Most drivers never notice spark plugs going bad. It happens slowly: a slightly rougher cold start here, a small drop in fuel economy there. On iridium plugs, that gradual wear can go 20,000 miles before it’s obvious enough to register.
Skipping the replacement interval has a specific consequence. A misfiring cylinder sends unburned fuel into the exhaust system, and that fuel ignites in the catalytic converter. A $40 set of plugs ignored long enough turns into a $1,500 cat replacement. That’s the actual cost of deferring this.
Quick answer
Copper plugs: every 20,000-30,000 miles. Platinum plugs: every 60,000 miles. Iridium plugs: every 80,000-100,000 miles. Check your owner’s manual — most vehicles from 2010 onward use iridium or platinum and specify 60,000-100,000-mile intervals. If your car is running rough or throwing a misfire code, check the plugs regardless of mileage.
Why it matters
As a spark plug wears, the electrode gap grows wider from normal erosion. Thousands of high-voltage arc events per minute slowly eat the electrode material. A wider gap means the ignition coil has to generate higher voltage to bridge it. Eventually the coil hits its limit and the cylinder starts misfiring, first intermittently, then on every cycle.
That misfiring cylinder dumps unburned fuel into the exhaust. The catalytic converter, which runs at 800-1,500 degrees F under normal operation, gets hit with raw fuel and burns even hotter. A few tanks of gas driving like this and the catalyst substrate cracks. Replacement runs $800-$2,500 depending on the vehicle.
Signs it’s due
- Mileage interval is approaching or past. This is the primary indicator. Don’t wait for symptoms on a modern plug.
- Rough idle when cold. The engine stumbles on startup and smooths out after warming. A classic sign of plugs that are past their service life.
- Hesitation or stumbling under acceleration. The engine feels sluggish pulling away from a stop or merging onto a highway.
- Check engine light with a P030X code. P0301 through P0308 identify misfires on specific cylinders. Pull the code before replacing parts randomly.
- Noticeable fuel economy drop over time. The ECU compensates for weak spark by adding fuel.
- Hard starting. Engine cranks longer than normal before firing.
How to do it
- Let the engine cool completely. Working on a hot engine risks burns and can damage aluminum cylinder head threads when installing cold plugs.
- Disconnect the ignition coil from the first plug. On coil-on-plug (COP) systems, unclip the connector and remove the retaining bolt. The coil pulls straight out.
- Use a spark plug socket (5/8″ or 16mm, with rubber insert) to remove the old plug. Turn counterclockwise. If it’s stuck, apply penetrating oil and wait. Forcing a stuck plug can snap it in the head, which is a much worse repair.
- Look at the old plug before tossing it. White chalky deposits mean the engine is running lean. Thick black soot means rich running or oil getting past the rings. That information matters before you close everything up.
- Check the gap on new plugs against your vehicle’s spec. Install finger-tight, then torque to specification, typically 18-22 ft-lb in aluminum heads. Do not overtighten.
- Reinstall coils and reconnect connectors. Start the engine. Clear any stored misfire codes with an OBD2 scanner after completing all cylinders.
DIY or shop
- DIY difficulty. Easy to moderate. An accessible inline 4-cylinder is a beginner job. V6 and V8 engines where rear plugs require removing the intake manifold are best left to a shop.
- DIY cost. $30-$80 for a full set of iridium plugs. Use OEM-spec plugs or equivalent. The few extra dollars per plug matter more than they seem when you’re looking at 60,000-mile service intervals.
- Shop cost. $150-$350 for a straightforward 4-cylinder. Up to $500-$600 on some transverse-mounted V6 applications with difficult access.
Sources
EPA — On-Board Diagnostics & Ignition System Emissions
DOE — Keep Your Car in Shape: Fuel Economy Tips
