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P0305 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Meaning

This error code denotes that the cylinder five has misfired. It is a generic OBD-II code that the engine control module (ECM) has experienced a misfire on cylinder number five of the engine. A misfire can occur when there is not enough fuel burning within the cylinder. Having enough fuel burning is critical in order for the engine to function properly. Without it, the combustion of fuel will not take place and the engine will not receive power. This is why this problem will need to be immediately addressed.

Causes

When code P0305 is detected, you will need to find the root of the issue. If you continue driving a vehicle that has an engine that continues to misfire, it can cause substantial damage to the car’s engine. Some of the causes of this code being detected are that the spark plug is not firing for cylinder number five due to fouling or cracks in insulation. It could also be due to a faulty ignition coil for cylinder number five has stopped firing. Another reason for this code to be detected is a plug boot leaks ignition spark to ground. Check to make sure that the valve cover is not leaking oil into the spark plug holes and shorting the spark plug firing. 

Symptoms

How do you know that you have a P0305 error code? There are several indicators that you have an issue as the Check Engine Light will be illuminated and the code is set in ECM memory. You may also notice the check engine light flashing as a misfire event takes place. This could indicate that catalyst damage has occurred. Another indicator is an engine misfiring that causes a buck or jerk and a rough idle. You may also notice that the engine may lack some power during acceleration while the misfire is active.

Diagnosis

First, you will need to scan codes and document the freeze frame data in order to verify the problem. You’ll then need to clear the engine and ETC codes to verify the issue. Take a road test, or multiple tests, to verify that there is a misfire to cylinder five. Once you have deciphered that that is the issue, remove the cylinder five ignition coil or plug wire and check for evidence of spark tracking on outside of the coil or plug boot. Also look for oil contamination from a leaking valve cover. Remove and check the cylinder spark plug and replace or switch to a different cylinder to see if misfire moves to another cylinder. If you do not find any ignition system or fuel system issues, there could be other mechanical reasons for your vehicle to misfire, such as: a leaking head gasket, broken valve spring, broken piston ring or the timing chain or belt has skipped a tooth and is off. 

Common mistakes

Some mistakes that you want to try to avoid when diagnosing your vehicle are failing to switch the coil to a different cylinder and rescanning to see if the misfire moves to another cylinder. If you forget to do this, you won’t be able to check if it is the coil that is bad. Also, avoid forgetting to switch one part at a time with a different cylinder. This is the best way to isolate the failed part and diagnose the cause of the problem. 

How serious is this?

If your scanner has detected this code, it is a severe problem. A misfiring cylinder can cause the engine to run excessively rich and can damage the catalyst. The misfire will cause the engine to run rough and hesitate on acceleration resulting in decreased performance and fuel economy.

What repairs can fix the code?

The best way to delete and repair this code is to replace the faulty spark plug or all spark plugs as a set if failure was due to a worn out plug. You may also want to replace the leaking valve cover gasket as well as replacing the failed coil, plug wires and plugs. These are the best ways to eradicate the issue.

Related codes

Other codes that relate to the P0305 codes are P0300, P0301, P0302, P0303 and P0304. When a P0300 code is detected, it means that there are two or more cylinders that are experiencing misfires. Error code P0301 specifies that cylinder one is misfiring. Likewise, with each error code P0302, P0303 and P0304 there is indication of an issue with that particular cylinder experiencing a misfire. 

Conclusion

If you find yourself having to diagnose this code, know that it is a serious issue that needs to be addressed immediately. But know that if correctly repaired, you should be able to have a properly functioning vehicle again.