Why Car Won’t Start After Changing Spark Plugs [Solved]

Let’s imagine a scenario. You’ve changed your vehicle’s spark plugs, and now, the engine isn’t starting well.

Normally, the performance in the ignition system should’ve been better. At this moment, the question in your mind is why your car won’t start after changing spark plugs.

Sometimes, not following the right way of replacing old spark plugs or using the wrong type of spark plugs that can’t create the required ignition might make it tough to start the car engine.

car won't start after changing spark plugs

While replacing old sparking plugs, we often forget to be cautious and make some blunder that causes many unwanted troubles. 

Because of this, many of you might think changing plugs on your own is a bad idea and consider going to a garage. Honestly speaking, if you know how to change the spark plugs, the thing’s easy as pie, you don’t have to spend bucks on hiring labor. 

Expert’s Note: When you find your new spark plugs aren’t working perfectly, simply try to reinstall them to know whether they are actually compatible or not. 

Why Car Won’t Start After Changing Spark Plugs [Solved]

The experience of starting a car after changing spark plugs suppose to be smooth. But changing spark plugs can turn out to be a bad idea too. You can face different sorts of problems after changing spark plugs if you don’t do everything accordingly. 

As you’re willing to troubleshoot and save both your money and effort, let’s focus on the reasons first. If you are able to identify the main flaw, the solution becomes quite easier. 

1. Blunder In Installation

The most common reason a new set of sparkers would trouble is to make any mistake while replacing the old ones.

blunder in installation

Suppose if you insert the plugs inside the input incorrectly, the car won’t start at all. Besides, connecting the spark plugs too tight or too loose can also end up in rough idling or constant engine misfire. 


The first thing in your hand is to disconnect all the cables and wires and prepare to reinstall the spark plugs.

The most common solution to the problem is by reinstalling the plugs but in the correct order. Thus, do some research on how you should put new spark plugs inside your car engine and play the Uno reverse card

2. Not Collecting All The Cables And Wires After Finishing Installation

In modern engines, you’d find more than one ignition coil in the ignitions system. In some cases, you could even find a separate coil for each sparking plug.

 collecting all the cables after finishing installation

So, the number of wires, cords, and other connections isn’t too small at all. Despite placing the new ignition plugs perfectly, a single connection attached to the wrong end can be harmful and make the engine idling. 


While changing spark plugs, you’ve to disconnect several wires in the ignition system.

Keep in mind or write down the right orders and inputs of those cords so that you don’t leave any mismatch after finishing the installation. 

3. Wrong Type Of Spark Plugs

Generally, there are three types of spark plugs in the market

  • Copper
  • Platinum and
  • Irridium. 
wrong type of spark plugs

It’s possible for a car model or engine to have its own preference for plug type. When you mount a type that’s not suitable or compatible with the engine, the car would stop working sooner or later. 


You should never install copper spark plugs in a car that only prefers platinum or iridium spark plugs. 

It’s just an example. Most engines have their own preferences regarding ignition parts. Check the user manual before making any purchase.

Or you can also contact the customer support of the manufacturer to know which type of OEM spark plugs you need to install. 

Sometimes, the engine can still remain idle even after using the right type of plugs. In that sort of situation, one should think of changing the spark plug model even if it’s preferred. 

Another reason might be the new plugs have already become fouled or damaged somehow. Anyways, try contacting the seller and ask for a replacement with a new model even if the previous purchase is under the engine’s preference list. 

4. Spark Plugs Not Perfectly Gapped

People who’ve purchased spark plugs before know that there are ungapped plugs in the market.

spark plugs not perfectly gapped

By any chance, when you buy an ungapped set and install it without gapping manually, you’d find spark plugs not firing in the combustion chamber. Ultimately, your car won’t start. 


If the spark plugs aren’t pre-gapped, ask the seller to gap them. Most of the time, the shop won’t charge too much for gapping if you buy more than one plug. 

5. Damaging The New Plugs

Recently, we’ve seen people damaging brand-new plugs during putting them inside the car engine.

It happens when someone doesn’t use the right type of tools while twisting the plugs inside the panels. Damaged plugs, even if they’re unboxed a while ago, can never do anything good to your vehicle.


Without using the right type of equipment, you can damage a new spark plug during the replacement process. 

use wrench for mounting new spark plugs

The best tool to use for mounting new spark plugs is a torque wrench. You should also keep a can of grease inside the toolbox to smoothly plugs into rough inserts. 

6. Failure Of Other Parts In The Ignition System

Apart from spark plugs, you’d find many other components of an ignition system (ignition coil, fuel injector, etc.).

Despite changing the old plugs, corrupted coils or fuel injectors can also stop a car from performing smoothly.

Though the issues might be impossible to confirm before changing the spark plugs, they can be badly effective after unplugging the connections for once. 


My experience says you need to call a mechanic in your home and let the experienced guy check everything thoroughly.

Although hiring a mechanic might cost 100$ (or even more), it’s the only way to identify where the main problem lies. 

7. Immobilizer Or ECM Malfunction

The computer or control module software in your car isn’t always correct. Due to some coding error or malfunction, the program can fail to read even the right immobilizer or other systems.

The incorrect reading makes the ECM give false Check Engine Light or engine error core along with freezing the whole engine. 


Reinstalling might not be enough in some situations. For that, you must reset the car computer and erase previous data and records so that it never shows any false error code again.

But you must do this resetting after you’re done with mounting plugs. 

Other Causes

Apart from all those causes, a car can suffer from tons of issues that might appear after installing new plugs that you can’t identify without a deep check. A hidden problem is hard to find even for a DIY car enthusiast. 

Now then, a person who’s very unlucky can fail to start the car by trying every method possible. He or she can find a hard start after the spark plug changes flawlessly. 

The only thing that remains for that guy is to contact the car manufacturer’s local customer support and describe the whole thing.

Only the authorized dealer or manufacturer can help to revive the car in its true form. 

Is It Safe To Drive A Car with Improper Or Badly Mounted Spark Plugs

No, it’s not safe to drive a car when you know you haven’t installed the spark plugs properly or the installed ones aren’t recommended. 

The worst thing that can happen is your engine getting some irreparable damages because of your lack of sincereness.

Besides, the common things you’d face when driving a car with improperly installed plugs are: 

  • Engine misfire,
  • Engine knocks,
  • Rough idling,
  • Hard starts,
  • Lazy accelerations, etc.


Why Does My Car Shake After I Changed The Spark Plugs?

The car is shaking because of rough idling or misfires. These issues would appear if haven’t inserted the spark plugs perfectly or the spark plugs have got some damage during the installation. 

Can New Spark Plugs Cause Rough Idle?

Of course, new spark plugs can cause rough idling when they aren’t perfectly gapped. On the other side, if the fuel injection system becomes older, it can also make problems despite using new plugs. 

Should you reset ECU after changing spark plugs?

Resetting the ECU or the sensors is a good idea. By erasing previous data, you allow the car computer to analyze freshly. 

Can bad spark plug wires cause a car not to start?

The bottom end of the spark plug wires is connected with the ignition coils. If the wires are fouled or burnt out, they won’t be able to produce the energy to let spark plugs create sparkles. Which actually ends in the car not starting properly. 


Now then, you’ve known why a car won’t start after changing spark plugs. Thus, you better be careful when changing your old plugs on your own.

We know it saves some bucks when you decide to DIY such small matters without thinking of hiring an expert. But for that, you need to act like an expert and finish things up professionally.

In fine, we hope you do things in the right way and seek expert advice before taking any serious steps. 

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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