If there’s any doubt, make sure the correct spark plugs are installed in the engine you're working on. Using the wrong spark plug can cause all kinds of problems. Automotive spark plugs have resistors to help suppress ignition noise. They also come in different heat ranges and thread lengths. Today’s engine control modules are sensitive and can set a misfire code if the wrong plug is used
The heat range depends on the design of the center electrode and the length of the ceramic insulator surrounding it. A hot plug is generally longer than a cold plug. A spark plug with a high heat range will run cleaner, but can cause ping, detonation and preignition problems. This is because spark plugs are self cleaning; the heat cleans the spark plug tip, especially while cruising at highway speeds. A cold spark plug helps with preignition, but can get dirty and foul out sooner, especially if the vehicle is only driven short distances or frequently idled.
Unfortunately, too hot of a tip can ignite the fuel charge at the wrong time, robbing the engine's power stroke of necessary energy, possibly resulting in damage.
Spark plug resistance can be tested using a digital multimeter. Spark plugs can leak voltage or foul out from excessive carbon deposits. Use a spark plug cleaner or some brake fluid to clean debris from the spark plug tip. Since automotive spark plugs have a resistor to prevent interference, there should be a resistance between the terminal and the center electrode of ~ 4000 – 6500 Ohms. Check manufacturer’s specifications before testing.