Signs Your Spark Plugs Need Replacing (The Top 9 Symptoms)


The effects of bad spark plugs can be varied, and can be the same as those caused by the failure of other engine components. It’s important that if you’ve got any of the following symptoms that you rule out the spark plugs first by taking them out and examining them.

Spark plugs are usually cheaper to replace than other engine parts, so even if they look ok, it might not be a bad idea to change them anyway.

1. Poor Fuel Economy

A very common symptom of bad spark plugs is a noticeable  drop in fuel economy. Contrary to other components that influence gas mileage, the effect of bad spark plugs is far more noticeable. Some studies have shown up to a 30% increase in fuel consumption due to bad spark plugs. That’s a huge increase, only comparable to bad fuel injectors

2. Engine Misfires

Engine misfires due to bad spark plugs represent a big issue that should be addressed as soon as possible. Usually an engine misfire will be accompanied by a check engine light, so you should get it checked out as soon as possible.

The incomplete fuel burning in the combustion chamber can easily lead to a severe misfire condition and all its associated outcomes:

  1. Intermittent stumbling sounds on the engine. 
  2. Possible engine knocking that could lead to drastic mechanical problems.
  3. Performance issues and power loss.
  4. Carbon deposits build-up on spark plugs (if not already present).
  5. Other ignition problems.

3. Engine Back Firing

This symptom is normally associated with an improper air to fuel ratio, specifically a lean air-fuel mixture. However, unburned fuel particles could also explode in the exhaust system due to its high temperature.

This can vary from vehicle to vehicle, but it is still possible and should be checked out as soon as possible so as not to damage the catalytic converter. If you have an OBD code reader you may see codes such as P0420 or P0430.

4. Engine Knocking (Bad Spark Plug Sound)

You can think of engine knocking as an extreme case of engine misfires. Bad spark plugs are one of the causes of engine knocking, and this knocking is the classic bad spark plug sound that you may have heard of.

You should worry when the typical knocking sound becomes audible because the risk of mechanical damage increases exponentially under this condition. The following is just a small list of engine components that can be damaged by consistent engine knocking:

  • Piston heads. The first component directly affected by engine knocking is the piston head. Depending on the intensity of the pinging, it may literally perforate the piston head.
  • Piston compression rings. In a similar way to piston heads, the piston rings also withstand increased pressures due to engine knocking. A broken compression ring is not as bad as a destroyed piston head but also requires a major engine repair.
  • Due to the increased pressure in the combustion chamber, the cylinder head itself could become perforated in some cases.
  • As you may guess, cylinder head valves could literally “melt” due to continuous engine knocking. Increased pressure is accompanied by an increased temperature that can easily surpass the valves’ material tolerance.
  • The combination of higher pressure and temperature in the combustion chamber could lead to accelerated wear of the valves, cylinder head, compression rings, and piston head which means metal debris and fuel contaminants will enter to the lubrication system causing wear on other engine components.

5. Lack Of Acceleration

The ignition system is one of the main systems responsible for acceleration. When a key component like the spark plug is faulty the acceleration performance drops significantly.

Depending on the spark plug condition, your car may experience as much as a 40% power loss.

Since few other components have such a high impact in engine performance. That’s why you should pay attention to their life expectancy. You may be running worn spark plugs without being aware of it.

6. Rough Engine Idle

It’s very hard for the engine ECU to compensate for issues related to bad spark plugs.

The reason is simple, the ECU can only modify fuel delivery through the fuel injector’s pulse modulation or change the ignition timing, but this doesn’t help little in this case.

When the combustion efficiency is hindered by faulty spark plugs, a rough idle is almost inevitable. The severity can vary quite a lot from one engine design to another. But this symptom will always be present.

7. Engine Is Difficult To Start

No matter the year of your vehicle, a classic symptom attributed to bad spark plugs are hard starts. If the spark plugs are bad enough, bad spark plugs can cause a car not to start at all.

Starting the engine requires several conditions, one of them a good air-fuel mixture ignition. When spark plugs are faulty this condition is not met or is partially met, which causes the hard starts.

8. Increased Exhaust Emissions

If the engine is not running as it should with the proper air/fuel mix, then you can expect an increase in exhaust emissions.

Depending on the spark plug condition, the associated side effects can cause a failed emissions test, and this might be a problem if you live in an area with tight emissions laws. 

9. Strong Gas Smell From The Tailpipe

Another common symptom of bad spark plugs is a gas smell from the exhaust fumes.

This symptom is related to the unburned fuel coming out from the combustion chamber. Fuel particles can detonate (back-fire) in the exhaust system but could also exit through the tailpipe.

This is where the strong gas smell comes from, however, be aware that bad spark plugs are only one of many possible causes for this symptom.