Spark plugs are small parts with a big job: they ignite the air-fuel mixture in your engine so it can run smoothly and efficiently. When spark plugs wear out, performance drops—and in many cases the vehicle starts misfiring, which can damage other components if ignored.

If you’re noticing rough running, poor fuel economy, or a check engine light, your vehicle may be telling you it’s time for spark plug replacement.

What Do Spark Plugs Do?

Spark plugs:

Create the spark that starts combustion in each cylinder
Help maintain smooth power and acceleration
Support fuel economy and lower emissions

Modern engines also use ignition coils (coil-on-plug systems are common), and worn plugs can strain coils over time.

10 Signs Your Spark Plugs May Be Worn

1) Rough idle (shaking at stop lights)

Worn plugs can cause incomplete combustion and a shaky idle.

2) Misfire feeling (hesitation or “stumble”)

A misfire can feel like a quick jerk, stumble, or uneven power—especially under load.

Important: if the check engine light is flashing, get it checked quickly.

3) Slower acceleration / less power

When spark is weak, the engine may feel sluggish, especially when merging or climbing hills.

4) Check engine light on (or flashing)

Spark plug wear can trigger misfire codes. A flashing light often means an active misfire that can be hard on the catalytic converter.

5) Poor fuel economy

Bad plugs can waste fuel because the engine isn’t burning the mixture efficiently.

6) Hard starting (especially in cold weather)

Worn plugs can make it harder for the engine to catch and start cleanly.

7) Engine surging or uneven cruising

If the engine can’t maintain consistent combustion, you might feel surging at steady speeds.

8) Strong fuel smell or failed emissions test

Misfires can leave unburnt fuel in the exhaust. That can increase emissions and cause a noticeable smell.

9) Jerking under acceleration

Under load, worn plugs may break down and cause noticeable jerking or hesitation.

10) It’s been a long time since replacement

Many modern plugs are designed for longer intervals, but they still wear. If you’re past your recommended interval—or you’re not sure when they were last done—it’s worth checking.

How Often Should Spark Plugs Be Replaced?

Intervals vary by vehicle and plug type. Some engines use long-life plugs, but driving conditions also matter:

Lots of short trips
Heavy loads or towing
Extreme cold/hot conditions
Stop-and-go city driving

If you want to avoid surprises, replacing plugs on schedule is cheaper than dealing with repeated misfires.

What Happens If You Ignore Worn Spark Plugs?

Delaying spark plug service can lead to:

Persistent misfires and poor drivability
Increased fuel consumption
Damage to ignition coils
Catalytic converter damage (especially with ongoing misfires)

A simple maintenance item can snowball into a bigger repair if left too long.

Spark Plugs vs Ignition Coils: What’s the Difference?

Spark plugs create the spark
Ignition coils supply the high voltage to fire the plug

Sometimes coils fail too—but worn spark plugs can increase the load on coils, making coil failure more likely.

What’s Included in a Spark Plug Service?

A proper spark plug service often includes:

Confirming correct plug type and gap/spec
Inspecting ignition coils and boots
Checking for oil in plug wells (valve cover gasket leaks)
Clearing codes and verifying no misfire after repair (when needed)
Road test / recheck

Book Spark Plug Replacement

If your engine is running rough, fuel economy dropped, or your check engine light is on, book an inspection. We’ll confirm if spark plugs are the issue and recommend the right fix—no guessing.

Call 403-258-3030 to book an appointment.

Location:
8304 Fairmount Dr SE
Calgary, AB, T2H 0Y8

Spark Plug Warning Signs: 10 Symptoms You Need Replacement (Before Misfires Get Worse)