* The condition of the spark plug: A worn or fouled spark plug will have a higher resistance than a new one. A heavily fouled plug might even act more like an open circuit under normal operating voltage.
* The applied voltage: The resistance will change with the applied voltage because of the dielectric breakdown of the air gap.
* The air gap: A larger air gap will require a higher voltage to break down, and thus appear to have a higher resistance at lower voltages.
While you can't give a single resistance value, a functioning spark plug will have a relatively low resistance *when the high voltage is applied and a spark jumps the gap*. Outside of that high voltage event, attempting to measure resistance with a typical multimeter will likely show a very high resistance (approaching infinity, similar to an open circuit) because the resistance of the air gap is normally too high for a multimeter to measure reliably.
Therefore, finding a high resistance with a multimeter is not necessarily indicative of a faulty spark plug; it's just reflective of the normal insulation properties of the gap. A faulty spark plug might show *consistently* high resistance even when high voltage is applied, or might show signs of other problems (like a short circuit).
