2016 Mercedes-Benz SL400 Spark Plug Replacement Cost

The spark plug turns high voltage electricity from the ignition coil into a strong spark inside the engine. This ignites the air-fuel mixture, creating power.

When spark plugs fail, the engine will struggle. The check engine light will come on, and it may appear either solid or flashing. The driver would likely notice one or more of the following: engine vibrations, rough idling, poor gas mileage, excessive cranking time needed to start the engine, and power loss or surging. In extreme circumstances, the engine may stall or fail to start.

The car will run with a spark plug problem, but it's not wise to keep driving it this way. If a spark plug isn't working, unburned fuel from the engine will get into the exhaust at a higher rate, causing problems for the catalytic converter. If the converter gets ruined by this unburned fuel, it will lead to a costly repair.

Spark plugs do eventually wear out, and failure is most common after 100,000 miles of use under normal driving conditions. But that can be greatly shortened by harsh conditions or leaking fluids.

If the spark plugs are suspected, they're checked for proper connection with the ignition coil. If that's OK, the plugs must be removed, and the spark plug gap must be checked. If the gap is too small because of corrosion, or too large because or deterioration, they will be replaced. If the spark plug is in good condition, a tester is used to find whether the spark plug is able to produce spark.

On newer vehicles, spark plug replacement involves removing the engine cover, ignition coil harness and ignition coils, and then finally the spark plugs. For most older vehicles, removing the spark plug wires and spark plugs is all that's required. Once the spark plugs are removed, the gap on the new spark plug is set, and the plug is installed using an anti-seize compound and proper torque. Prior to reinstalling the spark plug wire or ignition coil, dielectric grease is applied to the spark plug terminal to promote the transfer of electricity.

Following your carmaker's recommended maintenance means the spark plugs will be changed before they become faulty, which will save you money on diagnostic fees.

The cost of spark plug replacement can be surprisingly low or very high — it all depends on how many spark plugs need to be replaced, how hard it is to get to them, and how much diagnosis is required to identify them as the problem. 

Replacing your spark plugs can be one of the easiest or most frustrating experiences you can have. If your plugs are easy to access, it's usually a simple repair. But if the plugs are hidden away, requiring a lot of disassembly, you may be better off having a repair facility handle the job. If you choose to do it yourself, ensure you have the proper spark plug gap, use anti-seize compound on the threads, use dielectric grease on the spark plug terminal, and make sure the threads go in straight before attempting to tighten to the manufacturer's specifications.