Let's say you want to look up the spark plug for your 1996 Jeep Cherokee with a 4.0L engine. The first thing you do is find a spark plug catalog and check the date to make sure it is current. Then you look in the index for "Jeep." In this particular catalog, there is no listing by make and model in the index. The spark plug applications are broken down by Automobiles, Vans / Trucks & Buses, and several other listings. If you have an SUV or sport utility vehicle it may be listed either in the Car or Automobile section or in the Truck section depending upon the manufacturer. In this case, turn to the page starting Vans, Trucks & Buses.
Under Vans, Trucks & Buses, you'll find they are broken down into individual makes starting with Acura and working back to Volkswagen. Scan the pages until you find the heading Jeep. Under Jeep, you'll find the applications are further broken by model and year. And then 4-cylinder and 6-cylinder engines. Your Jeep is a 1996 Cherokee and has a 6-cylinder, so look under the appropriate heading. Read across the column from the L6 4.0L entry and find the number of the spark plug.