How can a mechanic identify a faulty spark plug?

To determine if a spark plug is faulty, one effective method is to short out each plug one at a time. This process involves removing the spark plug lead and temporarily grounding it. By doing this, the mechanic can observe if the engine runs differently or more smoothly with each spark plug disconnected. If disconnecting a specific plug causes a noticeable change in engine performance, that indicates the plug is functioning and the issue may lie elsewhere. Conversely, if a plug is shorted out and there is no change in engine performance, it suggests that the spark plug is likely faulty, as it is not contributing to the combustion process.

This method is highly effective because it directly tests the impact of each individual spark plug on engine performance, helping to isolate a faulty unit accurately.