What Are Spark Plugs?
Spark plugs are small but critical components in every petrol engine. Their job is to deliver a precisely timed electrical spark — generated by the ignition coil — that ignites the compressed fuel-air mixture inside each cylinder at exactly the right moment.
Diesel engines do not use spark plugs (they use compression ignition and glow plugs for cold starting). But every petrol-powered vehicle on New Zealand roads — from a Suzuki Swift to a Toyota Hilux petrol — depends on its spark plugs firing correctly.
How Do Spark Plugs Work?
The ignition coil generates a high-voltage pulse (typically 12,000–45,000 volts) that travels through the ignition lead or directly to the spark plug. Inside the plug, this voltage jumps across a small gap between the centre electrode and the ground electrode, creating a spark that ignites the fuel-air mixture.
The gap size is critical — too small and the spark is weak; too wide and it may not fire at all. As electrodes wear over time, the gap widens, requiring more voltage to jump. Eventually, the ignition system can't reliably fire the plug, causing misfires.
Types of Spark Plugs
| Type | Electrode Material | Typical Life |
|---|---|---|
| Standard (copper) | Copper core | 30,000–40,000 km |
| Platinum | Platinum tip | 60,000–100,000 km |
| Iridium | Iridium tip | 100,000–160,000 km |
Most modern NZ vehicles specify either platinum or iridium plugs. Using a cheaper plug grade than specified will result in shorter replacement intervals and potentially misfires before the scheduled change.
Signs of Worn or Failing Spark Plugs
| Symptom | Notes |
|---|---|
| Rough idle | One or more cylinders misfiring |
| Poor fuel economy | Incomplete combustion |
| Hard starting, especially when cold | Weak spark on ignition |
| Engine hesitation under acceleration | Misfire under load |
| Check Engine light (misfire codes P0301–P0304) | ECU detected misfires |
| Engine vibration at idle | Rough running from misfires |
Misfires aren't just annoying — they send raw fuel through the catalytic converter, causing it to overheat and potentially fail. An ignored misfire can turn a $20 spark plug problem into a $1,000+ catalytic converter replacement.
Inspecting Old Spark Plugs
The condition of removed spark plugs reveals a great deal about engine health:
- Light grey/tan deposits — normal wear
- Black, sooty deposits — rich mixture, oil burning, or misfires
- White/chalky deposits — lean mixture or coolant contamination (possible head gasket issue)
- Oily, wet plug — oil burning past piston rings or valve seals
- Blistered or eroded electrode — overheating or incorrect plug heat range
A good mechanic will report these findings and advise on whether there is an underlying issue beyond normal plug wear.
NZ Replacement Costs
| Service | Typical NZ Cost |
|---|---|
| 4-cylinder plug change (standard plugs) | $80–$160 |
| 4-cylinder plug change (iridium plugs) | $130–$250 |
| 6-cylinder plug change | $150–$350 |
| V8 plug change | $250–$500+ |
| Ignition lead replacement (if fitted) | $80–$200 additional |
Access varies widely by vehicle — on some Subaru engines, spark plug replacement requires significant ancillary removal and can take 2–3 hours. On a simple inline four-cylinder, it takes under an hour.
How Often Should You Change Spark Plugs?
Follow your manufacturer's service schedule. As a general guide:
- Standard plugs: every 30,000–40,000 km
- Platinum plugs: every 60,000–100,000 km
- Iridium plugs: every 100,000–160,000 km (or per manufacturer spec)
Many NZ drivers overlook plugs during major services because they're hidden away. If you're buying a second-hand vehicle and the service history is unknown, spark plugs are worth replacing as a precaution.
When to Book a Mechanic
- Check Engine light on with misfire codes
- Noticeable rough idle or engine vibration
- Fuel economy has dropped without obvious cause
- Hard starting in the morning
- More than the recommended interval since last plug change