Error Code P0301 means the vehicle’s computer has detected that cylinder number 1 of the engine is misfiring.


The term “engine misfire” or not firing properly, simply means the engine don’t have combustion in one or more of its cylinders. If the cylinder misfires at certain conditions, and not all the time, then it is called ‘intermittent misfire.’ In the case of Error code P0301, it is the first cylinder of the engine that is misfiring. Error code P0302 indicates a misfire in cylinder 2. P0303 means a misfire on cylinder 3, and so on. P0300 means random misfires in multiple engine cylinders.

The vehicle’s PCM or Power Control Module detects the ‘misfiring’ by keeping track of the engine speed. As soon as it detects the misfire, it will then set the Error code and turn the Check Engine light on in the dashboard.

Common Symptoms

  • Check Engine light is on
  • The engine is hard to start, shakes or run unevenly
  • Engine stumbles and/or hesitate
  • Rough run and/or jerks when accelerating
  • Performance problems such as signs of rough driving, dying at stop signs, hesitation, lack of power at acceleration
  • Decreased fuel economy
  • Sometimes, you may notice the smell of unburned fuel from the exhaust

Possible Causes

  • Poor fuel quality or running out of gas
  • Problems with ignition system components such as coils, distributor, ignition module and ignition wires, spark plugs
  • Dirty spark plugs
  • Faulty fuel injector
  • Lean air or fuel mixture caused by bad airflow sensor
  • Vacuum leak caused by cracked vacuum hose, cracked or disconnected vacuum line and cracked intake snorkel
  • Low compression
  • Low fuel pressure
  • Leaking cylinder of head gasket
  • Misaligned and misadjusted valves
  • Worn out valve train components
  • Improper set timing
  • Clogged exhaust
  • Stuck open purge valve (solenoid) and/or EGR valve

Common Misdiagnoses

  • Oxygen sensor
  • Fuel injector
  • Powertrain or Drivetrain problems

How to Check

The cause of misfire is usually easy to pin-point, especially if the problem is present at the time. The spark plug itself can tell a lot. Also, take a good look at the distributor cap, ignition coil, or rotor to see the faulty parts. Basic components such as compression, fuel pressure and timing should also be checked.

The engine should also be checked for vacuum leaks. While the STFT (short term fuel trim) and LTFT (long term fuel trim) should be checked with a scan tool to inspect whether the air and fuel mixture is too lean. Often times, bad mass air flow sensor causes the engine to run lean, which leads to misfire.

The pressure of the cooling system must be tested for leaks, especially on the head gasket.

Valve must be adjusted properly. Misadjusted valves results to misfires at cold start.

When misfires occur only at idle, both the EGR valve and purge valve must be checked, as there’s a good chance one may be stuck open.

Worn out camshaft lobes, stretched timing chain, and other components of the mechanical valve train that are not working properly causes misfire – which means they should be inspected thoroughly.

Crankshaft with broken or damaged tooth or camshaft sensor gear can cause misfiring. Check the sensor signal using oscilloscope to find out whether it is the CKP (crankshaft position sensor) or the CMP (camshaft position sensor) that causes the problem.

How to Fix

The misfire repair depends highly on the cause, thus, it is important to have the problem diagnosed properly. You need to know exactly what causes the misfire to identify what it really needs. You don’t want to spend big and throw new components at your car. If you plan to do the repairs yourself, it is still better to have it diagnosed by your dealer or from a reputable repair shop, where technicians can take a good look with your vehicle using up-to-date information, advanced scanning tools and TSB (technical service bulletin).

Obviously, the cost of repair depends on the severity of the problem.

To fix engine stumbling or hesitation, check all wiring and connectors that connects to the cylinders, such as the spark plugs. If ignition components have been in the car for quite some time, then it may be a good idea to replace them in your next regular maintenance schedule. Replace spark plugs and its wires, the distributor cap and the rotor (if applicable). Also, check the coil (coil packs). In some cases, it is the catalytic converter that causes the misfiring.

If you notice a rotten smell in the exhaust, then check the cat converter and replace if needed.

In some cases, the problem is caused by faulty fuel injectors.

Once you have repaired the problem that causes misfiring, the code will clear itself after a couple of miles of driving.

Is it okay to drive a car with misfiring engine?

NO! Driving a vehicle with misfiring engine is a terrible idea, as it can damage the engine’s catalytic converter, which is quite an expensive component of the engine’s vehicle emission control system.

Also, when one of the cylinders of the engine misfires, the unburned gasoline will head straight to the exhaust, causing it to overheat and melt the catalytic converter. As soon as the ECM (Electric Control Unit) detect high rate of misfire which can damage the catalytic converter, the Check Engine light will blink faster from your instrument panel. Many manufacturers highly advise not to drive your vehicle when you experience this Error Code, while others allow driving in moderate fashion. Make sure you have your vehicle thoroughly checked as often as possible. And read your owner’s manual for more information.

For more information about different error codes, check out our website. For professional mechanical assistance, feel free to Contact Us.