Every gas engine uses spark plugs to ignite the air-fuel mixture as the end-point in the ignition system. A spark plug has a terminal on one end, which the coil or wire fits into. On the other end, it has a pair of electrodes. The ignition system coil sends a high voltage spark across the electrodes at the appropriate time, which ignites the air-fuel mixture in the combustion chamber.
The size of the gap between the electrodes is critical, as that determines how much energy is required for current to spark across the gap. Each time a spark plug fires, there is a tiny amount of wear on the electrodes, which causes the distance across the gap to increase. The type of metal used in the electrodes makes the most difference in how much wear there is. Over time, the electrodes can also build up carbon deposits or become fouled with oil or fuel. Either way, spark plugs do wear out, and they have a scheduled replacement interval, which can be found in the owner’s manual.
Replacing a spark plug is usually pretty simple. The spark plug wire or coil is removed from the plug, and then the spark plug is unscrewed from its position on the engine. The gap on a new plug is checked, and then it’s screwed into place. Spark plugs are always recommended to be replaced as a set, as it’s important that all the cylinders are firing and contributing smoothly and equally.
How to Tell if You Might Need Spark Plugs
Reading the maintenance section of the owner’s manual and then replacing the spark plugs at the suggested interval is always the best approach. That can vary between 30,000 miles and 120,000 miles, depending on the vehicle and the type of spark plug used. It’s always best to replace spark plugs before they are needed.
When spark plugs are near the end of their lifespan, they can fire less efficiently and impact fuel mileage. If left too long, they can become fouled or fail to ignite the fuel mixture at all intermittently, which causes a misfire. That is usually first felt at idle, when it will appear as a rough-running engine, and sometimes the exhaust can smell like raw fuel. In most cases, it will also cause the check engine light to come on, as the PCM monitors the ignition system pretty closely.
A solid engine light is usually best checked as soon as possible. A flashing check engine light usually indicates a problem bad enough that the vehicle shouldn’t be driven. Many things can cause an engine light, and you don’t know what the reason is until the code is read. Any auto shop can read codes; many auto parts stores will also read engine codes if asked; and it’s fairly inexpensive for anyone to buy a code reader. Some of the more convenient ones plug into the OBDII port under the dash and use Bluetooth to pair with a smartphone.
The codes that might indicate a spark plug problem would be P030x codes. P0300 is for a random misfire, P0301 is for a cylinder 1 misfire, P0302 is for cylinder 2, and so on. Other things can cause misfires, but the first thing to check is the spark plugs, and if they are due for replacement, they are usually replaced before any other diagnosis is done. If that solves the problem, then that’s it.
In most shops, if a vehicle comes in with a check engine light for diagnosis, the work begins with a one hour diagnostic charge. If the problem is simply that the spark plugs are overdue for replacement, some shops would simply recommend that they be done first and only charge for the diagnosis if that doesn’t solve the issue.
Costs of Spark Plug Replacement
The cost of replacing spark plugs depends on several things, including how many spark plugs there are. Using $150 an hour as a typical shop rate, for a four cylinder engine, the average cost to replace the spark plugs is about $198. For a six cylinder engine, the average cost is about $400 (often requiring the removal of an intake plenum). For an eight cylinder engine, the average cost is about $360.
There are many exceptions, of course. On a 2010 Dodge Charger with a 5.7 liter V-8, for instance, there are two spark plugs per cylinder, for a total of 16 plugs, and access is restricted on 4 of them. The labor time is 1.6 hours, and OE spark plugs cost about $10 each. This makes the job about $400. The recommended plugs use standard copper electrodes, and have a 30,000 mile replacement interval.
For a 2007 Maxima with a 3.5 liter V6 engine, the labor time to replace the spark plugs is about 3.4 hours. On the right side of the engine (which is up against the firewall), the intake plenum lays over the plugs and has to be removed before they can be accessed. Using the OE platinum spark plugs, the job comes to about $600 most places. The spark plug replacement interval for that model and engine is 105,000 miles.
For an example on the cheaper side, to replace the standard spark plugs on a 2003 Honda Civic with a 1.7 liter engine, the cost usually comes to around $90. That uses a standard copper spark plug, with a replacement interval of 30,000 miles.
Different Kinds of Spark Plugs
The main improvement to spark plugs over the years has been the use of more durable metals in the electrodes. A standard spark plug uses a copper core plated with nickel alloy steel for its electrodes and is typically good for 30,000 to 50,000 miles. Newer plugs use platinum inserts on one or both of the electrodes. These are typically good for 60,000 miles. Iridium, or ruthenium spark plugs are another type, and can be good for more than 100,000 miles. The main advantage of the newer types is that they last longer; there is usually no difference in engine performance or miles per gallon.
In many cases, a vehicle that specifies standard spark plugs can be upgraded to platinum or other kinds of spark plugs, and it can still work fine. But it’s also common enough for non-OE spark plugs to have small differences in shape, in the value of the internal resistor, or in the heat range they are engineered for, all of which can cause problems. There are many vehicles that are quite finicky about spark plugs, and that don’t reliably run well unless they have the specific OE spark plugs. Even small differences can cause misfires in certain conditions. For that reason, many shops are reluctant to install spark plugs that aren’t OE.
What Else Might be Recommended
The most common thing is spark plug wires. Most vehicles are coil-on-plug nowadays and don’t use spark plug wires, but if spark plug wires are used, it’s common to replace them with the spark plugs. They can be fairly delicate and easily damaged when disconnected from the plugs, so it’s not a bad idea.
If the vehicle uses a cap and rotor, those are usually replaced on the same schedule as the spark plugs, and then they’re all replaced together as a “tune-up”, usually with a bundled price. Very few vehicles use a cap and rotor these days, though.
It’s also very common for spark plugs to be done at a service interval where other things need done as well, such as intake system cleaning, transmission services and so forth. At a dealership there is often a bundled-cost service, while other service facilities will separate out the costs. For the most part there isn’t a cost savings to one way over the other, and the more important thing is just to stick to a regular maintenance schedule regardless of who does the work.
What can go Wrong
Probably the most common problem that comes with spark plug replacement is shipping damage or a dropped plug that causes the gap to close up. If the gap isn’t correct, then the spark won’t be adequate, or won’t happen at all, causing a misfire. This is the reason to check the gap on each plug before installation.
The body of each spark plug has a metal part carrying the threads and the hex that it is screwed into, and then a ceramic insulation around its positive connection to the coil. If a plug is dropped, the ceramic insulation can crack, and then the voltage spike can follow the crack instead of bridging the gap in the combustion chamber. This also causes a misfire.
The threads in the cylinder head can strip, which is pretty much a worst-case scenario. Sometimes the thread can be cleaned up or repaired in place, other times the cylinder head may need to be removed for repair. The two practices that help avoid that are always starting the threads by hand and torquing the spark plug to the correct specification. A spark plug too tight can damage the threads, while a spark plug too loose can blow out, taking the threads with it.
One controversial topic is whether to use anti-seize on the spark plug threads. Some mechanics always do this in order to avoid the plug seizing in place. But anti-seize is abrasive and causes a little bit of wear on the threads when tightened down. It also interferes with torquing the spark plugs correctly, as the specs all assume dry threads.
There’s a wide variety of different service intervals depending on the engine and the kind of plug used. As a general rule, copper core spark plugs should be replaced every 30,000 miles, platinum spark plugs every 60,000 miles, and iridium or ruthenium spark plugs every 100,000 miles. Consulting the owner’s manual is the best approach.
Possibly, but many engines are very particular about what plugs they require, and can have misfires or run rough if they don’t have the recommended plugs. Most shops will prefer to install only OE plugs, or plugs as close to OE as possible, for that reason.
On most engines, yes, it’s a pretty straightforward job with few tools needed. 4 cylinder engines are usually the easiest. Many six cylinder engines are more difficult, often having an intake plenum that needs to be removed to access the plugs, and often having a transverse engine with the rear bank under the hood cowl. A general rule is that if you can see where the spark plugs go, you can probably replace them.
A spark plug socket is the main tool, made to be operated with a hand ratchet. These have a rubber insert to hold the spark plug in the tool, which protects the ceramic body and prevents the spark plug from being dropped. Most engines use a long spark plug tube where the part is installed, so most spark plug sockets are very long, as compared to standard sockets. A variety of common hand tools might also be necessary on some engines where other parts need to be moved out of the way.
In many cases, the owner’s manual has that information. In other cases, asking at a parts store or dealership parts department gets a good answer. Parts catalogs might list a dozen different spark plug options for a vehicle, but they will also usually specify which on the list is the OE spark plug.