engine misfire causes

P0300 Trouble Code – Cylinder Misfire

In OBD2 Trouble Codes by Magnus Sellén1 Comment

engine misfire causesWhat Does the P0300 Code Mean?

The P0300 Trouble Code indicates that one or more engine cylinder is misfiring. If the last digit of the trouble code is not zero, for example, the code generated is P0302, the number 2 suggests that the cylinder number 2 is misfiring.
The engine cylinder misfire causes the engine speed to fluctuate. If this fluctuation increases, it affects the crankshaft position sensor and the engine control module (ECM) can detect that one of the cylinders just misfired.
Engine misfire occurs due to an incomplete combustion process inside the cylinder. When the situation worsens, the engine jerks more often and the performance drops considerably. This misfiring triggers the P0300 trouble code.


P0300 Symptoms

Here are some common symptoms associated with the P0300 trouble code:
Difficulty in starting the engine
Check engine light illuminates
The engine does not start at all
Weak acceleration
Increase in fuel consumption


Possible Causes

The P0300 trouble code is caused due to many reasons. Some of the common causes include:
Damaged spark plugs
Faulty wires
Damaged coil
Oxygen sensor malfunction
Fuel injector malfunction
Burned exhaust valve
Defective catalytic converter
Defective throttle position sensor
Faulty PCM
Head gasket leakage
Vacuum leak

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Possible Solutions

Replace faulty spark plugs
Check and repair broken wire connections
Fix the damaged coil
Replace or service the oxygen sensor
Inspect the fuel injectors
Replace the catalytic converter
Replace the faulty head gasket
Repair vacuum leaks
Diagnose and repair the faulty PCM


How to Diagnose the P0300 Code?

A professional mechanic or technician will follow these steps to diagnose issues related to the P0300 code. You can perform these steps at home by yourself as well.

Plug in the Car Battery Charger

A car battery charger is extremely important when diagnosing the P0300 code. Since the diagnosis requires that you keep the engine running, this can cause the battery to drain and as a result, additional trouble codes will generate making the problem more complex. A car battery charger will ensure that your battery charge stays at its optimal level.

Connect the OBD2 Scanner

An OBD2 scanner is required to scan and diagnose the P0300 trouble code. With the OBD2 scanner, you can monitor and report trouble code related issues and fix them easily as well. For vehicles launched before 1996, an OBD1 scanner is used while every vehicle developed after 1996 makes use of the OBD2 scanner.
There are two types of OBD2 scanner, the first type is a regular code reader while the other one is an advanced scanner. The reader only reads basic codes and has simpler functions while the scanner can read enhanced codes and OEM codes as well.

Check for any sign of wiring damage

Before replacing any component, first ensure that all the wiring and connection is proper. Visually inspect the wiring and see for any signs of corrosion and fix it right away.

Check the Spark Plugs

Spark plugs do not wear out quickly but if the cylinder is misfiring, it is a good idea to check the spark plugs and notice any signs of damage. If the spark plug seems defective, replace it immediately. As a part of regular maintenance, you should replace the spark plugs after every 80,000 miles. You should also inspect your Ignition coils! You can find spark plugs on Amazon here: Spark plugs

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Inspect the distributor cap

If you have replaced the spark plugs and the problem still exists, check the fuel injector system for defects. If you have an old vehicle, check the distributor cap and the rotor button system for damage.

Check for other related trouble codes

If more than one trouble code is observed by the OBD2 scanner, take necessary steps to diagnose and repair them. After that take your car for a drive and test if the P0300 trouble code returns.

Replace the PCM

If you have followed all the steps mentioned above and the trouble code still exists, there is a possibility that the problem is with the PCM. The PCM may require reprogramming or replacement if it has gone bad.


How to Replace the Spark Plug?

Replacing the spark plug is an easy task and you can perform the replacement procedure at home as well. Before replacing the spark plugs, make sure that the vehicle’s temperature is cool and was not driven recently as the components can get extremely hot.

Step 1: Open the hood and locate the spark plug

Open the hood and you will notice a few wires leading to a different location in the engine bay. The spark plugs are positioned at the engine end of these wires.

Step 2: Pull the wire plug

Start by pulling the wire plug and at the bottom, you will see the spark plug installed. Make sure you do not apply too much pressure on the plug.

Step 3: Remove the spark plug

Insert the socket wrench inside and gently rotate to loosen the spark plug. Remove the old spark plug and notice its condition.

Step 4: Install the new spark plug

Using the socket wrench, install the new spark plug and tighten it properly. Then install the wire plug like before and you are good to go.

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Spark Plug replacement cost

The average cost to replace the spark plug comes between $110 and $300. The part itself cost between $50 and $90, while the labor cost is estimated between $60 and $200.


Should I Repair the P0300 Trouble Code Right Away?

The P0300 trouble code should be fixed right away as it can cause drivability issues and may cause harm to engine components


Recommended Tools to Fix P0300 Code

Car battery charger
Digital multimeter to test voltage and resistance readings
OBD2 Scanner to monitor and diagnose the code
Spark plugs


Further Questions?

If you have any further questions related to the P0300 trouble code or if the above solutions did not solve your problem, comment below and our trained mechanic will assist you ASAP.

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

  1. GWM H6 City Turbo. Misfires. Low power. Turbo that does not kick in. Stinking smell some times. Vehicle was checked by GWM but problem still persist. Plugs was replaced together with a coilpack on cyclinder 1

  2. Due to a very high demand and high ammount of comments, you have to wait for some time for your car questions to get answered. If you want to get fast answers from a certified master technician you can ask your questions here:
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