How to Tell if Your Spark Plug Wires Are Failing

Mia Bevacqua
June 22, 2018

spark plug wires

Spark plug wires, sometimes called ignition cables, aren’t as common as they used to be, but some cars made in the last couple decades still have them. If they go bad, your engine will likely suffer in terms of performance and fuel efficiency.

A faulty spark plug wire can cause several problems:

1. Your check engine light is on: If the engine’s computer detects an issue that could increase vehicle emissions, such as a misfire caused by a bad spark plug wire, it turns on the check engine light.

2. Your engine is running poorly: An arcing or high-resistance plug wire will prevent the spark plug from firing properly. This results in incomplete combustion, causing the engine to misfire. As a result, the car may hesitate and shudder.

3. Increased emissions: Bad spark plug wires can increase the hydrocarbon emissions from your vehicle. This can cause it to fail a state emissions inspection.

4. Worse fuel economy: Incomplete combustion usually creates poor fuel economy. A bad spark plug wire may result in more frequent trips to the gas station.


Get it diagnosed by a professional

How spark plug wires work

Spark plug wires deliver high voltage to the spark plugs, which then ignite the air-fuel mixture in the engine’s cylinders. Depending on the vehicle design, each wire connects between either the distributor cap or an ignition coil on one end, and a spark plug at the other.

Distributor ignition system: With this design, voltage flows from the ignition coil, through a spark plug wire, to the distributor. As the distributor rotor turns, it delivers voltage from the center terminal to the spark plug wires. Energy then flows through the spark plug wire to the plug, causing it to fire.

Distributor-less ignition system: Typically, in this type of system, each ignition coil pack provides electricity to two spark plugs. High voltage flows from the coil pack through the spark plug wires to the plugs, causing them to fire.

Most vehicles made in the last 10 to 20 years don’t have spark plug wires. Instead, an individual coil pack sits directly on top of each of the spark plugs.

» LEARN MORE: Get an estimate for your spark plug wire replacement

How to fix the problem

Before condemning the spark plug wires, a thorough diagnosis should be performed. A professional can do this using equipment such as a scan tool and oscilloscope.

Spark plug wires are expected to wear out eventually, often after 60,000 to 100,000 miles, though that can vary by manufacturer. They can also become faulty ahead of schedule.

Oil or coolant leaks can damage ignition wires. To prevent reoccurring problems, the leaks will need to be repaired as well.

Faulty spark plug wires should be replaced. This job can be more difficult than it sounds. On some vehicles, the wires are difficult to get to, and replacement should be left to a trained technician.

If your wires are easier to access, you can tackle this chore yourself. Just take caution, as the ignition system carries high voltage. Mark the spark plug wires according to which position they sit on the distributor or which ignition coil they attach to, and to which cylinder they supply power. 

Always use parts that are as good as or better than the original manufacturer’s equipment (also called OEM). This way, you won’t run as much risk of having to replace them again soon.


Photo credit: Ben Ostrowsky via Flickr


Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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