What is OBD-II Code P0303 – Cylinder 3 Misfire Detected

OBD-II Code P0303: Cylinder 3 Misfire Detected

As a car owner, it is important to know what OBD-II codes are and what they mean. Whenever a malfunction occurs in your car’s engine, the OBD-II (On-Board Diagnostics) system detects the problem and stores a corresponding code that helps mechanics determine the cause of the issue. Code P0303 is one such code, indicating a misfire in cylinder 3. In this article, we will discuss what causes this error, how to identify it, and how to fix it.

What is Cylinder 3 Misfire, and What Causes It?

Misfires occur when the air-fuel mixture in the engine does not ignite properly, causing the engine to run roughly or vibrate abnormally. A misfire in cylinder 3 means that there is an issue in the third cylinder, usually caused by one or a combination of the following:

1. Faulty Spark Plug
A spark plug that is corroded, worn out, or has become fouled can cause a misfire in the engine. In cylinder 3, the electrode tip of the spark plug may have worn out, making it unable to spark properly.

2. Bad Ignition Coil
An ignition coil that isn’t working correctly can also cause a cylinder 3 misfire. Ignition coils are responsible for generating high voltage electrical current, which is used to ignite the air-fuel mixture in the cylinder. If the coil is failing, it cannot provide enough spark to ignite the mixture in cylinder 3, leading to a misfire.

3. Clogged Fuel Injector
A clogged fuel injector can lead to a misfire in the engine. The fuel injector sprays fuel into the cylinder along with air to mix and combust to produce energy for the engine. If the fuel injector in cylinder 3 is dirty or clogged, it cannot mix fuel and air properly, which causes the engine to misfire.

4. Low Compression
Low compression in any cylinder of an engine can cause a misfire. Low compression usually occurs due to worn or damaged piston rings, valves, or cylinder walls. In cylinder 3, low compression can also result from a blown head gasket that allows coolant or oil to mix with the air-fuel mixture, causing misfiring.

How to Identify and Confirm the Error

Usually, the Check Engine Light (CEL) comes on when there is a fault in the engine management system. When code P0303 appears, it means that the OBD-II system has detected a misfire in cylinder 3. Other signs of a cylinder 3 misfire include:

1. Rough Idling:
The engine may vibrate or shake forcefully when idling.

2. Loss of Power:
The engine may have difficulty accelerating or may not reach the same level of power that it should.

3. Poor Fuel Economy:
Fuel consumption of the vehicle may increase due to the misfire in cylinder 3.

It is essential to diagnose the issue correctly to avoid related problems like engine damage or reduced fuel efficiency. To confirm the error, a mechanic will:

1. Check the Spark Plug:
The mechanic will remove the spark plug from cylinder 3 and examine it to determine if it is dirty or has worn out. If the spark plug is faulty, replacing it should fix the misfire issue.

2. Check the Ignition Coil:
The mechanic will test the ignition coil in cylinder 3 to determine if it is providing enough voltage to generate a spark. If the coil is failing, a replacement coil can be installed to solve the misfire.

3. Test the Fuel Injector:
The mechanic will connect a fuel pressure gauge to the fuel rail and check if cylinder 3 is getting enough fuel or not. If the fuel injector is found to be clogged, the mechanic will clean or replace it.

4. Conduct Compression Testing:
The mechanic will perform a compression test on cylinder 3 to check if there are issues with the cylinder wall or valves. If the compression is low, the engine may require an overhaul, new pistons or valves, or a head gasket replacement.

How to Repair the Misfire Detected in Cylinder 3

Since cylinder 3 misfire can have different causes, the recommended solution will depend on what is causing the misfire.

1. Replace Faulty Spark Plugs
Faulty spark plugs are the most common cause of a misfire in cylinder 3. Replace the spark plugs to solve the issue. Carry out a thorough tune-up of the engine to optimize its performance.

2. Replace Bad Ignition Coils
If the ignition coil fails, it can cause a misfire. Replace the coil of cylinder 3 to fix the issue.

3. Clean or Replace the Fuel Injector
If the fuel injector is clogged or dirty, cleaning or replacing it should restore normal fuel flow and eliminate misfires.

4. Fix Cylinder or Valve Issues
If the compression test indicates that there is low compression in cylinder 3, the engine may require a rebuild or replacement. If the head gasket is blown, it should be replaced immediately.

Frequently Asked Questions (FAQs):

Q. Can a misfire cause engine damage?
Yes, a misfire can create additional stress and strain on the engine, causing damage to internal components like cylinder walls, valves, head gasket, and pistons.

Q. What is the difference between code P0303 and P0300?
P0300 is a generic code, indicating random misfires in multiple cylinders, while P0303 is specific to cylinder 3 misfires.

Q. Can a bad fuel injector cause a misfire in all cylinders?
Yes. A bad fuel injector can cause a misfire in all cylinders if it is not injecting fuel evenly.

Q. What is the cost of repairing a misfire in the engine?
The cost of repairing a misfire depends on the cause of the error. The cost of replacing spark plugs or an ignition coil is usually low, while repairing a blown head gasket or a damaged piston can be expensive.

Q. How frequently should spark plugs be replaced?
Spark plugs should be replaced every 30,000 to 50,000 miles or as indicated by the manufacturer. Fuel additives can help keep the spark plugs clean in-between replacements.


There are a variety of reasons why code P0303 may appear in your vehicle’s onboard diagnostic system. The code signifies a misfire in cylinder 3, which can be caused by factors like a faulty spark plug, bad ignition coil, clogged fuel injector, or low compression in cylinder 3. Identifying the issue and solving it quickly will prevent further damage to the engine and improve the car’s fuel efficiency. If you’re not comfortable performing repairs yourself or aren’t sure of the cause, it’s best to take your vehicle to a mechanic who can diagnose and solve the problem.

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