How to test a spark plug
A weak or dead spark plug can make an engine feel lazy, misfire under load, or refuse to start cleanly. A car spark plug test helps separate an ignition fault from a fuel or compression problem before parts are replaced. Testing can be done with the plug removed for a visual inspection, with a spark tester attached to the plug wire, or by swapping a suspect plug to another cylinder and watching whether the symptom follows it. The most useful conclusion comes from combining an observed spark with the plug's physical condition and the engine's behavior when warm, cold, or under acceleration.
Direct answer
The essential way to check a car spark plug is to ground its threaded metal body against the engine while the ignition system is active, then look for a strong blue spark at the electrode gap. A steady snapping spark usually means the plug can fire outside the cylinder, although that does not prove it fires correctly under compression. A yellow, weak, or intermittent spark often points to a fouled plug, a wide gap, or a high-resistance plug wire or coil issue. No spark may indicate a bad plug, but the ignition coil, wiring, or trigger signal can also be responsible.
A practical variation is to swap the questionable plug into a different cylinder and see whether the misfire moves with it or stays in the original location. If the fault follows the plug, the plug itself is the stronger suspect. If the original cylinder still misfires with a known-good plug, the problem may be in that cylinder's coil, wire, injector, or mechanical condition. Because many cars use individual coil-on-plug ignition, a test result should be interpreted alongside diagnostic codes and the cylinder that sets a misfire code.
How to proceed
Work on a cool engine and disconnect the coil or injector connector if the vehicle design requires it before removing the plug. Use compressed air or a brush to clear debris from the plug well so nothing falls into the cylinder. Remove the plug carefully, note any oil or fuel wetting on the porcelain and threads, and inspect the center electrode and ground strap for erosion, cracking, or heavy deposits. Compare the gap with the manufacturer specification when that information is available, but avoid bending fine-wire electrodes unless the plug type is confirmed safe to adjust.
Install the plug into a spark tester and attach the plug wire or coil boot securely. Clamp the tester's ground lead to a clean engine ground, then have the engine cranked without starting it. Watch for a crisp spark at the tester gap while listening for an even cranking rhythm. A no-spark result should be repeated with a known-good plug or tester before blaming one component. Since ignition systems can deliver a painful shock, keep hands away from the tester and wiring while the engine is cranking, and confirm safe disable procedures when cranking a modern vehicle.