be the right choice, but they are not the same purchase. The compare prices and warranties guide walks through what to read. Prices on PartIndex carry the date they were recorded, so confirm the current price and fitment with the retailer before you buy.
Remove plugs from a cool engine, especially one with an aluminum head, so you do not damage the threads. Start each new plug by hand. Ignition systems carry high voltage, so do not touch coils or wires while the engine is running.