.. and even if they ohm out , change them if the spring ends are discoloered /burnt.
The problem is because they are between the coil and the plug. Benz used a spring friction contact that just does not hold up. Whereas the other three plugs have Snap connectors . They also can't get rid of engine heat as they are encapsulated.
It is common practice to change them out w/plug change as the cost is nil and they are known to cause misfires on these waste spark systems.
Bad ones also tax the ECU driver circuits and fried ones are starting to show up with regularity, so I recommend changing them just for insurance...