Remove the spark plug from the engine
Inspect the plug for cracks, heavy carbon buildup, oil fouling, or worn electrodes
Check the electrode gap with a feeler gauge
Set the gap to the manufacturer specification if needed
Use a spark plug tester to verify spark under pressure
Connect the spark plug to the ignition lead or tester
Ground the plug body properly
Crank the engine and look for a strong blue spark
Replace the plug if there is no spark, weak spark, or inconsistent spark
Check plug resistance with a multimeter if the plug is resistor type
Compare the resistance reading to the manufacturer specification
Test each spark plug individually
Reinstall or replace the spark plugs as needed