Here Are Top 10 Most Common OBD Codes and What They Mean

Modern vehicles are equipped with On-Board Diagnostics (OBD) systems that monitor various components to ensure optimal performance.

When an issue is detected, the system generates a diagnostic trouble code (DTC), which can be read using an OBD scanner.

In this article, we’ll explore 10 common OBD codes and decipher their meanings.

1. P0300 – Random/Multiple Cylinder Misfire Detected

When your OBD system throws a P0300 code, it indicates a random or multiple cylinder misfire. This can be caused by faulty spark plugs, ignition coils, or fuel injectors.

Prompt attention is necessary to prevent engine damage.

2. P0420 – Catalyst System Efficiency Below Threshold

A P0420 code points to potential problems with the catalytic converter. This could be due to a failing converter, oxygen sensor issues, or an exhaust leak.

Ignoring this code may result in increased emissions and reduced fuel efficiency.

3. P0171/P0174 – System Too Lean (Bank 1/Bank 2)

If your OBD system detects a lean air-fuel mixture, it will trigger a P0171 or P0174 code. Possible causes include vacuum leaks, faulty oxygen sensors, or a clogged fuel injector.

Addressing this issue promptly can prevent engine damage and improve fuel efficiency.

4. P0442 – Evaporative Emission Control System Leak Detected (Small Leak)

A P0442 code indicates a small leak in the evaporative emission control system, often related to the fuel cap, hoses, or charcoal canister.

Resolving this issue is crucial to maintaining proper emission levels and fuel economy.

5. P0401 – Exhaust Gas Recirculation (EGR) Flow Insufficient

This code signals a problem with the EGR system, which recirculates exhaust gases to reduce nitrogen oxide emissions. Causes may include a clogged EGR valve, faulty EGR sensor, or vacuum leaks.

Ignoring this code can lead to increased emissions and reduced engine efficiency.

6. P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

A P0128 code indicates that the engine is not reaching the optimal operating temperature. Potential causes include a malfunctioning thermostat, low coolant levels, or a faulty coolant temperature sensor.

Addressing this issue is essential for engine performance and fuel efficiency.

7. P0500 – Vehicle Speed Sensor “A” Malfunction

The P0500 code points to problems with the vehicle speed sensor, affecting the accuracy of speedometer readings. Issues with the sensor or its wiring may lead to erratic speedometer behavior.

Prompt diagnosis and repair are necessary for safe driving.

8. P0562 – System Voltage Low

When the OBD system detects low system voltage, it triggers a P0562 code. This may be caused by a weak battery, faulty alternator, or poor electrical connections. Addressing this issue promptly is vital to prevent stalling and potential damage to electronic components.

9. P1610 – Immobilizer/Security System Malfunction

A P1610 code indicates a problem with the vehicle’s immobilizer or security system. This may result in difficulty starting the engine or the inability to start it at all.

Professional assistance is often required to diagnose and resolve this issue.

10. P0700 – Transmission Control System Malfunction

The P0700 code is a generic transmission-related code that signals an issue with the transmission control system. This code often accompanies other transmission-related codes, and prompt attention is crucial to prevent further damage and ensure smooth vehicle operation.


Understanding common OBD codes is essential for vehicle owners, as it empowers them to address issues promptly and prevent more significant problems.

Regular maintenance and prompt attention to OBD codes contribute to the longevity and efficient performance of your vehicle. If you encounter any of these codes, it’s advisable to consult with a qualified mechanic to diagnose and resolve the underlying issues.

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