Error Code P0303 is described as Misfire Detected, Cylinder number 3. This means the vehicle’s computer has detected a misfire in the engine’s cylinder, specifically the cylinder #3.


Error Code P0303 is a common error code that indicates a misfire problem in the engine’s cylinder, specifically cylinder #3. Most modern vehicles, particularly vehicles made  in 1996 on, are equipped with a computer that comes with a misfire detection system and sends reports to the scanning tool about the trouble with the engine’s cylinder.

An engine misfire means a lack of combustion, which can be a result of different issues, such as low compression, lack of spark, metering, unmetered air entering the engine or poor fuel quality. When misfire occurs, the embedded computer on the vehicle will automatically trigger the Check Engine light and show a definite number. The last digit of this number points out the exact cylinder at fault. For example, if the Check Engine light comes with a number combination of P0301, that means the cylinder #1 is misfiring. P0302 means misfire on cylinder #2 and so on.

Common Symptoms

  • Check Engine light is on
  • The vehicle is harder to start than usual
  • Stumbles or hesitates upon acceleration
  • In most cases, the vehicle will demonstrate performance problems like lack of power on acceleration, dying at stop signs, rough riding, hesitation and bad fuel consumption.

Possible Causes

  • Problems on the Evaporative Emission System (EVAP), i.e. vapor leak, which results to the engine
  • Exhaust Gas Recirculation (EGR) problems, i.e. restricted ports, EGR valve leak
  • Running out of gas or poor fuel quality
  • Defective MAF (Mass Air Flow) sensor
  • Wrong throttle position
  • Base ignition problem such as low compression, bad timing, and valve train issues
  • Ignition system issue such as defective spark plug, wiring related problems, ignition module and wiring problems, and ignition sensor or wiring issues
  • Faulty Camshaft sensor or Crankshaft sensor
  • Faulty computer system

How to Check

When the Check Engine light comes out and presents the error code P0303, the first thing you need to do to diagnose is take note of the freeze frame information and duplicate the conditions that set the code through a test drive. This includes road speed, engine load, RPM, and throttle position. The error code refers to a specific cylinder misfire, which means it needs a more thorough diagnosis.

For a cylinder misfire, you have to change faulty components, such as coils and spark plugs. This is to isolate the exact cause of the misfire. Also, it is crucial to note other error codes, as the misfire may result from other faulty components.

Here are four common types of engine misfire and how to check for the error code P0303.

Ignition Misfire

Inspect the Ignition system and all connected components for signs of heat damage and wear and tear. A spark plug that is in good condition must not be blackened with soot, whitened by overheating combustion chamber or greened by the coolant. Rather, it should maintain a sandy color.

Cables and coils of the ignition system should be free of arcing. Scope the Ignition System if necessary; make sure it is firing even voltages, which should be around 8V to 10V for every cylinder.

Remove the distributor cap and rotor (if necessary) from the engine. Check the contact points and terminals for signs of wear and tear, corrosion build up or arching.

Though OBD II vehicles usually comes with computer-controlled trimming, make sure it’s the right specifications, even if it is using individual coils.

Lean Misfire

To check for a lean misfire, take a note of the Long Fuel Trim values, as it determines how the PCM (Powertrain Computer) works to compensate the balance of the air/fuel ratio. If result shows more than 10% in a one cylinder bank and not on the other, this can be a sign of vacuum leak, crack or defect on the manifold on the specified bank. You will know what causes the computer to do the compensation by checking the “numbers” of the Fuel Trim from the full range of operating conditions. Vehicle engines that are in good shape have around 1% to 3% Long Term Fuel Trim numbers, either positive or negative.

Mechanical Misfire

There are two ways to check for this misfire: a compression test and an engine idle manifold vacuum test. For a complete and smooth combustion, there should be consistent compression readings (about 10% of each other), a minimum of 120 PSI for every cylinder and with at least 17-inches of steady vacuum.

Powertrain Misfire

Cars can trigger this error code when badly warped and out-of-bound rear brake drums jerk the whole powertrain violently while the vehicle slows from highway speeds. To diagnose the vehicle for this misfire, it is important to bring your vehicle to a professional for a more thorough inspection and pinpoint the cause of the misfire. In some cases, wrongly perceived mechanical misfires that starts from the driveshaft, transfer case, transmission or front/rear differential can result in a replacement of the whole engine.

How to Fix

For cars that don’t have many symptoms, a simple reset of the code and retest will get rid of the error.

If symptoms are present thoughout, such as engine stumbling or hesitation, you must check for damages on the wiring and connectors to the cylinder, such as the spark plugs.

Ignition components must be regularly maintained, or even replaced, as part of a regular maintenance check. This includes the distributor cap, wires, rotor (if applicable) and spark plugs. Otherwise, you should inspect the coil (coil packs).

If you notice the smell of rotten eggs from the exhaust, then that’s a tell-tale sign of damage in the catalytic converter, and it means it needs replacement.

In some rare cases, the error code may be triggered by faulty fuel injectors.

For more information about this error code and other engine misfire error codes, feel free to browse through our website. For professional mechanical help,  Contact Us.