Spark plugs are the most underrated part in a car. They cost $5-25 each, last for years, and when they go, they destroy your $1500 catalytic converter. This guide covers when to replace, how to choose, and why ignoring a bad plug is a false economy.
What plugs do
Every cylinder in your engine has a plug that creates the spark that ignites the air-fuel mixture. No spark = no combustion = engine misfiring, losing power, burning more fuel, and dumping raw gasoline into your catalytic converter — which kills it over 1000-3000 km.
The three plug types
Copper (standard)
Cheapest ($3-6 each), lifespan 30 000-50 000 km. Great conductivity, but the electrode wears fast. OK if your manual specifies it and you don't mind frequent service.
Platinum
Platinum electrode, harder. Lifespan 80 000-120 000 km. Price: $8-15 each. Good cost/longevity compromise for most vehicles.
Iridium
Top tier. Very fine iridium electrode, more efficient ignition, lifespan 150 000-200 000 km. Price: $12-25 each. Required on many modern turbo and direct-injection engines. If your manufacturer specifies iridium, don't install platinum or copperto save money — you'll cause engine-management issues.
When to replace
Always follow your owner's manual interval first. If you don't have it:
- Copper: every 30 000-50 000 km
- Platinum: every 100 000 km
- Iridium: every 150 000 km or more
Symptoms of a plug near end of life:
- Engine hesitates or misfires under acceleration
- Longer cranking than before, especially cold
- Rough idle
- Fuel consumption creeping up with no obvious cause
- "Check engine" light with P0300-P0308 misfire codes
- Gasoline smell from exhaust after a cold start
Why an $8 plug can cost you $400
A worn plug causes misfires. Each misfire sends unburned gasoline into the exhaust. That gas hits your catalytic converter at 400°C, re-ignites inside it, and melts the internal ceramic honeycomb. A new cat costs $400-1500 depending on the vehicle.
The occasional misfire won't kill a cat — the real problem is when you ignore the check-engine light and drive for months. That's where the false economy costs 100x more than the preventive fix.
Common mistakes to avoid
- Ignoring the gap — plugs come pre-gapped, but verify with a gap gauge. Wrong gap = rough engine.
- Over-torquing — an over-tightened plug cracks the head or deforms the electrode. Use a torque wrench and respect the manual spec (typically 25-30 Nm).
- Mixing brands/types — install all 4 (or 6, or 8) plugs of the same brand and type at the same time.
- Forgetting anti-seize — a very thin coat on the threads prevents seizure over 100 000 km. Too much affects torque, so use sparingly.
Need plugs for your car?
We stock plugs for popular makes in Quebec. Call us with your make, model, and year — we check what the manufacturer specifies and give you a price. See also our auto parts in Laval page or our starting-problem diagnostic guide if your issue is actually electrical.