Maintenance Tips, Tricks And Hacks For Your Vehicle

Can Bad Spark Plugs Cause a P0420 Code? (Answered!)




P0420 is one of the most common error codes. It means your catalyst system efficiency is below the threshold near engine bank one which indicates that the catalytic converter is not functioning efficiently.

At this point, the vehicle is increasing the output of harmful pollutants into the atmosphere. But, can bad spark plugs cause a P0420 code?

Key Takeaway

  • Bad spark plugs can cause a P0420 due to the increased amount of unburnt fuel going into the catalytic converter and causing it to work harder.
  • You should consider replacing your spark plugs if you notice signs such as the check engine light turning on, difficulty starting the car, poor gas mileage, slow acceleration, rough idling, or engine misfires.

Can Bad Spark Plugs Cause a P0420 Code?

Can Bad Spark Plugs Cause a P0420 Code

Yes. Bad spark plugs can cause a P0420 code because when one or more spark plugs are faulty, they are not able to ignite the air and fuel mixture in the combustion chamber so the unburnt mixture, along with the Nitrogen Oxide and Carbon Monoxide from the combustion chamber will go into the catalytic converter.

This puts extra load on the catalytic converter which is designed to convert NOx and CO into less harmful toxins, however, the unburnt fuel puts extra load on the catalytic converter and can definitely cause a P0420 code.

Car owners don’t even know if their engine has spark plugs. Gasoline engines have spark plugs whereas diesel engines are equipped with glow plugs. Spark plugs are located in the cylinder heads and have a single purpose: to ignite the air and fuel mixture in the combustion chamber when ordered by the vehicle computer.

When one or more spark plugs go bad, the air-fuel mixture that needs to be ignited by the spark plug and burned in the combustion chamber will leave unburnt in the catalytic converter.

The catalytic converter is designed to deal with the nitrogen oxide and carbon monoxide that forms during combustion. NOx and CO, combined with unburnt fuel, puts extra load on the catalytic converter, and most of the time a check engine light is thrown with a P0420 code.

Inside a catalytic converter, there is a ceramic honeycomb with layers of metals such as platinum, rhodium, and palladium.

Next to the catalytic converter is an oxygen O2 sensor which monitors the output i.e. how clean are the gases that exit the vehicle through the tailpipe. If there is a lot of unburnt fuel because of bad spark plugs, the O2 sensor will detect this abnormality and cause a P0420 code.

How Do You Know When To Replace Your Spark Plugs?

Can Bad Spark Plugs Cause a P0420 Code (Answered!)

A spark plug gets its name because it literally creates a spark that jumps from the center electrode to the ground electrode. This happens inside the cylinder which ignites the fuel and forces the piston down.

It’s a pretty important part to get the car to run properly. Now, bad spark plugs don’t project a spark as hot and might not even spark every time that they’re supposed to. That could cause your car to misfire, hesitate especially under a load, it might idle rough, or you could get a check engine light and a P0420 error code.

When visually inspecting a spark plug, check the ceramic and make sure there are no cracks. Look for any deposits on the electrode, if there are any, replace the spark plug. Lastly, measure the gap. In most cases, if the spark plug is bad, you can even notice it without measuring it.

With preventive maintenance, you can prevent expensive repairs down the road. But, when exactly should you replace your spark plugs? Well, there isn’t a one-fits-all answer. This is because each vehicle is different and therefore the spark plugs have a different lifespan.

To exactly know when you should change your spark plugs, you should open the owner’s manual and go to the service intervals page. In general, most spark plugs are designed to last between 30,000 and 60,000 miles says Way.

How To Fix a P0420 Code Caused By Bad Spark Plugs

Can Bad Spark Plugs Cause a P0420 Code (Answered!)
  • Make sure the engine is cool.
  • Locate the spark plugs.
  • Disconnect the wires.
  • Remove the bad spark plugs.
  • Install the new spark plugs.

A P0420 code means that the catalytic converter is not running efficiently. Before going to the mechanic shop and spending thousands of dollars on catalytic converter replacement, read this. Did you know that most of the time, a P0420 code is caused by bad spark plugs?

So, instead of spending a lot of money on a new catalytic converter, try to replace the bad spark plugs. Here is how to fix a P0420 code caused by bad spark plugs:

1. Required materials for the job

  1. Spark plugs
  2. Ratchet
  3. Spark plug socket
  4. Dielectric grease
  5. Gaping tool for spark plugs

2. Make sure the engine is cool

Before you remove anything, you want to make sure that you’re working on a cool engine. If your engine is hot and you remove the spark plugs, you could damage the threads holding the spark plugs in.

Plus, you risk getting burnt on your hands from a hot surface. With that being said, leave the engine to cool down for at least an hour before you start working on it.

3. Locate the spark plugs

Can Bad Spark Plugs Cause a P0420 Code

In order to locate the spark plugs, you will have to remove the plastic cover on the engine. Spark plugs are located on top of the engine and for each cylinder your engine has, there is one spark plug.

So, a six-cylinder engine will have a total of six spark plugs. They might be distributed evenly on two sides of the top of the engine. You will see a wire going to each spark plug.

4. Disconnect the wires

When you are removing the wires, you don’t want to grab the wire and yank it from the actual wire because that could damage the wire. So be careful, especially if you’re gonna reuse the wires. What you want to do is grab it by the boot and pull it out from there.

Sometimes the wiring on spark plugs can be hard to remove. This is where nose ring pliers help tremendously. All you have to do is grab the boot. It gives you the extra leverage to pull it right out.

5. Remove the spark plugs

Get a nice extension and connect a spark plug socket to it. The spark plug socket has a little rubber boot that holds the spark plug in place so you can pull it out. It also protects the ceramic insulation from getting cracked. Once you are set in place, break it loose and loosen the rest of the way by hand.

6. Compare the old spark plug with the new

It’s always important to compare the old spark plugs with the new spark plugs to make sure that they’re the same exact part. This is especially important on spark plugs because if your spark plug is extra tall, the top of the piston could hit the spark plug and it could damage your engine.

So once you know you have the correct plugs for your car, the next thing that you need to do is make sure that the gap of the plug is the correct size. If the gap is too wide, you might not get any spark at all. If the gap is too small, you might get a weak spark and it might not ignite the fuel efficiently.

Even if your spark plugs come pre-gapped, they can get messed up in shipping. So, it’s always a good idea to check the gap because it’s so important.

To find the correct gap spec, you could check your owner’s manual or you could go under the hood of your car and look for an emission sticker. Once you get the gap measurements, measure the gap using a gapping tool.

7. Install the new spark plugs

Grab your new spark plugs and apply some dielectric grease to them. Dielectric grease is an insulator, so you don’t want to apply it to the threads of the spark plug.

The only place where you should apply dielectric grease on the spark plugs it’s the ceramic part. Spread a light coat of grease on the ceramic and then grab a Q-tip and apply some inside the spark plug boot.

The dielectric grease is going to prevent moisture from causing corrosion. It is going to be very easy to remove the boot if needed.

Place the new spark plug into the spark plug socket and connect a long extension to it. Slowly place the spark plug into the cylinder head and tighten it. It is important to tighten down your spark plugs correctly. If you tighten them too much, you could damage the threads. If you don’t tighten them enough, they could become loose and blow out the engine.

Never use an impact gun to tighten a spark plug.

So, what I like to do is first tight them by hand until I can’t turn the spark plug socket anymore. Then, grab a torque wrench and set it to 15 pounds. Repeat this for all spark plugs. Then, reconnect the wiring and reinstall the engine cover.


Q: What is a P0420 code?

A: The P0420 code is a diagnostic trouble code that indicates a problem with the efficiency of the catalytic converter in the vehicle’s exhaust system.

Q: What are the symptoms of a P0420 code?

A: Some common symptoms of a P0420 code include reduced engine performance, increased fuel consumption, and the illumination of the check engine light.

Q: What are the possible causes of a P0420 code?

A: The possible causes of a P0420 code include a faulty catalytic converter, damaged or leaking exhaust system, oxygen sensor malfunction, engine misfire, or engine mechanical problems.

Q: How can I diagnose a P0420 code?

A: Diagnosing a P0420 code typically involves checking the oxygen sensor readings, performing a visual inspection of the catalytic converter, checking for exhaust leaks, and testing the engine for misfires.

Q: Can a P0420 code go away on its own?

A: While it’s possible for a P0420 code to go away temporarily, it generally indicates a persistent problem that needs to be addressed. Ignoring the code can lead to further damage to the vehicle’s exhaust system.

Q: How much does it cost to fix a P0420 code?

A: The cost to fix a P0420 code can vary depending on the cause of the issue. Repairs could range from replacing the oxygen sensor or catalytic converter to addressing other engine or exhaust system problems. It is recommended to consult a professional mechanic for an accurate diagnosis and cost estimate.

Q: Can I drive my vehicle with a P0420 code?

A: It is generally not recommended to drive the vehicle with a P0420 code, as it may lead to further damage to the exhaust system and reduced engine performance. It is advised to have the issue diagnosed and repaired as soon as possible.

Q: How long does it take to fix a P0420 code?

A: The time it takes to fix a P0420 code can vary depending on the specific cause of the issue and the availability of parts. In some cases, it could be a relatively quick repair, while in others, it may require more extensive troubleshooting and replacement of components.

Q: Can I reset the P0420 code by disconnecting the battery?

A: Disconnecting the battery may temporarily reset the check engine light and clear the P0420 code. But, it will not fix the underlying problem. The code will likely reappear once the issue is detected again during the vehicle’s self-diagnostic process.

In Conclusion

Bad spark plugs can cause a P0420 code, but it is not the only reason this code may appear.

There are other possible causes such as faulty oxygen sensors, catalytic converter problems, and exhaust leaks.



Vide Polowenski, Senior Mechanic

The information in this article is current and up-to-date in accordance with the latest mechanic SOPs.

Please take the time to leave a comment if this article has helped you in any way, you need additional help, or you have a suggestion.

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