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P0300 Code: Cylinder Misfire - What it Means, Causes & How to Fix

By: CowboyBenWade

The P0300 means that the fuel and air mixture in one or more cylinders is not igniting at the right time or at all, and this has caused the crankshaft to slow down. It is 'not firing on all cylinders', and the vehicle will hesitate, accelerate slower, and produce less power.

What it means when a cylinder(s) misfires

When fuel & gas ignite in the cylinder and push the piston down, and that force turns the crankshaft. Each cylinder exerts a little 'kick' on the crankshaft with each cycle, and all of them together make it spin with enough twisting force (torque) to spin the tires and make the car move. If only 6 out of 8 cylinders are firing, the crankshaft will spin slower and with less force than if all 8 cylinders where firing properly. This is called a misfire.

The computer knows how much air and fuel is going into the engine and when the spark plugs are firing (or supposed to be firing). Therefore it's expecting the crankshaft to be spinning at a certain speed given all of these data points. The computer also measures the crankshaft's actual RPM. When the expected and actual RPM diverge by more than 2%, the vehicle will throw a P0300 code, indicating a misfire. This means that one or more cylinders are not exerting enough 'kick' on the crankshaft, and it has slowed down. If the discrepancy between the expected and actual RPM differs by more than 10%, the check engine light will blink on and off, indicating an extremely serious misfiring problem. Continuing to drive could cause wear on the cylinders that are still working because they are bearing all the load. Furthermore, the vehicle may not even be drivable if the misfire is too severe.

What causes P0300?

Engine need three things to run - air, fuel, and a spark. A problem in any of these three components can cause a misfire and result in a P0300. The following problems can cause a misfire by themselves, or several of these conditions can work together to cause a misfire. These are in no particular order.

Clogged fuel injectors - not enough gasoline is going into the engine, and the fuel mixture is not igniting or not burning completely.
Leaking fuel injectors - if an injector(s) is free flowing, the cylinder won't ignite at all.
Worn spark plugs - spark plugs have a gap where electrical current jumps across to ignite the fuel and air. Over time, this gap wears and the spark can't arc across it anymore.
Bad spark plug wires - Sometimes spark plug wires can get burned and the spark plug never receives any electrical current.
Faulty ignition coil - The coil translates the low voltage 12v battery current into thousands of volts that are suitable for igniting the air/fuel mixture. Most vehicle have one coil per cylinder, and the spark plug wire plugs into the coil. A bad coil can cause a cylinder to misfire and throw a P0300.
Distributor cap (older vehicles) - Older vehicles used distributors instead of coils. It's a gear driven by the crankshaft that mechanically fires the cylinders at the right time. Turning this distributor will advance or slow when the spark fires. If the nodes in the distribute cap are not picking registering when the crankshaft is turning, the timing will be off.
Loss of compression - Leaking head gaskets, burned exhaust valves or weak valve springs can prevent the ignition chamber from sealing properly, and the cylinder will not be able to compress air as designed. It will just leak out, and the cylinder won't fire.
Faulty camshaft sensor - Camshafts are responsible for opening and closing intake and exhaust valves. Modern vehicles use sensors on the camshaft to tell when the valves are open and when to inject fuel. A bad camshaft sensor will cause the injectors to fire at the wrong time or not at all. Cylinders may or may not fire or they may fire weakly. This is a good cause of multi-cylinder misfires. Some vehicles have 4 camshafts, so if the computer gets wrong information about their position relative to each other, the cylinder may misfire.
Faulty crankshaft sensor - Sensors constantly measure the actual speed of the crankshaft, and the computer compares that to the expected speed of the crankshaft. Feeding inaccurate crankshaft speed to the computer will not necessarily cause a misfire, but it can throw the P0300 code anyway. The car can be running fine and still show this code if crankshaft sensor is bad.
Vacuum leaks - too much air going into the engine can cause misfiring, resulting in sporadic idling. This is the most common symptom of a vacuum leak.
MAF (Mass Airflow Sensor) is bad - This will give engine's computer inaccurate information about how much air is entering the engine, and it could result it too much or too little gas being injected, which could result in a misfire.

What are the symptoms of P0300?

This code means that one cylinder or mutliple cylinders are not firing on time or at all. As a result, your previously smooth running engine will experience the following:

Skipping or hesitation especially when accelerating
Difficult to start
Rough idle and dying easily when idling
Lack of power
High fuel consumption

How to fix P0300

A mechanic will review freeze frames and check live data to see which cylinders are misfiring. He will then check spark plug wires and coils, replace worn cables, worn plugs and the coil if needed; these are the most obvious and easiest parts to rule out.

Related Codes
P0300 is the generic code for a misfire in any cylinder. When this code present, cylinder specific misfire codes will also present in the format P030x with x being the cylinder number. Additionally other codes outside of the 300 range can also be the cause of the misfires. Sensors don't measure part failure; they measure downstream symptoms of malfunctions. So when diagnosing P0300 and any other code, ask yourself, "Which part could cause this pathology in my engine?" Other codes can point more directly to specific parts than P0300. Other codes that can cause P0300 may include:

300 range codes

P0301 - Loss of compression, lack of spark in cylinder 1
P0302 - Loss of compression, lack of spark in cylinder 2
P0303 - Loss of compression, lack of spark in cylinder 3
P0304 - Loss of compression, lack of spark in cylinder 4
P0305 - Loss of compression, lack of spark in cylinder 5
P0306 - Loss of compression, lack of spark in cylinder 6
P0307 - Loss of compression, lack of spark in cylinder 7
P0308 - Loss of compression, lack of spark in cylinder 8

Non 300 range codes

P0171 - can be caused by clogged fuel injectors and can cause misfires.
P0172 - can me caused by leaking fuel injectors which can lead to misfires.

Sensor failure and P0300

It can seem like any and all part failures or sensor failures can cause a misfire. It's a general code that means there's a problem with the delivery or supply of any of the three things an engine needs to run well - air, fuel, and spark. Knowing how an engine works goes a long way in being able to figure out what's wrong when this code presents. Sensors are often the cause because they can throw off the entire engine event when nothing mechanically is wrong. And sensors are an easy fix.

Complete and partial misfire

Sometimes the spark just comes too late, the fuel still ignites, but the exhaust valves are already starting to open so the force from the explosion is lost out the exhaust. The fuel/air mixture can also not burn completely, resulting in a weak piston stroke. Sometimes this is caused by a timing issue - the spark is coming too late or too early.

How to simulate a misfire

Unplug one of your spark plugs and the engine will throw P0300 and P030x with x being the cylinder where you unplugged the spark plug wire.

About the Author

CowboyBenWade does interviews and lectures on a huge variety of topics, and he is an expert in a multitude of fields. His knowledge includes but is not limited to: engineering, chemistry, aviation, cooking, and farming.

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