How to Check if Spark Plugs are Good or Bad?

[SanJasper]

Methods to check the condition of spark plugs: Observe the color of the spark plug—grayish-yellow indicates normal condition and no need for replacement. If the center electrode shows signs of corrosion, it indicates poor condition and requires replacement. After removal, install the spark plug wire and ground the side of the spark plug to observe the strength of the spark and whether it jumps at the center of the spark plug. Weak sparks or sparks jumping around the electrodes also indicate the need for replacement. The spark plug is a crucial component of a gasoline engine's ignition system. It introduces high voltage into the combustion chamber and creates a spark by jumping the electrode gap, thereby igniting the combustible air-fuel mixture in the cylinder. A spark plug mainly consists of a terminal nut, insulator, terminal stud, center electrode, side electrode, and shell, with the side electrode welded onto the shell.

[VanDallas]

When I usually repair cars, I can judge the condition of the spark plugs just by looking at the electrode part: use a spark plug wrench to remove it after the engine cools down. If the center electrode is burnt black or the gap widens beyond the manual's specified 0.6 to 1.0 mm, it indicates aging and needs replacement; if the electrode is covered in oil sludge or carbon deposits, it means the air-fuel mixture is too rich or there's an oil issue, and it needs immediate cleaning. A clean electrode with light brown deposits is normal, but if the color turns white, it’s a sign of overheating. I always keep a vernier caliper handy to measure the gap—too narrow or too wide can cause unstable ignition. When installing new ones, don’t forget the tightening torque, usually around 25 Nm.

[SanLeila]

I always enjoy tinkering with the engine myself. To check if the spark plugs are good, I use simple tools: buy a dedicated tester for a few dozen bucks, attach it to the spark plug wire, start the engine, and see if it flashes a blue spark—bright means it's good, no flash or a yellow one means it's bad. Alternatively, observe the car's condition; if the engine shakes at idle and lacks power during acceleration, it's likely a spark plug has failed. Regular checks are crucial. Ordinary nickel alloy ones need replacement every 20,000 kilometers, while platinum ones can last up to 60,000 kilometers. Remember to wear insulated gloves when removing them to prevent electric shocks—safety first.

[CalvinLynn]

If your car has issues, start by checking the spark plugs. The method is quite simple: turn off the engine and let it cool for half an hour, then remove a high-voltage wire and gently pull the spark plug to check for looseness. If the car is hard to start or you hear metal knocking sounds, the spark plugs may be heavily carbon-fouled. Good daily driving habits can extend their lifespan—avoid frequent hard acceleration. If you're still unsure, take it to a professional shop to measure the resistance with specialized tools. The normal range is 1 to 6 kilo-ohms. If it's out of range, replace the part to save money and hassle.