Spark plugs create the spark that ignites the air and fuel in each cylinder, so when they wear out you may notice a rough idle, hesitation when you accelerate, harder starts, lower fuel economy, or a flashing or steady check engine light from misfires. Replacement timing depends on your engine and plug type, so your owner's manual is the best guide. Hillside Auto Clinic at 26122 Western Ave, Lomita, CA 90717 can inspect your plugs and ignition system; call (310) 956-5794 or schedule.
What spark plugs do
Every time a cylinder fires, the spark plug sends a small electrical arc across a gap at its tip. That spark lights the air-fuel mixture and pushes the piston down. A four-cylinder engine has at least four plugs, and V6 and V8 engines have six or eight (some engines use two per cylinder). If even one plug is weak, that cylinder can misfire and the whole engine runs worse.
Signs your spark plugs may be worn
- Rough or shaky idle, especially at stoplights on Western Ave or Pacific Coast Highway.
- Hesitation or stumbling when you press the gas or merge onto the 110 or 405.
- Harder starting, or the engine cranks longer than usual before it catches.
- Lower fuel economy than you're used to for the same driving.
- Check engine light, often with misfire codes. A flashing check engine light usually means an active misfire—ease off and get it checked soon.
- Loss of power or the car feeling sluggish going up hills like those around Palos Verdes.
These symptoms can also come from ignition coils, fuel injectors, vacuum leaks, or sensors, so it's worth diagnosing the cause instead of guessing.
Why spark plugs wear out
Over time the electrode at the tip erodes and the gap widens, which makes it harder for the coil to push a strong spark across. Plugs can also get fouled with carbon, oil, or fuel deposits. Oil on a plug can point to a leaking valve cover gasket or internal engine wear, and heavy carbon can point to rich running or lots of short trips. Reading the old plugs often tells a technician something useful about the engine's health.
How often should spark plugs be replaced?
There isn't one number for every car. Older copper plugs were replaced much more often, while many modern engines use iridium or platinum plugs designed for long service intervals. Your owner's manual or maintenance schedule lists the right interval for your engine. If you've bought a used car and don't know when the plugs were last changed, an inspection is a good way to find out.
Spark plugs and ignition coils
Most newer cars put an ignition coil directly on top of each plug (coil-on-plug). A worn plug makes the coil work harder, and a failing coil can cause the same misfire symptoms as a bad plug. When plugs are replaced, the coils, boots, and connectors are usually checked too. On some engines, especially V6s where the rear plugs sit under the intake, plug replacement takes more labor, which is another reason to plan it ahead of time.
What happens if you put it off
Driving with misfiring plugs can waste fuel, make the engine run rough, and in some cases send unburned fuel into the exhaust, which can overheat and damage the catalytic converter. Misfires can also cause a car to fail a California SMOG check. Replacing worn plugs on schedule is usually far simpler than dealing with what they can damage later.
Simple ways to stay ahead of it
- Check your owner's manual for the spark plug interval and note your current mileage.
- Pay attention to new rough idling, hesitation, or a drop in fuel economy.
- Never ignore a flashing check engine light.
- Ask about spark plug condition if your car is near its interval or you don't know its history.
Hillside Auto Clinic is open Monday–Friday 8 AM–6 PM and Saturday 10 AM–3 PM (closed Sunday). Visit us at 26122 Western Ave, Lomita, CA 90717, call (310) 956-5794, or email hillsideautoclinic@gmail.com.
Call or schedule
Noticing a rough idle, hesitation, or a check engine light? Call (310) 956-5794 or book online. Have your year, make, model, and current mileage handy, and tell us what you've been noticing.
FAQ
What are the first signs of bad spark plugs?
Common early signs are a rough idle, hesitation when accelerating, harder starts, and lower gas mileage. A check engine light with misfire codes is another clue.
Can bad spark plugs cause a check engine light?
Yes. Worn or fouled plugs can cause misfires, which the engine computer detects and reports with a check engine light. A flashing light usually means an active misfire that should be checked soon.
How long do spark plugs last?
It depends on the engine and the plug material. Many modern iridium or platinum plugs are designed to last a long time, while older copper plugs need replacing sooner. Your owner's manual lists the interval for your car.
Is it safe to drive with a misfiring engine?
It's best not to keep driving for long. Misfires can waste fuel, make the car hesitate in traffic, and can damage the catalytic converter over time.
Should ignition coils be replaced with spark plugs?
Not automatically. Coils are usually inspected when plugs are changed, and they're replaced only if testing or symptoms show they're failing.
