Skip the auto shop - Our mechanics make house calls

P0303 OBD-II Trouble Code: Cylinder 3 Misfire Detected

Check Engine Light

P0303 code definition

P0303 is a generic diagnostic trouble code (DTC) indicating an issue with the firing of cylinder 3. This code may be seen with other misfire codes as well as lean / rich condition codes.

What the P0303 code means

The P0303 DTC is set when the ECU detects multiple misfires from cylinder 3. Misfires cause the ECU to set the Check Engine Light and put the vehicle into a failsafe mode to prevent damage. The Check Engine Light will remain until the code is repaired.

What causes the P0303 code?

  • Faulty spark plugs
  • Fuel injection issues
  • Plug wires, cap and rotor, or coil pack issues (depending on what vehicle is equipped with)
  • Mechanical engine issue
  • Intake air leaks

What are the symptoms of the P0303 code?

P0303 has a variety of symptoms. In some cases, the driver may only notice the Check Engine Light and no other symptoms. In other cars, the vehicle may run poorly, misfire, stall, become hard to start, and experience poor gas milage.

How does a mechanic diagnose the P0303 code?

There are many different systems that cause the P0303 code, so it is critical to follow all steps in the proper order:

  • The mechanic will hook up a scan tool to check for any current, history, or pending codes

  • Next, all freeze frame data will be noted for use on the test drive

  • Codes will all be cleared and a test drive will be taken

  • Now, a visual inspection will be performed looking for wear, broken components, leaks, and damaged wiring

  • A scan tool will be used to check the misfire counter for cylinder #3 during operation

  • Long-term fuel trims will be checked to verify if any fuel or intake issues are present

  • In some cases, the spark plugs may be pulled and inspected

Common mistakes when diagnosing the P0303 code

Like any other diagnosis that is done, all steps must be followed to avoid mistakes that waste time and money. The most common mistakes are overlooking fuel issues or a leaking air intake as the cause of the misfire. Though the spark plugs themselves are the most common cause, they are not the only cause for P0303.

How serious is the P0303 code?

P0303 can cause drivability symptoms, such as lack of power or the engine running poorly, that may make it more uncomfortable to drive. In these scenarios, P0303 should not prevent the car from being driven to a safe location. Once the vehicle is in a safe location it can be diagnosed and repaired promptly.

What repairs can fix the P0303 code?

Repairs for P0303 are listed in the order of most to least likely:

Additional comments for consideration regarding the P0303 code

A scan tool that can view live engine data from sensors is very important when diagnosing P0303. This allows technicians to see long-term fuel trims, intake air sensor readings, fuel injector operation and misfire counters for cylinders. All of this data is used to help determine which system is causing P0303.

Spark plugs are normally replaced at service intervals provided by the vehicle manufacturer. They do not last the life of the vehicle and are one of the most common causes of misfires in vehicles, especially ones with higher milage. If vehicles are equipped with a distributor that has a cap and rotor system, these will be replaced with regular service intervals as they are mechanical parts that wear over time. On vehicles with spark plug wires, the wires are usually replaced when the plugs are done, as heat from the engine causes the wiring to fail over time.

Newer vehicles may be equipped with a coil pack setup. These generally last longer than plug wires or the cap & rotor. When the coil packs fail, they can be slightly different to diagnose. The easiest way is to move the suspected coil pack to another cylinder; if the misfire changes to the new cylinder, then the coil pack is the cause. Coil packs are normally replaced as a set.

Need help with a P0303 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.


Post a question and get free advice from our certified mechanics.


More related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
How to Transfer a Car Title in Idaho
In order to prove ownership of a car, you must have the title. However, when a car is sold, given away or inherited, the title needs...

Related questions

Q: codes p0101 and p0303 simultaneously

The code P0101 is for a problem with the MAF sensor and code P0303 is for a cylinder number 3 misfire. The first thing to check is to make sure there is no vacuum leaks or leaks between the MAF...

Q: CEL transmission slipping

Hi there. You definitely appear to have a voltage issue with the vehicle. From what you are can be a bad connection at the battery, a failing alternator, or most commonly a bad ignition switch. I usually start by checking...

Q: Vehicle shuts off intermittently at idle

The P0113 code relates to a problem with the intake air temperature (IAT) sensor. The computer is seeing a reading that is outside of its normal parameters and is causing the computer to set the code. This could also be...