What indicates that there could be a problem with an engine's ignition system?

An irregular engine idle can suggest an issue with the ignition system because it typically indicates that the combustion process isn’t occurring consistently. The ignition system is responsible for creating the spark that ignites the fuel-air mixture in the engine's cylinders. If the ignition system is malfunctioning, it could lead to uneven firing of the cylinders, causing the engine to idle roughly or inconsistently.

In contrast, excessive fuel consumption or increased exhaust emissions can be related to other factors in the fuel system or engine performance issues but do not specifically point to ignition system problems. High oil pressure readings primarily relate to lubrication issues, not ignition. Therefore, irregular engine idle serves as a more direct indicator of potential ignition system problems, making it the most relevant choice for diagnosing this type of issue.