You can tell if a spark plug is weak by the symptoms.
The symptoms and signs of a weak spark plug include rough idling, hard starting or no starting, decreased fuel efficiency and misfiring of the engine.
Taking the spark plug out and looking for any excessive wear or buildup of carbon can also allow you to determine better if the spark plug is bad.
You can also use a spark tester to test the spark plugs output.
Cracks in the porcelain of the spark plug is also a sign that the spark plug is bad as it's a sure sign of electricity leakage in the spark plug.
On lawnmowers and other small engines a spark plug should be changed once a year or every few years depending on how much use the small engine sees.
On vehicles spark Plugs need to be replaced as often as every 100,000 miles in most cases.
Although some vehicles may need their spark plugs replaced every 30,000 miles to 40,000 miles.
Platinum spark plugs and iridium spark plugs last longer than regular spark plugs in most cases.
To keep your engine running smoothly and help improve fuel economy it's always good to replace the spark plugs every 30,000 miles to 50,000 miles although I have had them last 100,000 miles.
I like to do routine maintenance on my vehicles and replace the spark plugs at least every 50,000 miles which can take me around 5 years to rack up that amount of miles.
Some vehicles they recommend that you change them every 7 years or 105,000 miles and some should be changed out sooner.
The signs and symptoms of worn spark plugs include hard starting, rough idling, engine misfire, shaky idling, reduced or poor acceleration and decreased fuel efficiency.
If your engine is hesitating or sputtering it can be a sign of worn or faulty spark plugs.
Frequent stop and go traffic driving or towing of heavy loads and operating the vehicle in extreme conditions can also shorten the life of the spark plugs.
Spark plugs in small engines such as lawn mowers should be changed ideally every season although some can last longer.