P0303 OBD II Trouble Code | Autance

If the P0303 code is keeping your Check Engine Light shining, you’re probably wondering what it means. Short answer? The…

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P0303 OBD II Trouble Code | Autance © P0303 OBD II Trouble Code | Autance

If the P0303 code is keeping your Check Engine Light shining, you’re probably wondering what it means. Short answer? The P0303 Diagnostic Trouble Code (DTC) indicates a misfire on a cylinder 3. The long story? We cover it in detail down below.

Meaning and Cause of P0303 Code

Like all DTC codes, this code too means that something is wrong with your vehicle’s interior body. When the sensors inside your car detect a fault of some sort, the OBD-II system displays a DTC. In this case, because the DTC is P0303, your OBD-II is letting you know that there is a misfire condition on cylinder 3. If you’re wondering what a misfire is, it’s simply a lack of combustion that can be caused by a myriad of reasons, including poor fuel quality, lack of spark, low compression, etc.

To put it simply, when your OBD-II system detects several misfires from cylinder 3, your Check Engine Light will turn on, alerting you that something is wrong. The light will shine until the problem is repaired.

What causes a Misfire, or the P0303 code?

There can be various reasons for the P0303 code, but some of the most common include:

  • Faulty spark plugs
  • Faulty plug wire
  • Faulty coil pack
  • Fuel injection problems
  • Incorrect fuel pressure
  • Burned exhaust valve
  • Sensors or computer issues
  • Poor fuel quality or running out of gas

Related Post: Best OBD2 Scanner

Symptoms of the P0303 Code

Just like there can be various reasons for the P0303 code to show up, there can also be a lot of different symptoms. Saying that, some drivers will only see their Check Engine Light shining but won’t experience any other symptoms. Other drivers, on the other hand, may experience a variety of different signs that something’s wrong with their vehicle. Common symptoms include:

  • The engine is harder to start
  • The vehicle may run poorly
  • The vehicle may stall, misfire
  • Choppy acceleration
  • The engine may stumble
  • Poor gas mileage.

Diagnosing and Repairing the P0303 Code

Because there can be many causes for the P0303 code, it’s important to check the vehicle properly and thoroughly. To do this, you’ll need a good mechanic, first and foremost. This is crucial because misdiagnoses are quite common with this code. For example, components of the ignition system are often completely replaced in error. Granted, these components usually are the culprits, however, they’re far from being the only possible cause of the P0303 code. A misdiagnosis will cost you more money, plus can lead to various drivability problems.

Although this DTC can cause driving problems, such as the engine running poorly, you should be able to drive to a safe location. Once there, the vehicle can be diagnosed and repaired pretty quickly.

To diagnose the P0303 DTC, a mechanic will (should) do the following:

  • Mount a DTC scan tool to check for any current and pending codes (if you want to do this yourself, just get a DTC reader or a scan tool, but bear in mind that scan tools are more recommended)
  • Perform inspection and look for damaged wiring, broken components, general wear, and tear, etc.
  • Repair or replace corroded, damaged or disconnected connectors, wiring and components
  • Reset the system after repairing what needs to be repaired. If nothing needed repairing, mount the DTC tool scan or reader to the connector and note all stored codes as well as freeze frame data because this information can be useful when diagnosing intermittent conditions.
  • Perform the test drive and see if the code returns.

After the test drive, there are two possible scenarios: the code doesn’t return immediately, or the code is stored but no symptoms manifest. In the former case, the vehicle could have an intermittent condition which is usually a challenge to diagnose. Sometimes, the condition needs to be allowed to worsen before it can be properly diagnosed. In the latter scenario, the mechanic will have to reset the PCM and then see if the code returns.

In any case, a proper diagnosis and repair require a very thorough inspection. It’s important to follow a certain procedure so to not overlook the problems and make mistakes, which waste time and money. The most common mistakes include overlooking simpler causes of the misfire, such as a leaking air intake or fuel issues. Of course, as mentioned, the spark plugs are the most common cause of the misfire, but certainly not the only one.

To sum it up, the most common repairs for P0303 include:

  • Replacing spark plugs on all cylinders
  • Cap and rotor, coil pack or spark plug wiring replacement
  • Repairing intake air leaks
  • Repairing fuel problems
  • Fixing mechanical engine problems.

Additional Information About the P0303 DTC

When diagnosing the P0303 code, a DTC scan tool is very important as it can view live engine data from sensors. For this reason, it’s better to let a professional take a look at your vehicle and not try to diagnose the problem yourself (scan tools are more expensive than regular DTC readers). Live engine data from sensors give a good look into the overall state of the vehicle and also allow the mechanic to see long-term fuel trims, fuel injector operation, intake air readings and misfire counters for cylinders.

Again, spark plugs are among the most common causes of misfires in vehicles, especially cars with higher mileage. Normally, they’re replaced at service intervals provided by your vehicle’s manufacturer as they don’t last a lifetime. If your car is equipped with a distributor with a cap and rotor system, these are also replaced at service intervals, as they too are susceptible to wear and tear. As for spark plug wires, they’re usually replaced when the spark plugs are done so no worries on that end either.

What about the newer vehicles with coil pack setups? A coil pack (which replaces the distributor) usually lasts much longer than plug wires or cap and rotor, but when it becomes a problem, it can be harder to diagnose. For this reason, the suspected coil pack is usually moved to another cylinder to see if the misfire occurs there. If it does, that coil pack is the problem and needs to be replaced.


  1. P0303 OBD-II Trouble Code: Cylinder 3 Misfire Detected, Your Mechanic
  2. P0303 – Check Engine Light Diagnostic Trouble Code Description, Engine-Light-Help
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