Most Common OBD-II Trouble Codes Explained .
(Complete Guide for US Car Owners)
If your Check Engine Light has turned on, an OBD-II scanner can read a diagnostic trouble code (DTC) that helps identify the problem. Since 1996, all cars sold in the United States use the OBD-II standard, meaning many common codes have the same basic definition across brands like Ford, Toyota, Honda, Chevrolet, Nissan, BMW, Mercedes-Benz, Volkswagen, and others.
This guide explains the most common OBD-II codes, what they mean, common symptoms, likely causes, and links to detailed troubleshooting.
What Is an OBD-II Code?
OBD-II (On-Board Diagnostics II) is the standard diagnostic system used in modern vehicles. When the engine or emissions system detects a fault, it stores a trouble code and usually turns on the Check Engine Light.
Most Common OBD-II Codes
P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
Usually caused by a failing catalytic converter, a faulty oxygen sensor, or an exhaust leak.
P0300 – Random/Multiple Cylinder Misfire
Indicates engine misfires that may be caused by worn spark plugs, ignition coils, fuel injectors, or vacuum leaks.
P0171 – System Too Lean (Bank 1)
The engine is receiving too much air or not enough fuel. Common causes include vacuum leaks, dirty MAF sensors, or weak fuel pumps.
P0172 – System Too Rich (Bank 1)
The engine is receiving too much fuel. Faulty fuel injectors, MAF sensors, or oxygen sensors are common causes.
P0128 – Coolant Temperature Below Thermostat Regulating Temperature
Often caused by a thermostat stuck open or a faulty coolant temperature sensor.
P0335 – Crankshaft Position Sensor Circuit
May cause hard starting, engine stalling, or a no-start condition.
P0340 – Camshaft Position Sensor Circuit
Usually related to a faulty camshaft position sensor, damaged wiring, or timing issues.
P0430 – Catalyst System Efficiency Below Threshold (Bank 2)
Similar to P0420 but affects Bank 2 on V-type engines.
P0442 – EVAP System Small Leak Detected
Often caused by a loose fuel cap, cracked EVAP hoses, or a leaking charcoal canister.
P0455 – EVAP System Large Leak Detected
A loose or missing fuel cap is one of the most common causes, but larger EVAP leaks can also trigger this code.
P0135 – Oxygen Sensor Heater Circuit Malfunction
Usually caused by a failed oxygen sensor heater element or damaged wiring.
P0500 – Vehicle Speed Sensor Malfunction
May affect speedometer operation, transmission shifting, and cruise control.
P0520 – Engine Oil Pressure Sensor Circuit
Can indicate a faulty oil pressure sensor, wiring issue, or low oil pressure.
P0562 – System Voltage Low
Often caused by a weak battery, failing alternator, or poor electrical connections.
P0700 – Transmission Control System Malfunction
A general transmission fault code that usually requires reading transmission-specific codes for further diagnosis.
Can You Drive With a Check Engine Light?
Some codes indicate minor issues, while others can lead to severe engine or transmission damage if ignored. If the Check Engine Light is flashing, stop driving as soon as it is safe and have the vehicle inspected.
Frequently Asked Questions
Are OBD-II codes the same on all cars? Generic P0xxx codes have the same definition on all OBD-II compliant vehicles, although the exact repair may differ by make and model.
Can I clear OBD-II codes myself? Yes, using an OBD-II scanner. However, if the underlying problem is not fixed, the code will usually return.
Do I need a mechanic for every code? Not always. Some issues, such as a loose fuel cap, can be fixed easily, while others require professional diagnosis.