EXPERT IN AUTO PARTS
7 April 2025 (updated) Table of Contents
You may be able to determine whether a car spark plug has seen better days just by looking at it. There are often obvious signs of wear, damage or contamination, which will be outlined in the guide below. We’ll also discuss how to test a spark plug using a multimeter.
Before we dive into that, we should establish the ground rules for your safety. Before inspecting or testing the spark plugs, you should make sure to wear appropriate protective gear, such as safety glasses and protective work gloves. Do not attempt to remove the components while the engine is still hot and make sure the ignition is switched off at all times while handling the components.
Spark plugs are located at the top of each cylinder in your engine, threaded into the cylinder head, with spark plug wires leading to the top of the engine. In newer vehicles with "coil-on-plug" designs, you'll see plastic modules directly attached to each spark plug instead of wires.
The number of spark plugs depends on the number of cylinders in your engine. On a 4-cylinder engine, you'll find the spark plugs arranged in a row along the top or side of the engine. For inline 6-cylinder engines, the spark plugs are positioned on the top or side of the engine head. In V6 and V8 engines, the spark plugs are evenly distributed on both sides of the engine, matching the V-formation of the cylinders.
For a proper test, you'll need several specific tools that help diagnose the condition and performance. A thorough inspection requires both visual assessment and electrical testing.
Required tools include:
These tools allow you to remove, inspect, clean, and test your spark plugs to check if they're functioning correctly or need replacement. Testing spark plugs can reveal problems with your ignition system and overall engine performance.
Ensure that the car is parked on a hard flat surface and the engine is cool. Make sure the engine and ignition are switched off and the key is removed from the ignition before proceeding. Check the manufacturer's repair guide for specific safety instructions.
You’ll need to disconnect the spark plugs from the ignition system. Older vehicles typically have a wire connecting the plugs to the ignition, whereas newer vehicles normally have a separate ignition coil for each spark plug and use coil-on-plug connectors. Coil-on-plug connectors usually feature a locking tab that can be released using a screwdriver.
Using the correct size spark plug wrench and socket, gently unscrew the spark plug from the cylinder head and remove the component.
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Look for the following signs once the plugs have been removed:
If the ground or centre electrode is broken or the ground electrode has been flattened, shortening the gap between the two, the components will not be able to produce a normal spark. This can occur when the wrong size spark plug is installed.
Melting or white deposits are a sign of thermal damage caused by engine overheating, an unbalanced air-fuel mixture, timing faults or an incorrect heat range.
Some deposits can be expected but if the plugs are unusually dark and sooty, they could be over contaminated. Carbon fouling is often the result of a bad air filter, a rich air-fuel mixture, or poor driving habits.
This could indicate that oil is leaking into the engine cylinder head.
This is often the result of a flooded engine.
While most spark plugs work effectively throughout their service life provided that there are no underlying issues, it is still recommended to only use parts from trusted brands, such as NGK spark plugs, to ensure long-lasting efficiency.
You can test your spark plugs using a multimeter. Simply set the device to the 20K ohms setting and place the probes on both ends of the plug; one on the terminal end and one at the tip of the centre electrode.
You should see a reading between 5 and 15. If there’s no reading, the internal components may be damaged. If there’s a reading when one probe touches the terminal end and the other touches any other metal part of the plug, this indicates a malfunction.
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Testing spark plugs without a multimeter is straightforward using visual inspection and simple tools. First, remove the spark plug using a socket wrench with the proper size socket. Examine the plug's appearance – a healthy plug should have a light brown or greyish electrode with minimal deposits. Black, sooty deposits indicate a rich fuel mixture, while white, chalky deposits suggest the engine is running too hot.
To check for spark, use an inline spark tester connected between the ignition coil and spark plug. Alternatively, you can remove the plug, reconnect it to the ignition cable, ground the threaded portion against the engine block, and have someone crank the engine while you watch for a visible spark across the electrode gap.
Listen for regular clicking sounds during cranking, which indicates the ignition system is firing properly.
You can perform limited spark plug checks without removal. Listen for misfires or rough idling, which may indicate plug issues. Use an engine diagnostic scanner to detect misfire codes. For a more direct test, use an inline spark tester connected between the ignition coil and plug – a visible spark confirms the ignition system is working.
However, important assessments like examining electrode wear, measuring gap width, and checking for deposits require removing the plugs. Without removal, you can only infer potential problems rather than confirming specific issues with the spark plugs themselves.
A bad spark plug can certainly trigger the check engine light. When spark plugs fail, they cause engine misfires that disrupt the combustion process. Modern vehicles have sensors that detect these misfires, and when they occur repeatedly, the engine control module records the issue and illuminates the check engine light.
Common diagnostic trouble codes related to spark plug problems include P0300 (random/multiple cylinder misfire) or specific cylinder misfire codes like P0301 (cylinder 1 misfire). These codes alert you to investigate the ignition system, with faulty spark plugs being a primary suspect. Replacing worn or damaged spark plugs often resolves these check engine light warnings.
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