Understanding the Top 10 Most Common OBD-II Fault Codes in US Vehicles

by Garage Pro

In modern vehicles, the OBD-II (On-Board Diagnostics) system plays a crucial role in detecting and reporting problems. It helps in identifying issues through OBD fault codes, which can be scanned using an OBD-II scanner to understand the root cause of problems. In this blog post, we’ll dive into the top 10 most common OBD-II fault codes in US vehicles based on data collected from over 100,000 scans.

A detailed image showing an OBD-II scanner in use, connected to a car's OBD-II port. The scanner's screen displays multiple diagnostic codes such as P0420 and P0300. The background includes the lower part of the car's dashboard and driver's seat.

Top 10 OBD-II Fault Codes and Their Meanings

Below is a detailed breakdown of the most frequent OBD-II fault codes, including their descriptions, symptoms, causes, and potential solutions.

1. P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

  • Description: This fault code indicates that the catalytic converter is not functioning properly and is below its efficiency threshold.
  • Symptoms: Faulty catalytic converter, Oxygen sensor malfunction, Exhaust leaks, Fuel system problems, Ignition system problems.
  • Solutions: Replace the catalytic converter.

2. P0300 - Random/Multiple Cylinder Misfire Detected

  • Description: This code indicates that the engine is misfiring on multiple cylinders.
  • Symptoms: Rough idling, Poor engine performance, Decreased fuel efficiency.
  • Solutions: Check the spark plugs, fuel injectors, or ignition coils for issues and replace them as needed.

3. P1000 - OBD Systems Readiness Test Not Complete

  • Description: This code indicates that the OBD-II system's readiness test has not been completed.
  • Symptoms: No noticeable symptoms but the check engine light may be on.
  • Solutions: Drive the vehicle under varying conditions to allow the system to complete the tests.

4. P0171 - System Too Lean (Bank 1)

  • Description: The engine's air/fuel mixture is too lean, meaning there is too much air and not enough fuel.
  • Symptoms: Decreased power, Rough idling, Hesitation during acceleration.
  • Solutions: Inspect for vacuum leaks or issues with the mass airflow sensor (MAF), oxygen sensor, or fuel injectors.

5. P0442 - Evaporative Emission Control System Leak Detected (Small Leak)

  • Description: There is a small leak detected in the evaporative emission control system.
  • Symptoms: Check engine light, Decreased fuel efficiency.
  • Solutions: Check the gas cap for tightness or inspect the EVAP system for leaks.

6. P0340 - Camshaft Position Sensor Circuit Malfunction

  • Description: This code indicates a malfunction in the camshaft position sensor circuit.
  • Symptoms: Difficulty starting the engine, Engine stalling, Poor engine performance.
  • Solutions: Replace the camshaft position sensor or repair the wiring.

7. P0456 - Evaporative Emission System Leak Detected (Very Small Leak)

  • Description: There is a very small leak detected in the evaporative emission control system.
  • Symptoms: Check engine light, Poor fuel economy.
  • Solutions: Inspect and tighten the gas cap, or check for leaks in the EVAP system.

8. P0011 - "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)

  • Description: The camshaft timing in bank 1 is over-advanced, indicating a timing issue.
  • Symptoms: Hard starting, Poor engine performance, Rough idling.
  • Solutions: Check the camshaft timing and adjust if necessary, or replace the camshaft position sensor.

9. P0174 - System Too Lean (Bank 2)

  • Description: Similar to P0171, this code indicates that the air/fuel mixture in bank 2 is too lean.
  • Symptoms: Rough idling, Poor acceleration, Reduced fuel efficiency.
  • Solutions: Inspect for vacuum leaks, clean the mass airflow sensor (MAF), or check the fuel injectors.

10. P0301 - Cylinder 1 Misfire Detected

  • Description: This code indicates a misfire in cylinder 1.
  • Symptoms: Engine roughness, Decreased power, Check engine light.
  • Solutions: Inspect and replace the spark plug, ignition coil, or fuel injector for cylinder 1.

Frequency of the Top 10 Fault Codes

The following chart shows the frequency of occurrence for the top 10 most common OBD-II fault codes in US vehicles. These codes were gathered from over 100,000 scans and provide a scientific overview of the most common issues drivers face.

bar chart visualizing the top 10 most common OBD-II fault codes in US vehicles

Conclusion

If you are experiencing any of these fault codes in your vehicle, it’s essential to address the issue before it worsens. You can easily scan and diagnose your car using the GaragePro Advanced Plus OBD Scanner, a powerful tool that gives you real-time insights into your vehicle’s health.

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Disclaimer: The solutions mentioned above are general fixes that are only indicative in nature and may not be specific to your vehicle’s issue. A vehicle may need different repairs depending on its physical condition, presence of other codes and repair history among other things. Always ensure you diagnose the problem accurately before attempting repairs.

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