How To Check Spark Plug Wires

13 May 2017
 Categories: , Blog


If your vehicle engine seems sluggish, or you experience decreased fuel mileage, or an erratic idle, test the spark plug wires. Spark plug wires usually don't need replacing too often, but they should be checked occasionally to ensure optimal performance.  

The spark plug wire carries voltage from the ignition to the spark plug. Over time, the wires can get damaged. It is simple to check spark plug wires by following these tips.

Prepare to Work

For this project, you need:

  • work gloves
  • flashlight
  • rags
  • spray bottle
  • multimeter
  • spark plug pliers
  • spark plug
  • boot grease
  • jumper cable

Turn the engine off, and let it cool, Park the vehicle on a flat surface in a lit area, and engage the parking brake.

Prop the hood open and look for a group of four to eight colored wires. The spark plugs are  commonly attached to the end of the wires. If you are still uncertain of the location of the spark plug wires, consult your owner's manual.

Inspect the wires with a flashlight. Check for cuts, burns, and examine the area between the boot and spark plug for corrosion. Test wires for brittleness by bending them. 

Check  for Arcs

Start the engine, and listen for arcs or snapping sounds that may indicate an electrical leak. Ground an insulated screwdriver with a jumper wire, and run it over the coils and boot. Test the insulation, arcs by running a damp rag over it. If any part of the wires produces an arc, replace them. 

Fill a spray bottle with water, and mist the wires, especially near the boots. If you get an arc near the boot, turn off the engine, and let it cool. Take off the spark plug boot, and examine it for carbon tracking, which indicates the spark plug also needs replacing.

Test Wires with a Multimeter

If there is no arc or visible wire damage, test the wires with a mulimeter. Ensure the engine is off, and it has cooled. 

Place the pliers as close as possible to the boot, and  detach the wires with a gentle twisting movement. Detach the wires from the retaining clips.

Find the resistance rating for spark plug wires in your manual, which is commonly 3,000 to 12,000 OHMS. Touch the probes on the multimeter to the metal contacts..You should get  a reading within the resistance rating.

Before you reattach the wires, apply some boot grease on the end of the wire boot. This makes them easier to remove the next time you check them. If the spark plug wires are defective, and you don't trust your skill, get a  mechanic to  replace them. Talk with a company like Professional Automotive for more information.