Spark plug replacement is one of the most commonly delayed maintenance items — and the consequences of skipping it are more serious than most drivers realize. Here's what happens to your engine when spark plugs are overdue.
Stage 1: Performance Decline (Slightly Past Due)
The first sign is subtle: fuel economy drops by 5–10%, throttle response feels a little sluggish, and the engine might take an extra second on cold starts. Most drivers attribute this to the vehicle "getting older" rather than recognizing it as a maintenance indicator.
Stage 2: Misfires Begin (Significantly Past Due)
As electrode wear increases, the spark becomes inconsistent. You'll notice rough idle, engine stumble during acceleration, and possibly a P0300 misfire code. At this stage the check engine light often illuminates.
Stage 3: Ignition Coil Damage
This is where it gets expensive. When a spark plug has excessive gap (from wear), the ignition coil has to work much harder to fire it. The increased voltage demand stresses the coil. Over time, the coil overheats and fails — turning a $15 plug into a $80–$200 coil replacement.
Stage 4: Catalytic Converter Damage
Persistent misfires send raw, unburned fuel into the exhaust system. Catalytic converters are not designed to handle unburned fuel — it overheats and destroys the catalyst. A catalytic converter replacement costs $800–$2,500 or more.
The Math Is Simple
A set of iridium spark plugs costs $40–$80 for most vehicles. Neglecting them can cascade into coil failures and catalytic converter replacement worth thousands of dollars. Change your plugs on schedule.
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