Testing GMC Sierra spark plugs isn't always as easy as it might sound & is an important step when diagnosing misfires like those associated with OBD Code P0300, P0301, P0302, & P0303 among other maintenance & repair tasks. This article aims to help you learn how to test the spark plugs on your vehicle without having to take it to a mechanic.
What is a Spark Plug?
Before we get into how to test the spark plugs on your GMC Sierra, let's briefly take a look at what a spark plug is & what it does. There's not a lot you need to know here but the basics are important if you want to maintain a vehicle. In short, a spark plug is one component of a gasoline engine (notably not present in diesels) that delivers an electrical spark into your engine cylinder, igniting the fuel-air mix and starting the explosion that drives the piston.
How do Spark Plugs Work?
Spark plugs are almost always inserted into the top of the cylinder assembly, at least in small engines. They typically screw into a threaded slot on the top of the cylinder – this is the "spark plug hole" you'll see in the engine block. The spark plug is connected to the ignition system, which is responsible for delivering the spark to the spark plug.
The spark plug typically recieves 20,000 to 30,000 volts from the ignition system, which connects to the spark plug through a thick & heavily-insulated wire. The spark plug turns this voltage into an electric "spark" in the combustion cylinder by way of its tip that rests directly inside. There's a few things that can go wrong with spark plugs – essentially all of which boil down to the plug not jumping the "gap" into the cylinder & igniting the fuel.
Tools You'll Need to Test a Spark Plug on Your GMC Sierra:
There are only two tools you really need to test a spark plug on a GMC Sierra – an appropriately-sized socket wrench (typically a 5/8" socket) & a spark plug tester. Any old socket wrench should do, but purpose-designed ones are available for purchase, like this one from Amazon.
You'll also need a spark plug tester to check your GMC Sierra's spark plug function. A spark plug tester is a neat (and cheap) little device that checks a plug's electrical functioning. It works by running the electrical current from your spark plug through itself – if it lights up, your spark plug is working properly. If not, either your ignition system or your spark plug is faulty. We recommend getting these online as they can sometimes be hard to find locally. Here's one option for a spark plug tester.
How to Use a Spark Plug Tester on a GMC Sierra:
Attach your spark plug tester to the ignition wire – with the other end of the tester connected to the plug itself (Sitting in your engine, of course). Fire up your engine & take a look at the tester. If it lights up, that plug isn't the issue & you can turn the engine off & repeat the process elsewhere if needbe.
However, if you don't see any light, that spark plug is likely faulty & you'll need to replace it. There is a distinct possibility that your ignition system is faulty, but that's a much more complicated issue to diagnose & fix, and goes beyond the scope of this article. If you're not sure, you can always take your vehicle to a mechanic for a diagnosis.
If the Spark Plug is Faulty, How Do I Replace it?
If you encounter a suspected faulty spark plug, you'll need to replace it. Before you go out & buy a new set, though (spark plugs should all be replaced at the same time), you should check to ensure it's not just too dirty to function. Remove the plug & visually inspect it for oily or fluffy black deposits on the tip. Don't fret if the tip is covered in a light gray or brown residue – this is normal.
You might get away with just cleaning the plug if the only issue is that's its dirty. However, if the plug either doesn't have the problematic residue or fails even after cleaning, you'll need to replace it. Spark plugs are relatively cheap individually, but the more cylinders you have, the more it will cost to buy a set. Budget about $8-10 per plug and buy one for each cylinder in your vehicle.
When it comes to buying spark plugs for the GMC Sierra, you'll want to make sure you get the correct type. This is a list of options that should work well for your GMC Sierra
Our Typical Caveats & Disclaimers:
There are a number of mistakes you can make when diagnosing spark plug issues on a GMC Sierra. It'd be hard to screw up too badly, but be careful. We hope this article has been helpful, but we'd like to remind you – Use the information above at your own risk. This article is not meant to be a substitute for professional advice. If you're not comfortable with taking the steps outlined above, we recommend taking your GMC Sierra to a mechanic or dealership to have them diagnose & repair the issue.