How to tell if spark plug wires are bad?

How to tell if spark plug wires are bad?


Spark plug wires play a crucial role in the ignition system of a vehicle. They are responsible for delivering the electrical current from the ignition coil to the spark plugs, which ignite the fuel-air mixture in the engine. Over time, spark plug wires can wear out or become damaged, leading to poor engine performance. In this article, we will explore how to tell if spark plug wires are bad and when they need to be replaced.

Signs of Bad Spark Plug Wires

1. Engine Misfires: One of the most common signs of bad spark plug wires is engine misfires. Misfires occur when the spark plug wires fail to deliver the necessary electrical current to the spark plugs, resulting in incomplete combustion. This can cause the engine to run rough, experience a loss of power, or even stall.

2. Rough Idle: Faulty spark plug wires can also lead to a rough idle. If you notice that your engine is idling inconsistently or vibrating excessively when at a stop, it could be a sign of bad spark plug wires. The irregular flow of electrical current can disrupt the engine’s smooth operation at idle.

3. Poor Fuel Economy: Another indication of bad spark plug wires is a decrease in fuel economy. When the spark plug wires are not functioning properly, the engine may not be able to burn the fuel efficiently, resulting in decreased mileage. If you find yourself making more frequent trips to the gas station without any apparent reason, it may be worth checking the condition of your spark plug wires.

4. Engine Stalling: Faulty spark plug wires can cause the engine to stall unexpectedly. As the electrical current is not consistently reaching the spark plugs, the combustion process can be disrupted, leading to engine stalling. If your engine frequently stalls, especially during acceleration or at high speeds, it is crucial to inspect the spark plug wires.

5. Visible Damage: Physical damage to the spark plug wires is an obvious sign of their deterioration. Inspect the wires for any cracks, cuts, or burns. Additionally, check for loose connections or corrosion at the ends of the wires. Any visible damage should be addressed promptly to ensure proper ignition system performance.

Testing Spark Plug Wires

If you suspect that your spark plug wires may be faulty, there are a few tests you can perform to confirm their condition. However, it is important to note that these tests require some technical knowledge and the use of specialized tools. If you are not comfortable or experienced in working with automotive electrical systems, it is recommended to seek assistance from a qualified mechanic.

1. Visual Inspection: Start by visually inspecting the spark plug wires for any signs of damage, as mentioned earlier. Look for cracks, burns, or other visible issues that could indicate a problem. Additionally, check the connections at both ends of the wires to ensure they are secure and free from corrosion.

2. Resistance Test: Using a digital multimeter, you can measure the resistance of each spark plug wire. Consult the vehicle’s service manual for the manufacturer’s specified resistance values. If the measured resistance deviates significantly from the recommended range, it may indicate a faulty wire.

3. Insulation Test: Insulation breakdown can also cause spark plug wires to malfunction. To test the insulation, you can use a high-voltage insulation tester. This tool applies a high voltage to the wire, checking for any leaks or breakdowns in the insulation. Again, refer to the manufacturer’s specifications for acceptable insulation values.


Recognizing the signs of bad spark plug wires is crucial for maintaining optimal engine performance. Engine misfires, rough idle, poor fuel economy, engine stalling, and visible damage are all indicators that your spark plug wires may need to be replaced. Performing visual inspections and conducting tests such as resistance and insulation checks can help confirm their condition. If in doubt, it is always best to consult a professional mechanic to ensure the proper functioning of your vehicle’s ignition system.