Felt like my V-6 was running on five cylinders. Suspected a bad coil -- have one for each spark plug. OM confirmed my diagnosis. Helped me decide to replace all six now -- hard to determine which one was bad now and others would likely have same problem soon. Saved money on labor over doing one at a time. OM helped me make the right decision. My 10-year-old engine is running as smoothly as when it was new!