P0304 Fault Code
You can find information about the P0304 fault code on this page.
It is one of the most popular fault codes that people search for on this website (in the top 50).
We have given it a seriousness of 8/10 with 1 being the least serious and 10 being the worst.
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Fault code P0304 is the code used to represent a misfire in cylinder 4. This happens when the engine control module (ECM) has detected cylinder 4 is not firing properly.
This code is part of a series of codes used to inform of a misfiring cylinder. These run from P0300 up to the number of cylinders found within the vehicle. For example, a vehicle with a V12 engine will have P0312. P0300 represents a random misfiring of multiple cylinders.
Misfiring can be caused by a variety of reasons. The result is the misfiring cylinder is not producing sufficient power to operate properly. If a cylinder does not have enough fuel, air, or heat, a misfire can occur.
Some of the most common manufacturers that suffer this type of code are Jeep and other General Motors brands, Ford, and Mazda.
Other Signs of This Problem
A misfire typically comes with several noticeable indicators that can inform you of the issue. These all relate to the sound and feel of the engine at idle and under acceleration.
For those with a fine-tuned and experienced ear, a misfire can be heard as an absence of cylinder noise. The engine can also sound strange.
More noticeable signs include rough idling or even trouble staying running. Additionally, the Check Engine light will illuminate along with a P0304 fault code being reported by the ECU.
Another less noticeable symptom will be decreased fuel economy as there is fuel wasted without garnering power from it.
What Could Be The Problem?
The causes of this type of issue are relatively localized to the cylinder in question. For any single cylinder that is misfiring, it is important to look at the systems supplying heat, air, and fuel, as well as the computers controlling these ratios.
For petrol engines, the heat supply is of particular concern. Diesel engines do not use spark plugs, so if a misfire is occurring it is likely a result of the air-fuel ratio or timing.
Faulty spark plugs and other heating elements could certainly be at fault. If a spark plug is dirty or otherwise damaged, it will not provide sufficient heat to ignite the cylinder. However, this is reserved for petrol engines.
In terms of fuel, there could be fouling with the fuel injector for that cylinder or wiring issues that are causing the injector to perform poorly.
Vacuum leaks and other mechanical issues can cause this as well. If the cylinder is receiving too much or too little air it could also cause a misfire. Fault codes P0171 and similar are good indicators that there is an issue with the air-fuel ratio.
In more serious situations, timing may be the culprit. This could be indicated by a code like P0011 being reported alongside this P0304 fault.
Fixes for a misfire are dependent on the system that is causing the issue. This can mean the solution is relatively cheap and easy or rather difficult.
In a petrol engine, it is worthwhile to check the spark plug and ignition coil in question and compare it to a new one. If there is damage here, replacing the spark plug should work.
If there are any error codes relating to air leaks, be sure to find the source of these leaks and address them.
If your vehicle has them ensure the fuel injectors are not damaged or faulty. They need to be replaced if there is anything wrong.
Any issue that results in the engine acting in an unexpected way is considered very serious. Misfiring can lead to significant damage to the engine and its components.
Be sure to have this P0304 fault code fixed as soon as possible to prevent further damage.
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