The spark plug is considered one of the key parts of a gasoline engine. Its condition directly affects reliable starting, smooth cylinder operation, and efficient fuel consumption. However, even the most reliable spark plugs wear out over time. Let's take a detailed look at what symptoms can help you recognize a faulty spark plug, what to pay attention to, and how to check it yourself.

How spark plug problems manifest

First of all, you should pay attention to the car's behavior. If at least one spark plug fails, the engine immediately reacts to it. Here are the most typical signs:
  1. The engine runs unevenly.
    The car starts to vibrate, the RPMs "float," especially at idle. The reason is that one or more cylinders do not receive a spark, and the fuel mixture in them does not ignite.
  2. Difficult starting.
    If the engine starts with difficulty, especially in the morning or in rainy weather, this may indicate that the spark plug is not producing a stable spark.
  3. Reduced power and responsiveness.
    The car accelerates sluggishly and responds weakly to pressing the gas pedal. In such a situation, you should think about how to find out whether the spark plug is working and check each one separately.
  4. Increased fuel consumption.
    Due to incomplete combustion of the mixture when there is no spark, the engine begins to require more gasoline.
  5. Pops and detonation.
    If the spark plug does not ignite the mixture at the right moment, the fuel may ignite already in the exhaust system — then pops or "shooting" sounds come from the muffler.

Ways to check the performance of a spark plug

There are several methods that allow you to find out whether the spark plug is working and whether it produces a spark:

1. Diagnosis by sound and vibration

While the engine is running, remove the high-voltage wires from the spark plugs one by one (this must be done carefully, using insulation).
  • If after disconnecting the wire the engine continues to run the same — the spark plug is most likely faulty.
  • If the RPMs drop — the spark plug is fine.
The method is simple but requires caution: do not touch bare wires to avoid an electric shock.

2. Visual inspection of the spark plug

Unscrew the spark plug and carefully examine its appearance:
  • Normal condition of the insulator — light brown or grayish tint.
  • Black carbon deposits indicate an over-rich mixture or a weak spark.
  • White coating — a sign of overheating or too lean a mixture.
  • Oily trace — evidence of oil entering the cylinder.
By appearance, you can determine that the spark plug has failed or needs replacement.

3. Checking the spark

Connect the spark plug to the high-voltage wire, press its metal part against the engine ground, and crank the starter. If a bright blue spark jumps between the electrodes — the spark plug is working. If the spark is weak, yellowish, or absent altogether — the part needs to be replaced. This method helps check whether the spark plug is functioning, but it must be done with caution, away from flammable substances.

How to recognize a faulty spark plug by the car's behavior

Sometimes you can identify the problem without any tools. Symptoms to pay attention to:
  • The engine "misses" — it does not run on all cylinders.
  • The exhaust pipe smells of unburned gasoline.
  • When accelerating, the car jerks and makes lunges.
  • The Check Engine indicator lights up on the dashboard (especially on modern cars).
If several of these signs are observed at once, it is highly likely that at least one spark plug has failed.

Checking the spark plug with a multimeter

A more accurate option is measuring the resistance of the spark plug using a multimeter.
  1. Set the device to resistance measurement mode (Ohms).
  2. Place one probe on the central contact, the other on the spark plug body.
  3. The normal resistance reading is from 4 to 6 kOhm. If the resistance is absent or significantly exceeds the norm, the spark plug is faulty.
This method is especially relevant for spark plugs with a built-in resistor, where it is difficult to visually identify the problem.

When it's time to replace spark plugs

Even with proper operation, a spark plug loses its properties over time. The average service life depends on the type:
  • Copper spark plugs — up to 30,000 km.
  • Platinum and iridium spark plugs — up to 90,000–120,000 km.
However, with poor fuel quality, incorrect ignition timing, or oil leaks, replacement may be needed earlier. Regular condition checks and knowing how to determine whether a spark plug is working will help avoid serious engine damage and excessive fuel consumption.

Summary: how to recognize a faulty spark plug

To accurately understand that a spark plug is not working, you need to pay attention to symptoms such as unstable engine operation, reduced power, increased fuel consumption, and pops. The check can be done visually, by spark, or with a multimeter. The main thing is to approach diagnostics competently: follow safety measures, do not touch wires with your hands, and use only those spark plugs recommended by the manufacturer. Now you know how to correctly determine that a spark plug is not working, how to find out and check its condition, and also when it should be replaced.