How to Test a Spark Plug in 5 Easy Steps

Ever wonder why your engine sputters or stalls? Most of the time a bad spark plug is to blame. You don’t need a garage to find out – just a few tools and a bit of patience. In this guide we’ll walk through the testing process so you can decide if the plug needs cleaning, repairing, or replacing.

What You Need Before You Start

Grab a spark plug socket (usually 5/8" or 13/16"), a ratchet, a spark plug gap gauge, and a multimeter that can measure resistance. If you have a magnetic spark plug tester, that’s a nice shortcut, but it’s not required. Make sure the engine is cool – a hot engine can burn your fingers and give false readings.

Step‑by‑Step Testing Process

1. Remove the plug. Disconnect the battery terminal to avoid any accidental sparks. Use the socket and ratchet to loosen the plug, turning counter‑clockwise. Keep track of where each plug came from – the engine cylinder order matters.

2. Inspect the tip. Look for carbon build‑up, oil fouling, or a cracked insulator. Heavy deposits mean the plug is dirty, but they also hint at deeper engine issues like a rich fuel mixture.

3. Check the gap. Insert the gap gauge into the spark plug’s electrodes. The correct gap is usually printed on the plug’s metal shell; typical cars need 0.6‑0.8 mm. If the gap is too wide or narrow, gently bend the electrodes until it matches the spec.

4. Test resistance. Set your multimeter to the ohm (Ω) setting. Touch one probe to the terminal at the top of the plug and the other to the metal base. A healthy plug shows between 5,000 and 15,000 Ω. Anything significantly higher means the internal conductor is broken.

5. Spark test (optional). If you have a plug tester, plug it into the spark plug, reconnect the battery, and crank the engine. You should see a bright, blue spark. A weak or orange spark signals a failing plug.

After testing, clean a reusable plug with a wire brush, reinstall it (or replace it if it failed any step), and tighten to the manufacturer’s torque – usually around 13‑20 ft‑lb. Reconnect the battery and give the engine a short run. If the idle smooths out and acceleration improves, you’ve solved the problem.

Testing spark plugs regularly – every 20‑30 k miles or when you notice rough idling – can save you costly repairs later. It’s a quick check that anyone can do, and it keeps your engine firing on all cylinders.