What are the most common OBDII codes?

Onboard Diagnostics II (OBDII) codes, also known as Diagnostic Trouble Codes (DTCs), are standardized codes used by vehicles to indicate specific issues or malfunctions. These codes help mechanics and technicians diagnose and repair problems in a vehicle's engine, transmission, emissions, and other systems. The most common OBDII codes can vary depending on the make and model of the vehicle, but I can provide you with a list of some frequently encountered codes and their general meanings:

1. P0300 Random/Multiple Cylinder Misfire Detected: Indicates that the engine has misfires occurring in multiple cylinders.

2. P0420 Catalyst System Efficiency Below Threshold: Suggests that the catalytic converter is not working efficiently.

3. P0171 System Too Lean (Bank 1): Points to a lean condition in the fuel mixture on bank 1 of the engine.

4. P0174 System Too Lean (Bank 2): Similar to P0171 but for bank 2 of the engine.

5. P0442 Evaporative Emission System Leak Detected (Small Leak): Indicates a small leak in the vehicle's evaporative emission control system.

6. P0455 Evaporative Emission System Leak Detected (Large Leak): Suggests a larger leak in the evaporative emission control system.

7. P0507 Idle Air Control System RPM Higher Than Expected: Indicates that the engine's idle speed is higher than expected.

8. P0601 Internal Control Module Memory Checksum Error: Points to a malfunction in the vehicle's internal control module.

9. P0715 Input/Turbine Speed Sensor Circuit Malfunction: Indicates a problem with the input or turbine speed sensor circuit in the transmission.

10. P0720 Output Speed Sensor Circuit Malfunction: Points to a problem with the output speed sensor circuit in the transmission.

Remember that these codes are just examples, and there are many more OBDII codes that cover a wide range of issues. When a specific code appears, it's important to refer to the vehicle's repair manual or consult a professional mechanic to accurately diagnose and address the problem. Additionally, OBDII codes can have different meanings for different vehicle manufacturers, so it's important to consider the manufacturer-specific definitions as well.