5 Signs of Bad Spark Plug Wires You Should Know

Written by

Charles Bolte


Vernon Hoppe 

signs of bad spark plug wires

Faulty spark plug wires are one of the many issues that can affect your vehicle and prevent it from performing optimally.

Luckily, there are several ways you can have an early diagnosis to prevent them from causing worse car problems. The signs of bad spark plug wires include engine misfiring, engine hesitation, rough idling, and check engine light.

Symptoms of Bad Spark Plug Wires

Bad spark plug wires will interfere with the car’s electrical system and thus are hard to go unnoticed. Knowing about the signs and symptoms will come in handy for a timely diagnosis, even before going to a professional for a proper check.

Here’s how to tell that your vehicle has bad spark plug wires:

1. Engine misfiring and hesitation


One of the first signs you’ll notice when your vehicle has bad spark plug wires is engine misfiring and hesitation.

Engine misfiring happens as a result of incomplete combustion. Faulty ignition cables will fail to transmit the required spark from the distributor, which may cause misfiring in the cylinder.

According to the National Institute of Automotive Service Excellence, engine misfiring can cause 30% more fuel consumption.

It is most noticeable when the engine is under heavy throttle demand, and you notice the car has reduced its power. This engine hesitation happens due to a lack of proper conduction of electricity from the distributor to the spark plugs.

Another sign of misfiring is a popping or sneezing sound when the engine is under load.

2. Rough idling


Rough engine idling is a sign of a bad spark plug wire sound, characterized by shaking and vibration, and occurs due to imbalance from one cylinder to the next.

It can be a result of many culprits, like clogged air filters, dirty fuel injectors, or a bad ignition coil.

Faulty spark plug wires can also cause an inconsistent fuel burning rate, causing a rough idle. So, to be certain the spark plug wires are the culprit causing the rough idle, first, inspect them.

3. Check engine light


Most modern cars are equipped with a variety of sensors that detect issues affecting performance and send signals to the car’s Engine Control Module (ECM).

Faulty spark plug wires can cause misfires which in turn will cause a misfiring code to be sent and the check engine light to be illuminated on the dashboard. This handy diagnostic aid will alert you that something needs your attention.

4. What Causes Bad Spark Plug Wires?


Several factors can cause bad spark plug wires. The major culprits include the following in the engine:

  • Heat – When the car roars to life, the engine heats up. Ideally, spark plug wires are heavily insulated to withstand extremely high temperatures.

However, since they are located close to the engine’s exhaust manifold, they are prone to eventually experience degradation caused by the extreme heat.

Subsequently, the heat damages the spark plug wire insulators, exposing the wires to lose voltage to the ground.

  • Vibration – The constant shaking of the engine can eventually damage spark plug wires. This is because, over time, the vibration will cause wear to the spark wire’s internal filaments.

The consequent resistance in the wires can lead to further degradation and overheating, leading to the wire’s insulator melting.

  • Abrasion – Another leading cause of bad spark plug wires is the friction caused by the wires rubbing against sharp or rough edges. Over time, this can cause damage to the spark plug wires’ insulator.

The damaged areas on the insulator will cause the voltage to be ground.

5. How to Fix Bad Spark Engine Wires?

To avoid much bigger problems to the vehicle’s engines, it is best to replace bad spark plug wires. Here’s how you can replace old, weary, or bad spark plug wires.

Step 1: Remove the bad plug wire set


When planning to remove and replace the spark plug wire sets, do so systematically to avoid confusion and accidentally switching the wires. Handle one wire at a time, starting with the longest.

Twist the boot and loosen it from the spark plug. Don’t pull the wire; just rotate the boot to remove the wire from the distributor cap or spark plug.

Step 2: Install the new spark plug wire set


Follow these steps:

  1. Match the longest wire set on the vehicle with the longest in the new set. Ensure you handle one wire set at a time so as NOT to confuse the order of the spark plug wires.
  2. Push the new spark plug wire boot to the spark plug. Push the wire in the spark plug with a screwdriver while holding the boot end. Do not use force. Instead, push gently while twisting, and the boot end will snap firmly over the spark plug insulator, and you should hear a “click” sound.
  3. Put the other end onto the module, ensuring it is properly fixed and the wire cannot dislodge from the cap.

What Should I Do if My Car Has Signs of Bad Spark Plug Wires?

If your car is exhibiting signs of bad spark plug wires, you should avoid driving the vehicle. Diagnose and have the faulty wires replaced as soon as possible.

You can do it yourself if you have the know-how, have some time to spare, and will be careful not to mismatch the spark plug wires.

Alternatively, you can call a professional mechanic to do it for you.

How to Diagnose Bad Spark Plug Wires

After you notice the signs of bad spark plug wires, it would be wiser to further check spark plug wires and be sure before replacing them.

Here are ways to diagnose and test spark plug wires:

#1. Without a multimeter, do a physical inspection of the cables for visual damage like a cut or broken spark plug wire insulator or melting along the cable.

#2. Perform a resistance test with a multimeter


Attach the multimeter probe to each end of the spark plug wire and read the total resistance.

According to the Society of Automotive Engineers, the recommended maximum resistance is 12,000 ohms per foot.

  • Note: Confirm the exact recommended specifications on the tune-up manual because some OEMs differ on the maximum resistance.

#3. Check for shorts along the spark plug cable using a 12-volt test light

With a test light, you can diagnose whether the spark plug wires are faulty.

Find grounding on the engine and attach the test light. Start the engine and slowly move the test light along the spark plug wires.

If there is any breakage on the insulation, then you will see sparks jumping from the cable to the test light.


#4. Use a spark tester to perform a spark test

  • Unattach a spark plug cable from any plug.
  • Take the spark tester and attach one end to the cable and the other to the engine ground. Roar the engine and check the spark tester gap.
  • A blue-white spark is a good spark, meaning the spark plug wire is not faulty. A weak spark will be orange or red and could mean the fault is in the spark plug wire, ground, loose spark plug wire, or too much resistance.


Consequences of Ignoring Bad Spark Plug Wires

Choosing not to replace the bad spark plug wires can lead to their breaking down, which will cause too much electrical resistance. This will, in turn, degrade the spark plug leading to misfires, poor gas mileage, and terrible engine performance.

Faulty spark plug wires can also cause unburned fuel to overflow into the catalytic converter, damaging it.

When to Replace Spark Plug Wires


Depending on your car model, most manufacturers recommend replacing spark plug wires after 30,000 to 50,000 miles.

However, weather conditions, driving habits, or even the engine type can influence the time frame. So, if you experience signs of bad spark plug wires, you should have them checked on time.

Frequently Asked Questions


Spark Plug Wires and Spark Plugs: what is the difference?

Spark plugs are electrical devices that produce a spark to ignite the air/fuel mixture in the cylinders that power a vehicle engine.

On the other hand, spark plug wires are the cables that carry the electrical current from the spark plugs to the ignition system.

How often should I replace my spark plug wires?

Many car makers recommend replacing spark plug wires after 30,000 to 50,000 miles. Indeed, you should check and replace it sooner if you drive the car often.

What is the average cost of spark plug wire replacement?

The average cost to replace spark plug wires is approximately $150 to $350. However, if you want high-performance cables, you might have to spend more.


For your car to perform more efficiently and save you from future visits to the mechanic, ensure you are replacing spark plug cables when they are faulty or after the recommended period in your owner’s manual.

This guide will help you to know what the signs of bad spark plug wires look like so you can diagnose early enough and enjoy a smoother drive.

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