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Engine-Codes – Mercury – P0303 Mercury

P0303 Mercury Code - Cylinder 3 Misfire Detected

Quick Answer

When a vehicle's onboard computer detects a misfire in cylinder 3, it triggers the diagnostic trouble code (DTC) P0303. Common causes include Faulty spark plugs: Worn or damaged spark plugs can lead to incomplete combustion in the cylinder, Malfunctioning ignition coil: A faulty ignition coil can cause weak or no spark, resulting in a misfire, Clogged fuel injector: A clogged fuel injector can disrupt the fuel delivery to the cylinder. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P0303 Mercury indicates Cylinder 3 Misfire Detected.
Common causes
Common causes include Faulty spark plugs: Worn or damaged spark plugs can lead to incomplete combustion in the cylinder, Malfunctioning ignition coil: A faulty ignition coil can cause weak or no spark, resulting in a misfire, Clogged fuel injector: A clogged fuel injector can disrupt the fuel delivery to the cylinder.
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Sep 12, 2024
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Code P0303 Mercury Description

When a vehicle's onboard computer detects a misfire in cylinder 3, it triggers the diagnostic trouble code (DTC) P0303. This code indicates that the engine's control module has detected a misfire in the third cylinder, meaning that the fuel-air mixture in that cylinder did not ignite properly.

OBDII Code P0303 Mercury - Cylinder 3 Misfire Detected
P0303 Mercury Code - Cylinder 3 Misfire Detected

Common Causes of P0303 Mercury

  1. Faulty spark plugs: Worn or damaged spark plugs can lead to incomplete combustion in the cylinder.
  2. Malfunctioning ignition coil: A faulty ignition coil can cause weak or no spark, resulting in a misfire.
  3. Clogged fuel injector: A clogged fuel injector can disrupt the fuel delivery to the cylinder.
  4. Vacuum leaks: Leaks in the intake manifold or vacuum lines can affect the air-fuel mixture.
  5. Low compression: Issues with the engine's compression, such as worn piston rings or valves, can lead to a misfire.

Symptoms of P0303 Mercury

  1. Rough idle: The engine may run unevenly or vibrate excessively at idle.
  2. Hesitation or stumbling: The vehicle may hesitate or jerk during acceleration.
  3. Decreased fuel efficiency: The vehicle may consume more fuel than usual.
  4. Check Engine Light: The Check Engine Light may illuminate on the dashboard.

How to Fix P0303 Mercury

  1. Diagnose the issue: Use a scan tool to read the DTC and pinpoint the misfiring cylinder.
  2. Inspect spark plugs and ignition coil: Check the condition of the spark plugs and test the ignition coil for proper operation.
  3. Test fuel injector: Inspect the fuel injector for clogs or malfunctions.
  4. Check for vacuum leaks: Conduct a smoke test or visual inspection to identify any vacuum leaks.
  5. Compression test: Perform a compression test to check the engine's cylinder compression.

Cost to Fix P0303 Mercury

The cost of repairing a cylinder 3 misfire can vary depending on the specific cause of the issue. On average, the cost of replacing spark plugs and ignition coils can range from $100 to $300, while addressing a clogged fuel injector or vacuum leak may cost between $150 and $500. If the problem is related to low compression, the repair costs can be higher, potentially exceeding $1000. Keep in mind that labor rates and diagnosis time can also impact the total repair costs. It is advisable to consult with a trusted mechanic or auto repair shop for an accurate estimate based on your vehicle's make and model.

What does this mean?

Tech Notes for P0303 Mercury

The P0303 code means that cylinder #3 is misfiring or is randomly misfiring. Start by checking for intake leaks and if no leaks are found the next step is to replace the spark plugs on cylinder #3. If the problem persists more tests need to be done to diagnose the problem. See "Possible Causes" for the most common problems.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

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