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:
- Spark plug replacement for all cylinders
- Replacing the cap & rotor, coil pack, or spark plug wiring as necessary
- Intake air leaks repaired
- Repairing fuel issues
- Mechanical engine issues fixed
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?
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