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10 Bad Spark Plug Symptoms & Replacement Cost

A spark plug plays a significant role in internal engine combustion and the daily function of your vehicle. Without it, there will be nothing to ignite the air-fuel mixture and set engine pistons in motion.

But because it is tiny and out of sight, it can be easy to miss out on giving it appropriate care and attention. This neglect often proves to be a very costly mistake down the road.

Recognizing bad spark plug symptoms at the onset is unquestionably necessary for vehicle owners. To avoid unforeseen expenses and on-road troubles, drivers must not allow jerkiness, fouling/discoloration, and engine hesitation to translate to fuel, ignition, or emissions system component failure.

Although non-exhaustive, here are the 10 most common bad spark plug symptoms:

  1. Misfiring or jerkiness
  2. Surging, stalling, rough idling, or starting difficulties
  3. Engine knocking
  4. Poor fuel economy
  5. Throttle unresponsiveness or lack of acceleration
  6. Overheating
  7. Catalytic converter failure
  8. Illuminated Check Engine Light
  9. Failed emission tests
  10. Fouling or contamination

Never put off spark plug replacement and maintenance, especially if OEM recommendations tell you to do so. It makes more sense to employ pre-emptive measures in contrast with experiencing ignition-related problems before taking action.

Savvy car owners need only an hour to replace spark plugs with the right tools. But if you feel uncomfortable taking on bad spark plug symptoms on your own, seek help from a trusted mechanic at once to avert complications.

Bad Spark Plug Symptoms

Old Spark Plugs

1. Misfiring or Jerkiness

One of the most prevalent symptoms of bad spark plug wires is misfiring or jerkiness. If left unattended over an extended period, misfiring can lead to catalytic converter (view on Amazon) damage. Just make sure this is not tied to a bad O2 sensor, as both issues share this tell-tale indicator.

2. Surging, Stalling, Rough Idling, or Starting Difficulties

The reason behind these four scenarios is often assumed to be an empty fuel tank or a fully discharged/dead battery as opposed to a faulty spark plug. While both are not entirely wrong, they are not the only causes of vehicle performance problems or starting difficulties.

Ignition is necessary to bring an engine to life, and the spark plug is responsible for carrying out this task. That makes a compromised or fouled spark plug the main culprit behind a car that would suddenly not start.

In any of these situations, one or more cylinders end up not firing correctly, messing up power output and resulting in energy loss. What makes matters worse is that when they occur, they cause vibrations to resonate throughout the vehicle, damaging the affected spark plug and engine components further.

3. Engine Knocking

Because bad spark plugs cause components like oxygen sensors to malfunction as a proper emissions device, a car ends up with surplus fuel and vapor inside its combustion chamber. These remnants typically catch fire and detonate when the vehicle accelerates.

Ever wondered about that knocking sound coming from your engine when you depress the accelerator pedal? What I just described is the cause behind that unusual noise.

4. Poor Fuel Economy

According to Automotive Service Excellence (ASE), a faulty spark plug reduces fuel economy by roughly 30%. And even with the correct heat range, a spark plug can still go bad.

Several factors could lead to this, such as excessive/incompatible fuel additives, advanced engine wear, or the plug simply reaching the end of its lifespan. Because the spark plug is not functioning the way it should, it consequently forces your car engine to consume more fuel – hence, the drop in gas mileage.

5. Throttle Unresponsiveness or Lack of Acceleration

I cannot stress enough how vital a working spark plug is to vehicle operations. When a plug turns defective, it reduces gas mileage and adversely affects internal engine combustion.

Your power mill then becomes unresponsive to throttle inputs or when you depress the accelerator pedal. This situation requires immediate attention, as it could put you and your passengers in harm’s way when switching lanes or while driving on the interstate.

6. Overheating

A multitude of things can trigger your engine to overheat. But when it comes to spark plugs, this symptom almost exclusively signifies a tightening problem. One of two things could happen.

If the spark plug is loose, it will likely overheat and eventually cause pre-ignition or backfiring. If it is too tight, the insulator and internal components may become damaged, resulting in operating problems.

7. Catalytic Converter Failure

A spark plug may be an electrical device, but it plays a crucial role in fuel economy and emissions. When dirtied or fouled, it would cause the air-fuel mixture to run too rich or too lean, adversely affecting the catalytic converter’s ability to control the amount of air in the car exhaust.

The spark plug does not do this directly, however. What it does is confuse your vehicle’s onboard computer and O2 sensors. These components, in turn, upset the catalytic converter’s ability to alternate between rich and lean mixtures and reduce toxic emissions.

The bottom line is it would be a while for a defective spark plug should this be the case, as this outcome does not happen overnight. Moreover, this symptom is not something you would want, as it is usually accompanied by a host of equally troublesome problems like coolant leaks or leaky valves.

8. Illuminated Check Engine Light

An activated CEL is your vehicle’s secondary reaction to bad spark plugs. Why secondary? Because a spark plug does not necessarily throw fault codes all the time.

Certain situations, however, would guarantee a lit Check Engine Light. Among these scenarios are misfiring, fuel system malfunction, and cat failure (heck, defective spark plugs can even trigger O2 sensor codes!).

9. Failed Emission Tests

More of an aftermath than a symptom, failed emissions testing is usually linked to a faulty sensor or a compromised catalytic converter. However, it can also be associated with ignition and detonation concerns.

When a spark plug goes awry, it fails to burn gases inside the combustion chamber. These unburned gases eventually find themselves in the exhaust system and exit the tailpipes as toxic fumes (in the form of black exhaust smoke).

10. Fouling or Contamination

Ideally, this tell-tale sign should never occur. Or if it were to happen, it should only do so in its early stages. But realistically, vehicle owners (myself included) discover this a tad too late – usually when they already have fault codes or are confronted with performance problems.

A regular, preventive inspection of your spark plugs should be enough to save you from experiencing loss of acceleration power or engine hesitation. Sadly, it can be easy to dismiss turning this practice into a habit.

It also does not help that some spark plug types only warrant replacement every 30,000 to 50,000 miles (or even longer), giving some drivers a wrong sense of complacency when servicing and visually inspecting these components.

Causes of Spark Plug Failure

Spark Plug Ignition

As driving with bad spark plugs can cause a slew of headaches, it is better to take preventive measures than suffer the above symptoms and their impending problems. This feat is doable as long as you adhere to scheduled maintenance and inspect different system components.

However, you may not always beat bad spark plug symptoms (or the resulting problem) to it with these steps. Thankfully, you would still be able to tell what may have caused your spark plug to become defective based on its appearance:

Grayish Tan to White Color Around Insulator at the Firing End

  • Cause: Normal spark plug wear or operation (naturally occurs when the plug reaches its service limit)
  • Remedy: Replace the spark plug in line with OEM recommendations

Rounded Center/Ground Electrodes or Excessive Spark Plug Gap

  • Cause: General wear and tear
  • Remedy: Get a replacement spark plug with the same heat range.

Spark Plug Gasket Insufficiently Crushed

  • Cause: Incorrect tightening of spark plug
  • Remedy: Tighten spark plug per OEM-recommended torque values

Encrusted Light Brown Deposits on the Center/Ground Electrode

  • Cause: Excessive fuel additives and general engine wear
  • Remedy: Confirm spark plug heat range is correct and check for engine wear

Reddish/Brown/Purple Deposits or Discoloration on the Core Nose Insulator

  • Cause: Use of fuel additives
  • Remedy: Replace spark plugs and ensure additives are compatible with ignition and emission systems and used in the right proportions

Cost of Replacing a Spark Plug

Estimated spark plug replacement costs are based on a vehicle’s year, make, and model, type and condition of the engine, choice of spark plug, and labor fees (if the job is outsourced to a professional mechanic).

Below are the average pricing ranges for each spark plug type:

  • Copper: E3 spark plugs cost between $5 and $230; a Champion Copper Plus 302 plug is around $3
  • Iridium: $7 – $70
  • Platinum: $3 – $30
  • Silver: $3 – $30

Collectively, spark plug prices fall between $10 and $100 (sans spark plug wire and labor costs). With labor fees, expect an additional $66–$250 minimum. Prices will vary depending on your vehicle, how far gone the bad spark plug issue is, and if repairs outside your spark plug and ignition system are needed.

If you are mechanically inclined, you can keep your expenses to a minimum by doing the replacement yourself using a spark plug gap tool  (view on Amazon) and HAZET 4766-3 154 mm 12-Point Profile Spark Plug Wrench (view on Amazon).

How Long Do Spark Plugs Last?

Spark Plug

Barring complications, the answer to this question depends on the spark plug type and car make and model. For secondhand wheelers, the car’s accumulated mileage is factored in.

Collectively, spark plugs can last from 12,400 to 150,000 miles. Performance and luxury models, however, may have spark plugs with shorter intervals.

That said, you should heed your OEM’s recommendations regarding spark plugs. This will only change if you opt for aftermarket spark plugs different from what came stock with your car. At this point, the spark plug’s composition determines its longevity. Your vehicle model only comes in second.


These are the most affordable and best-performing options in the market. Their copper core coated with nickel alloy make for great electricity conductors. But it is for this same reason they must change more often than the others – roughly every 12,400 miles (20,000 km).


NGK spark plugs like the NGK Iridium IX (view on Amazon) have a life expectancy of 40,000–50,000 miles (64,300–80,400 km) or higher if used on an unaltered motor. Meanwhile, the NGK Laser Iridium spark plugs typically last between 80,000 and 100,000 miles (128,700–160,900 km).

The latter has an iridium center and platinum (instead of nickel) ground electrodes of OEM design. These figures are equivalent to approximately 3,000–4,000 hours of engine uptime.


Spark plugs of this kind give a slightly weaker spark but will outlast copper and some iridium plugs by a considerable margin. The life expectancy of a platinum spark plug – whether single or double platinum tip – is within 40,000–50,000 miles (48,200–80,400 km) and is ideal for car owners who value longevity more than performance output.


Used in older European cars and motorcycles (specifically performance ones), this type features silver-coated electrode tips that fall behind copper in handling thermal conductivity.

Despite this, it does not last longer than platinum or iridium spark plugs as its metal composition is less durable – although it is slightly ahead of copper plugs at approximately 20,000 miles (32,100 km).

If you do oil changes frequently or your vehicle has short maintenance intervals, a copper spark plug would be the best pick out of the four. Otherwise, any of the other three options would do you well.

Is it Okay to Drive with a Bad Spark Plug?

While continuing to drive a vehicle with a worn spark plug may not be immediately life-threatening, I strongly advise against it as repercussions can be catastrophic (if not expensive) for your car.

As established in this guide, bad spark plug symptoms are often suggestive of accompanying problems in the fuel and emissions systems. That said, continuing to drive your car with a defective plug is like pushing your luck and asking for repair expenses (and your anxiety) to double.

Conclusion – 10 Bad Spark Plug Symptoms & Replacement Cost

To recap, here are the 10 most common bad spark plug symptoms:

  1. Misfiring or jerkiness
  2. Surging, stalling, rough idling, or starting difficulties
  3. Engine knocking
  4. Poor fuel economy
  5. Throttle unresponsiveness or lack of acceleration
  6. Overheating
  7. Catalytic converter failure
  8. Illuminated Check Engine Light
  9. Failed emission tests
  10. Fouling or contamination

The items in this list can be avoided if identified early on and addressed promptly. But like all other car problems, there are a few exceptions to this proverbial rule – as is the case with high-mileage, secondhand four-wheelers.

In these situations, it is best to replace defective spark plugs and rectify problem sources before they create more complications. More importantly, strictly adhere to routine maintenance and change intervals moving forward.