Modern vehicles use the OBD2 diagnostic system to monitor engine performance, emissions and electronic systems. When a fault is detected, the car stores a Diagnostic Trouble Code (DTC) such as P0420, P0171 or P0300.
On this page you will find the complete index of OBD2 error codes, along with explanations of how the diagnostic system works and links to detailed troubleshooting guides.
DiagnozaBAM provides technical resources for drivers, mechanics and automotive enthusiasts who want to understand vehicle diagnostics and fix problems faster.
What is an OBD2 Error Code
An OBD2 error code is a standardized diagnostic code generated by the vehicle’s Electronic Control Unit (ECU).
When the onboard system detects a malfunction, it stores a code and activates the Check Engine Light on the dashboard.
These codes help identify problems related to:
engine performance
fuel system
emissions components
sensors and electronic modules
The stored codes can be read using an OBD2 scanner connected to the diagnostic port of the vehicle.
Example codes:
P0171 – System Too Lean
P0420 – Catalyst System Efficiency Below Threshold
P0300 – Random / Multiple Cylinder Misfire
P0087 – Fuel Rail Pressure Too Low
Each code indicates a specific malfunction that requires further diagnosis.
OBD2 Code Structure Explained
All OBD2 diagnostic codes follow a five-character format.
Example:
Code structure:
P – Powertrain system (engine / transmission)
0 – Generic SAE diagnostic code
4 – Subsystem category
20 – Specific detected fault
This structure allows technicians to quickly identify which system is affected and what type of problem occurred.
Types of OBD2 Diagnostic Codes
OBD2 codes are divided into four main categories depending on the vehicle system involved.
Powertrain Codes (Pxxxx)
Powertrain codes refer to engine and transmission systems, including fuel injection, ignition, air intake and emissions components.
Examples:
P0100 – Mass Air Flow Sensor Circuit
P0171 – System Too Lean
P0300 – Random Misfire Detected
P0420 – Catalyst Efficiency Below Threshold
These are the most common diagnostic trouble codes found in modern vehicles.
Chassis Codes (Cxxxx)
Chassis codes relate to systems responsible for vehicle stability, braking and suspension.
Examples:
C0035 – Wheel Speed Sensor Fault
C0040 – ABS Sensor Circuit Issue
Body Codes (Bxxxx)
Body codes are associated with interior and comfort systems.
Examples include faults in:
airbag systems
seat modules
climate control
lighting systems
Example code:
B0020 – Airbag Deployment Loop Fault
Network Codes (Uxxxx)
Network codes indicate communication problems between electronic control modules inside the vehicle.
Examples:
U0100 – Lost Communication with ECM/PCM
U0121 – Lost Communication with ABS Module
These faults usually point to CAN bus communication issues or module failures.
Most Common OBD2 Error Codes
Some diagnostic trouble codes appear much more frequently than others because they relate to typical engine problems.
Common examples include:
P0171 – Engine running too lean
P0300 – Random cylinder misfire
P0420 – Catalytic converter efficiency problem
P0128 – Coolant temperature below thermostat regulation
P0455 – Large EVAP leak detected
Detailed diagnostic guides for each code are available in the articles listed below.
How to Diagnose an OBD2 Error Code
When a fault code appears, the diagnostic process usually follows these steps:
Connect an OBD2 scanner to the vehicle diagnostic port.
Read the stored diagnostic trouble codes from the ECU.
Identify the affected system such as fuel, ignition or emissions.
Inspect sensors and related components.
Clear the code and verify if the fault returns.
Accurate diagnosis often requires checking live sensor data and performing component tests.
OBD2 Diagnostic Articles
Below you will find detailed guides explaining individual OBD2 error codes, including causes, symptoms and repair solutions.
(New diagnostic articles are added regularly as part of the DiagnozaBAM technical knowledge base.)
👇
Frequently Asked Questions
What does an OBD2 error code mean
An OBD2 code identifies a malfunction detected by the vehicle’s onboard diagnostic system.
Can I drive with a Check Engine light
It depends on the fault. Some issues are minor, while others may indicate serious engine problems that require immediate attention.
Can OBD2 codes clear themselves
Temporary faults may clear automatically after several drive cycles, but persistent problems require proper diagnosis and repair.
Explore the DiagnozaBAM OBD2 Database
DiagnozaBAM is continuously expanding a complete technical database of OBD2 error codes used in modern vehicles.
Each diagnostic guide includes:
meaning of the code
possible causes
symptoms
diagnostic procedures
repair recommendations
This page acts as the central hub for understanding OBD2 diagnostics and troubleshooting vehicle problems.
