P0301 OBD-II Trouble Code: Cylinder 1 Misfire Detected

Diagnostic Trouble Code (DTC): P0301

P0301 code definition

Cylinder 1 misfire detected

What the P0301 code means

There are 4-6 cylinders in most vehicles. In order, each cylinder fires a spark plug in a smooth and steady manner. The spark plug sets a fire to the fuel, using a flammable fuel/air mixture. The crankshaft is then powered by energy that is released by the fuel.

The cylinders control the revolutions per minute (RPM) of the crankshaft. All of the cylinders work together to ensure the continuous rotation of the crankshaft. If a cylinder misfires, the crankshaft revolutions per minute will increase or decrease.

If the powertrain control module detects that there is an increase or decrease of the crankshaft revolutions per minute that is more than 2 percent, it will store the P0301 trouble code.

The Check Engine Light will come on if the percentage of increase or decrease in crankshaft revolutions per minute is between 2 percent and 10 percent. The Check Engine Light will blink if the percentage of the increase or decrease in crankshaft revolutions per minute is more than 10 percent.

A percentage increase or decrease in revolutions per minute greater than 10 percent indicates that the misfiring problem is more serious. The P0301 trouble code indicates that cylinder 1 is not getting the necessary spark resulting in a misfiring condition.

What causes the P0301 code?

• Faulty spark plug in cylinder 1
• No compression in cylinder 1
• Damage or excessive wear of spark plug wires and/or coils
• Damage or excessive wear of distributor cap (not applicable to all vehicles)
• Damage or excessive wear of rotor button (not applicable to all vehicles)
• Faulty fuel injector
• Clogged EGR valves or tubes
• Burned valves
• Inaccurate ignition timing
• Vacuum leaks
• Low fuel (rare)
• Leaking from head gasket
• Cracked distributor cap
• Faulty camshaft sensor
• Faulty crankshaft sensor
• Faulty mass air flow sensor
• Faulty oxygen sensor
• Faulty throttle position sensor
• Faulty catalytic converter
• Faulty powertrain control module

What are the symptoms of the P0301 code?

• Check Engine Light illuminated or flashing
• Vehicle is hard to start, or does not start at all
• Vehicle hesitates or stumbles during acceleration
• Vehicle engine runs rough
• Vehicle idles rough
• Vehicle lacks power during operation
• Vehicle stalls or dies while stopped
• Vehicle has fuel consumption increase

How does a mechanic diagnose the P0301 code?

• Collects all trouble codes and freeze frame data stored by the powertrain control module, using an OBD-II scanner

• Test drives the vehicle to see if the P0301 trouble code returns

• Inspects the spark plug wire for cylinder 1 for damage or excessive wear

• Inspects the spark plug cylinder itself for damage or excessive wear

• Inspects the coil packs for damage or excessive wear

• Inspects coil pack wires for damage or excessive wear

• Replaces spark plugs, spark plug wires, coil packs, and coil pack wiring if needed

• After replacing these components, if the P0301 trouble code returns, it may be necessary to inspect the fuel injectors and fuel injector wiring to see if they are in any way defective.

• They will inspect the distributor cap and/or rotor button for corrosion and/or cracks (Older vehicles have a distributor cap and rotor button system - this step is not applicable to all vehicles).

• Diagnoses and repairs any other codes that have been stored by the powertrain control module and are related to the P0301 trouble code.

• Repeats a test drive to see if the P0301 trouble code returns.

• After the test drive, if the P0301 trouble code returns, they will inspect the compression system of cylinder 1 (this is rare).

• There could be an issue with the powertrain control module, if the P0301 trouble code persists. This is rare, but it may require replacing or reprogramming.

Common mistakes when diagnosing the P0301 code

Before replacing the spark plugs, spark plug wires, coil packs, and coil pack wires, complete a visual inspection of the fuel injector wiring for damage.

Although it is rare for faulty fuel injectors to be the problem, completing a quick visual inspection could save some time. Make sure that the possibility of a faulty cylinder is ruled out as the cause of the problem.

It is also important to complete a diagnosis and necessary repairs of other related trouble codes, that have been stored by the powertrain control module. Any of these components could be the cause of the misfiring problem.

How serious is the P0301 code?

The P0301 trouble code is considered a serious one because of the drivability issues that exist when this trouble code has been stored by the powertrain control module. Driving may become hazardous and/or dangerous if the car randomly stalls or stops while the vehicle is being operated. The P0301 trouble code should be addressed as soon as possible.

What repairs can fix the P0301 code?

• Replacing faulty spark plugs
• Replacing faulty spark plug wires and/or coils
• Repairing or replacing EGR valves and/or tubes that are clogged
• Replacing burned valves
• Repairing vacuum leaks
• Repairing or replacing head gaskets that are leaking
• Replacing defective camshaft sensor
• Replacing defective crankshaft sensor
• Replacing defective mass air flow sensor
• Replacing defective oxygen sensor
• Replacing defective throttle position sensor
• Replacing defective fuel injectors
• Replacing defective catalytic converter
• Diagnose other related trouble codes and repair as necessary
• Replacing distributor cap and rotor button, in addition to the wires, coils and plugs
• Making necessary repairs to internal engine components
• If cylinder damage exists, it may be necessary to replace the engine
• Although rare, it may be necessary to replace or reprogram the powertrain control module.

Additional comments for consideration regarding the P0301 code

One quick way to make a determination as to whether the misfiring problem is the result of a faulty wire or coil pack is to switch the wire and/or coil pack with another one. If a trouble code for a different cylinder comes back, this is a good indication that the wire and/or coil pack is causing the misfire.

It is important to replace all spark plugs when repairing any misfire trouble code. Only replacing the spark plug for the misfiring cylinder may result in other cylinders misfiring afterwards. If there are other related codes stored in the powertrain control module, be sure to inspect and diagnose those codes.

It is also important to make sure the spark plugs have been precisely gapped. Neglecting to do so will cause a misfire. The spark plugs should be gapped according to the the manufacturer’s specifications. These specifications can be located under the vehicle’s hood on a sticker. Any local auto part store will have the gap specifications as well. A feeler gauge will be necessary to gap the spark plugs properly.

Need help with a P0301 code?

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