How to Test a Spark Plug the Right Way

A spark plug is one of the most critical components in your car’s ignition system. It ignites the air-fuel mixture inside the combustion chamber, ensuring smooth engine performance. When a spark plug wears out or fails, your engine may misfire, struggle to start, or consume more fuel than usual. Understanding how to test a spark plug can save you money on repairs and help you maintain your vehicle’s efficiency.

What Is a Spark Plug?

A spark plug is a small but powerful part that fits into your engine’s cylinder head. It generates a spark that ignites the compressed air-fuel mixture during combustion. This explosion drives the piston downward, creating the power that turns your engine.

Spark plugs are made up of a metal shell, ceramic insulator, and electrode. The electrode gap — the small space between the center and ground electrode — is where the spark jumps. Using the correct spark plug gap and type ensures optimal performance and fuel economy.

How Does a Spark Plug Work?

Every time you start your engine, the ignition coil sends high voltage to each spark plug. The electrical energy arcs across the plug gap, igniting the air-fuel mixture inside the cylinder. This process happens thousands of times per minute in modern engines.

If a spark plug becomes dirty, worn, or carbon-fouled, it may misfire. This can affect the performance of related components like the alternator, fuel filter, and camshaft, leading to poor combustion and reduced fuel efficiency.

Common Symptoms or Signs of Spark Plug Failure

Knowing the spark plug failure symptoms can help you identify problems early and avoid engine damage. Here are the most common warning signs:

Difficulty starting the car
Engine misfires or rough idling
Poor fuel economy
Reduced acceleration or power
Black or oily spark plug tips
Check Engine Light coming on

If you notice these signs, you might also want to check related systems such as the ignition coil, crank sensor, or fuel injector.

How to Test, Clean, or Replace a Spark Plug

Testing a spark plug is straightforward and requires basic tools such as a spark plug socket, ratchet, and a multimeter or spark plug tester.

1. Remove the Spark Plug

Allow the engine to cool down.
Disconnect the spark plug wire.
Use the socket and ratchet to remove the plug carefully.

2. Inspect the Plug

Look for:

Black carbon deposits (rich fuel mixture)
Oily residue (possible piston ring or valve issue)
Burnt electrodes (overheating or incorrect gap)

3. Test for Spark

Reconnect the plug to the ignition wire.
Ground the plug’s metal body against the engine block.
Crank the engine — you should see a bright blue spark.
- If no spark appears, check your ignition coil or wiring.

4. Use a Multimeter (Optional)

Set the multimeter to resistance (ohms) and measure between the top terminal and electrode.

A healthy plug usually reads 4,000–8,000 ohms.
High or infinite resistance indicates a faulty plug.

5. Clean or Replace

If dirty, clean with a wire brush and a small amount of engine oil on the threads before reinstalling. However, if the electrodes are worn or damaged, replacement is recommended.

When replacing spark plugs, check your car’s manual for:

Correct spark plug type and gap
Proper torque specifications
Whether to use anti-seize compound on the threads

Spark Plug Replacement Cost in South Africa

In South Africa, spark plug replacement costs vary depending on vehicle make and plug type:

Standard spark plugs: R50 – R120 each
Platinum or Iridium spark plugs: R150 – R400 each

Related Car Parts and Maintenance Tips

When replacing or testing your spark plugs, consider checking related engine parts for optimal performance:

Air filter and fuel filter (ensure proper airflow and fuel delivery)
Ignition coil and crank sensor (for consistent spark delivery)
Throttle body (cleaning improves fuel efficiency)
Timing belt, camshaft, and cylinder head (for smooth engine timing)

Conclusion: Why Spark Plug Maintenance Matters

Regularly testing and maintaining your spark plugs keeps your engine running efficiently, prevents costly repairs, and improves fuel economy. Whether you drive a Toyota, BMW, or Ford, always use genuine parts from trusted auto supply sources.

Looking for quality spark plugs or related components like ignition coils, air filters, or engine parts?
Find affordable, genuine car spares at car-parts.co.za today.