The modern car is a complex machine, but thankfully, diagnosing issues has become more accessible to the average car owner thanks to On-Board Diagnostics (OBD) systems. At the heart of this system are Obd2 Diagnostic Trouble Codes, often referred to as DTCs. Think of these codes as your car’s way of telling you exactly what’s going wrong under the hood. When your vehicle’s computer, the Engine Control Unit (ECU), detects a problem, it stores a specific fault code in its memory. By using an OBD2 scanner, which plugs into your car’s OBD port, you can retrieve these codes and begin to understand the nature of the problem.
These codes aren’t just random numbers and letters; they follow a structured format. Understanding this structure can give you a significant head start in diagnosing car trouble even before you consult a detailed code chart. Let’s break down how to decipher these obd2 diagnostic trouble codes.
Understanding the Anatomy of a DTC
Every OBD2 diagnostic trouble code is composed of five characters: one letter followed by four numbers. Each position provides crucial information about the fault.
The Initial Letter: System Area
The very first letter of the DTC indicates which major vehicle system is experiencing the problem:
- P – Powertrain: This is the most common category and relates to issues with the engine, transmission, and related drivetrain components.
- B – Body: Body codes pertain to systems within the car’s body, such as the airbags, power windows, and central locking.
- C – Chassis: Chassis codes refer to problems with the chassis systems, including braking, steering, and suspension.
- U – Network & Vehicle Integration: These codes indicate issues with the communication network within the car, often involving the Controller Area Network (CAN) bus and electronic control modules.
The First Number: Code Type
The first numerical digit following the letter further refines the code’s meaning:
- 0 – Standardized (SAE) Code: This signifies a generic code defined by the Society of Automotive Engineers (SAE). These codes are the same across all makes and models of vehicles, ensuring a degree of universality in diagnostics.
- 1 – Manufacturer-Specific Code: This indicates a code that is specific to a particular car manufacturer. While the basic OBD2 system is standardized, manufacturers can add their own enhanced codes for more detailed diagnostics of their vehicles.
The Second Number: Subsystem Area
The second number pinpoints the specific subsystem within the broader system identified by the first letter. For Powertrain (P) codes, the categories are:
- 0 – Fuel and Air Metering and Auxiliary Emission Controls: This covers issues related to fuel delivery, air intake, and components that assist in reducing emissions.
- 1 – Fuel and Air Metering: Focuses specifically on fuel and air mixture control.
- 2 – Fuel and Air Metering (Injector Circuit): More specifically related to the fuel injector circuits.
- 3 – Ignition System or Misfire: Concerns problems with the ignition system, including spark plugs, ignition coils, and engine misfires.
- 4 – Auxiliary Emission Controls: Deals with components like the catalytic converter, oxygen sensors, and EGR system.
- 5 – Vehicle Speed Control & Idle Control Systems: Relates to systems that manage vehicle speed and engine idle speed.
- 6 – Computer & Output Circuit: Indicates problems within the vehicle’s computer or its output circuits.
- 7 – Transmission: Specific to issues within the automatic transmission system.
The Third and Fourth Numbers: Specific Fault
The final two digits, the third and fourth numbers, are used to pinpoint the exact nature of the fault. These numbers are sequential and provide a highly specific identification of the problem within the subsystem.
Let’s take the example DTC P0303 to illustrate this breakdown:
- P = Powertrain fault (Engine or Transmission related)
- 0 = Standardized SAE code (Generic across manufacturers)
- 3 = Ignition system or misfire subsystem
- 03 = Specific fault – Misfire detected on cylinder 3
Understanding Diagnostic Trouble Codes Structure
OBD II Trouble Code Chart*
| Code | Code Identification |
|---|---|
| P0101 | Mass air flow (MAF) sensor circuit, range or performance problem |
| P0102 | Mass air flow (MAF) sensor circuit, low input |
| P0103 | Mass air flow (MAF) sensor circuit, high input |
| P0106 | Manifold absolute pressure (MAP) sensor circuit, range or performance problem |
| P0107 | Manifold absolute pressure (MAP) sensor circuit, low input |
| P0108 | Manifold absolute pressure (MAP) sensor circuit, high input |
| P0112 | Intake air temperature (IAT) circuit, low input |
| P0113 | Intake air temperature (IAT) circuit, high input |
| P0117 | Engine coolant temperature (ECT) circuit, low input |
| P0118 | Engine coolant temperature (ECT) circuit, high input |
| P0121 | Throttle position sensor (TPS) circuit, range or performance problem |
| P0122 | Throttle position sensor (TPS) circuit, low input |
| P0123 | Throttle position sensor (TPS) circuit, high input |
| P0125 | Insufficient coolant temperature for closed loop fuel control |
| P0131 | Oxygen sensor circuit, low voltage (pre-converter sensor, left bank) |
| P0132 | Oxygen sensor circuit, high voltage (pre-converter sensor, left bank) |
| P0133 | Oxygen sensor circuit, slow response (pre-converter sensor, left bank) |
| P0134 | Oxygen sensor circuit – no activity detected (pre-converter sensor, left bank) |
| P0135 | Oxygen sensor heater circuit malfunction (pre-converter sensor, left bank) |
| P0137 | Oxygen sensor circuit, low voltage (post-converter sensor, left bank) |
| P0138 | Oxygen sensor circuit, high voltage (post-converter sensor, left bank) |
| P0140 | Oxygen sensor circuit – no activity detected (post-converter sensor, left bank) |
| P0141 | Oxygen sensor heater circuit malfunction (post-converter sensor, left bank) |
| P0143 | Oxygen sensor circuit, low voltage (#2 post-converter sensor, left bank) |
| P0144 | Oxygen sensor circuit, high voltage (#2 post-converter sensor, left bank) |
| P0146 | Oxygen sensor circuit – no activity detected (#2 post-converter sensor, left bank) |
| P0147 | Oxygen sensor heater circuit malfunction (#2 post-converter sensor, left bank) |
| P0151 | Oxygen sensor circuit, low voltage (pre-converter sensor, right bank) |
| P0152 | Oxygen sensor circuit, high voltage (pre-converter sensor, right bank) |
| P0153 | Oxygen sensor circuit, slow response (pre-converter sensor, right bank) |
| P0154 | Oxygen sensor circuit – no activity detected (pre-converter sensor, right bank) |
| P0155 | Oxygen sensor heater circuit malfunction (pre-converter sensor, right bank) |
| P0157 | Oxygen sensor circuit, low voltage (post-converter sensor, right bank) |
| P0158 | Oxygen sensor circuit, high voltage (post-converter sensor, right bank) |
| P0160 | Oxygen sensor circuit – no activity detected (post-converter sensor, right bank) |
| P0161 | Oxygen sensor heater circuit malfunction (post-converter sensor, right bank) |
| P0171 | System too lean, left bank |
| P0172 | System too rich, left bank |
| P0174 | System too lean, right bank |
| P0175 | System too rich, right bank |
| P0300 | Engine misfire detected |
| P0301 | Cylinder number 1 misfire detected |
| P0302 | Cylinder number 2 misfire detected |
| P0303 | Cylinder number 3 misfire detected |
| P0304 | Cylinder number 4 misfire detected |
| P0305 | Cylinder number 5 misfire detected |
| P0306 | Cylinder number 6 misfire detected |
| P0307 | Cylinder number 7 misfire detected |
| P0308 | Cylinder number 8 misfire detected |
| P0325 | Knock sensor circuit malfunction |
| P0327 | Knock sensor circuit, low output |
| P0336 | Crankshaft position sensor circuit, range or performance problem |
| P0337 | Crankshaft position sensor, low output |
| P0338 | Crankshaft position sensor, high output |
| P0339 | Crankshaft position sensor, circuit intermittent |
| P0340 | Camshaft position sensor circuit |
| P0341 | Camshaft position sensor circuit, range or performance problem |
| P0401 | Exhaust gas recirculation, insufficient flow detected |
| P0404 | Exhaust gas recirculation circuit, range or performance problem |
| P0405 | Exhaust gas recirculation sensor circuit low |
| P0410 | Secondary air injection system |
| P0418 | Secondary air injection pump relay control circuit |
| P0420 | Catalyst system efficiency below threshold, left bank |
| P0430 | Catalyst system efficiency below threshold, right bank |
| P0440 | Evaporative emission control system malfunction |
| P0441 | Evaporative emission control system, purge control circuit malfunction |
| P0442 | Evaporative emission control system, small leak detected |
| P0446 | Evaporative emission control system, vent system performance |
| P0452 | Evaporative emission control system, pressure sensor low input |
| P0453 | Evaporative emission control system, pressure sensor high input |
| P0461 | Fuel level sensor circuit, range or performance problem |
| P0462 | Fuel level sensor circuit, low input |
| P0463 | Fuel level sensor circuit, high input |
| P0500 | Vehicle speed sensor circuit |
| P0506 | Idle control system, rpm lower than expected |
| P0507 | Idle control system, rpm higher than expected |
| P0601 | Powertrain Control Module, memory error |
| P0602 | Powertrain Control module, programming error |
| P0603 | Powertrain Control Module, memory reset error |
| P0604 | Powertrain Control Module, memory error (RAM) |
| P0605 | Powertrain Control Module, memory error (ROM) |
* Not all codes apply to all models
By understanding the structure of OBD2 diagnostic trouble codes, you are better equipped to diagnose car problems efficiently. When you encounter a check engine light and retrieve a DTC, this knowledge will help you narrow down the potential issues and guide your troubleshooting process, saving you time and potentially money on unnecessary repairs. Remember to always consult a reliable repair manual or database for the specific meaning of each code for your vehicle model.
