Secondary Ignition

More Information

Secondary ignition components are responsible for delivering high-voltage spark from the distributor or coil to the combustion chamber at precisely the right moment. Spark plugs are the most frequently replaced item here — most standard copper plugs are due every 30,000–45,000 miles, while iridium and platinum plugs can go 60,000–100,000 miles depending on the application. Ignition condensers and contact sets are primarily found on older points-based distributor systems and should be replaced whenever the points are serviced. Diesel glow plug wiring harnesses and power straps are specific to compression-ignition engines and often corrode or crack with age, causing hard starts in cold weather. When buying spark plugs, match the heat range, thread reach, and electrode gap to your engine's spec — the wrong plug can cause pre-ignition or fouling. For tube seals, fitment is highly engine-specific, so verify by year, make, model, and engine code before ordering.

Signs you need replacement

• Rough idle or engine misfire: A worn or fouled spark plug loses its ability to ignite the air-fuel mixture consistently, causing a noticeable stumble at idle or a P030X misfire code on OBD-II vehicles.
• Hard starting or long cranking on a diesel: A failed glow plug wiring harness or corroded power strap prevents glow plugs from reaching operating temperature, making cold starts difficult or impossible below 40°F.
• Oil smell inside the cabin or oil fouling on spark plugs: A cracked or worn spark plug tube seal allows oil to leak down into the plug tube, contaminating the plug and causing misfires — common on Honda, Toyota, and BMW engines with overhead cam designs.
• Poor fuel economy or sluggish acceleration: Spark plugs worn beyond their service life require a higher firing voltage, which strains the coil and reduces combustion efficiency — often showing up as a 10–15% drop in fuel economy before other symptoms appear.
• Engine points-style ignition misfiring or inconsistent timing: A failed ignition condenser on a vintage points distributor causes arcing across the contact points, resulting in a weak spark, rough running, and accelerated point wear.
• Check engine light with lean or random misfire codes: A cracked spark plug non-fouler or a plug misfiring due to incorrect heat range can trigger codes P0300–P0308, especially under load at highway speeds.

Frequently asked questions

• How often should spark plugs be replaced, and does the electrode material matter? Copper plugs typically require replacement every 30,000–45,000 miles. Single-platinum plugs extend that to around 60,000 miles, and double-platinum or iridium plugs can last 80,000–100,000 miles. Always follow your OEM service interval — some modern engines with coil-on-plug setups are spec'd for iridium only.
• Are OEM spark plugs worth the premium over aftermarket options? For most late-model vehicles — particularly those with direct injection or specific heat range requirements — OEM or OEM-equivalent plugs (NGK, Denso, Bosch as original suppliers) are the safer choice. Quality aftermarket plugs from those same brands work well too. Avoid cheap no-name plugs; electrode gap consistency and heat range accuracy vary significantly.
• What else should I replace when doing a spark plug service? On high-mileage engines, replace spark plug tube seals at the same time — especially on DOHC four-cylinders where oil intrusion into the plug wells is common. If you're servicing a distributor-equipped vehicle, replace the ignition condenser and contact set together. Labor to access plugs on transversely mounted V6 engines can run $150–$300, so bundling related parts saves money.