How to check spark plugs

Difficult starting, unstable engine operation, rough idling, and ignition skips are the signs of spark plug failure. Consequently, it is worth unscrewing the spark plugs to check their condition and performance if malfunction symptoms appear. To check the serviceability of the spark plugs, sometimes a visual inspection is enough, but if there are no visible defects and problems, use a multimeter, a tester, or at least the ability to spark formation.

Symptoms of bad spark plugs

Hard to Start. When spark is present but weak, the engine may take longer to start than usual. This can happen if a spark plug or several plugs are covered with soot.

Engine Misfires. Such a problem will be noticeable by unstable noisy engine operation, and the engine indicator light may illuminate. It is a signal of cylinder failure (no spark, no fuel). This can be the fault of a spark plug, ignition coil, spark plug wires, or injectors.

Higher fuel consumption. If the spark plugs are not working correctly, there is incomplete combustion and a decrease in engine power. This usually occurs when the electrodes are worn and the gap between them increases.

In addition to these basic symptoms, acceleration may be reduced when you depress the accelerator pedal, rough idle (unstable spark), fuel smell in the exhaust (when the fuel does not burn, fill the plugs or they are covered with sludge).

There is no hurry to change plugs with such problems. They may not be the problem, and the replacement costs up to $250 (the price of plugs up to $70 + replacement up to $180). That's why you should check them before deciding to replace them. Besides they could become defective not because of age (their resource can reach up to 100,000 km), and for other reasons.

Causes of spark plug failure

  • Natural wear and aging (the average life of plugs is 50-100 thousand km, depending on the type of plug). If the spark plug failed very quickly, the reason could lie in the poor-quality fuel or oil with additives. Significant spark plug wear can be observed in detonation and/or overheating of the engine.
  • A large amount of soot on the electrodes. This is usually the result of either an unbalanced fuel/air mixture or a faulty fuel system.
  • Oil on the electrodes. In turn, this can be caused by: an excessive amount of oil in the engine, worn piston rings, pistons, cylinder block, and valve guides. If the engine is equipped with a turbine, it may have partially failed.
  • Slag deposits. They are the result of additives in the engine oil.
  • Failure of the center electrode. This can occur due to improperly aligned ignition. Besides, there may be deposits in the combustion chamber, faulty valves, poor fuel quality, incorrect tightening torque of the spark plug in the cylinder head or improperly selected plug gauge.
  • Melted plugs. Usually, the electrodes are melted due to the fact that there are deposits in the combustion chamber, which constantly spontaneously combust. This situation can also occur if the valves are faulty or poor quality fuel is used.

How to check if spark plugs are bad

The inspection of spark plugs should begin with a visual inspection to check the gap between the electrodes, their integrity, the presence of carbon deposits, traces of fuel, oil, or coolant. The color and appearance of the soot may indicate a probable problem. If the spark plug has fuel or a melted side electrode, it may be the problem.

In some cases, the spark plug may still work. It may just need to be cleaned or the electrode gap adjusted to the desired setting.

Black soot, white, oily traces will indicate a problem with the engine or fuel system. So the answer to the question, of how to check spark plugs without removing them, will be — it is impossible! You will need a multimeter, a discharge tester or the coil/tip of the spark plug to further check the performance.

How to check spark plugs with a multimeter

How to Test a Spark Plug In One Minute

To check the spark plug with a multimeter you need to know the resistance value of a new spark plug. To do this, you can measure a new spark plug of the same type or find out its characteristics.

  1. Set the multimeter to 20 kOhm resistance test mode.
  2. Unscrew the spark plug from the engine and connect the probes of the meter, one to the upper tip and one to the center electrode.
  3. Look at the meter reading. Normal is 3 to 8 kOhm. If the resistance is much higher, the spark plug is bad, and it is better to replace it. If it is infinity — it is completely out of order, and replacement is obligatory.
Keep in mind that only plugs that have internal resistance are checked with this method, so there are models that have zero resistance, which are put on some engines.

It is also worth checking the spark plug for breakdown. To do this, put one feeler gauge on the tip and the other first on the threaded part and then on the insulator. The readings on the instrument panel should not change, otherwise, the spark plug is punctured and it is not working.

Pay attention that all plugs installed in the engine should have approximately the same resistance and if it is very different — better replace them, the whole set!

How to check for spark by yourself

Diagnosing and Repairing a Lack of Spark Plug Maintenance

The easiest way to check is to see if the spark is present and how powerful it is. To test the spark plug do the following:

  1. Turn off the fuel supply to the ramp. It is enough to disconnect the gas pump by removing the fuse from the block.
  2. Unscrew the spark plug from the engine that you are going to test.
  3. Put the spark plug tip from the ignition coil on the spark plug or insert it into the coil if it is individual for each cylinder.
  4. Ground the threaded part of the spark plug to the engine housing.
  5. Ask your partner to turn on the ignition and try to start the engine for a few seconds.
  6. Watch what color and strength the spark is.
  7. A light orange spark color, as well as no spark, indicates an inability to ignite the fuel mixture.
  8. If the spark is blue, large do not stitch into the insulator — the spark plug is probably malfunctioning.

Another way to check the sparking ability if you are afraid of electrocution or damaging the car ECU is to use a piezo element from a lighter.

  • take out the piezo element;
  • add a small wire to fix the wire on the tip of the plug and put the piezo element on the body of the plug;
  • press the piezo element and see if a spark appears.

If there is no spark, then the spark plug is broken!

The spark is not an indication that the spark plug is stable in the cylinder. If it is worn, there can be skips under pressure in the combustion chamber. Only a special tester will allow you to see them.


  • What should be the spark plugs resistance?

    If the spark plugs have a resistor, their resistance ranges from 2 to 5 kOhms. If the spark plug has no resistor, this parameter is 0 kOhm. When the spark plug is in the engine for a long time, its resistance increases.

  • What should a good spark plug look like?

    If the skirt of the central electrode is light brown or light gray in color, there are no traces of electrode wear, there are few deposits of soot — the spark plug is functioning normally.

  • How do I know if the spark plug works?

    The functioning of a spark plug can be evidently seen by its ability to give out a spark. To do this, one should unscrew the spark plug, connect it to the coil or put the tip on, touch the ground and try starting the engine (with the fuel supply disconnected). A blue strong spark means that the spark plug is working, and it is orange or weak indicates that the spark plug is worn.

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