Explanation of Diagnostic Trouble Codes structure
Explanation of Diagnostic Trouble Codes structure

Decoding OBD2 Diagnostic Trouble Codes: A Comprehensive Guide

For car owners and DIY mechanics, understanding what’s wrong with your vehicle can often feel like deciphering a complex code. Thankfully, modern vehicles are equipped with an On-Board Diagnostics (OBD) system, specifically OBD-II in most modern cars, that can pinpoint issues through Diagnostic Trouble Codes (DTCs). When your car’s computer, the Engine Control Unit (ECU), detects a problem, it stores a DTC in its memory. By using an OBD-II scanner, you can access these codes and gain valuable insights into the nature of the problem.

These codes are not random; they follow a structured format that, once understood, can provide a general indication of the issue even before consulting a detailed chart. Let’s break down the anatomy of a DTC.

Understanding the Structure of OBD2 Trouble Codes

Each DTC is composed of five characters: a letter followed by four numbers. Each position in this sequence reveals specific information about the fault.

The Initial Letter: System Designation

The first letter of the DTC indicates which vehicle system is experiencing the problem:

  • P (Powertrain): This pertains to issues within the engine, transmission, and related drivetrain components. These are the most common types of DTCs.
  • B (Body): Body codes relate to components found in the car’s body, such as the airbags, power windows, electric seats, and central locking system.
  • C (Chassis): Chassis codes refer to problems with the chassis system, including braking, suspension, and steering systems.
  • U (Network): Network or communication codes indicate issues with the vehicle’s communication network, such as the Controller Area Network (CAN) bus, which different electronic control modules use to communicate with each other.

The First Number: Code Type

The first number that follows the letter specifies whether the code is a standardized or manufacturer-specific code:

  • 0: Indicates a standardized SAE (Society of Automotive Engineers) fault code. These codes are common across all makes and models of vehicles, ensuring a level of consistency in diagnostics.
  • 1: Signifies a manufacturer-specific code. These codes are defined by the vehicle manufacturer and may refer to systems or issues unique to that brand or model.

The Second Number: Subsystem Identification

The second number further categorizes the fault by identifying the specific vehicle subsystem affected. These categories are generally structured as follows:

  • 0: Fuel and Air Metering and Auxiliary Emission Controls
  • 1: Fuel and Air Metering
  • 2: Fuel and Air Metering (Injector Circuit)
  • 3: Ignition Systems or Misfires
  • 4: Auxiliary Emission Controls
  • 5: Vehicle Speed Control & Idle Control Systems
  • 6: Computer & Output Circuit
  • 7: Transmission

The Final Two Numbers: Fault Specificity

The third and fourth digits provide greater detail, pinpointing the exact nature of the fault within the identified system and subsystem. These numbers are sequential and used to differentiate between various faults within a category.

Explanation of Diagnostic Trouble Codes structureExplanation of Diagnostic Trouble Codes structure

Let’s illustrate this with an example: P0303.

  • P: Powertrain fault – indicating the problem is engine or transmission related.
  • 0: Standardized fault code – meaning this code is common across different vehicle manufacturers.
  • 3: Ignition system or misfire – narrowing down the issue to the ignition system.
  • 03: Misfire on cylinder 3 – specifying the exact problem: a misfire detected on the third cylinder.

This detailed breakdown enables mechanics and car owners to quickly narrow down potential issues and start the diagnostic process more effectively.

OBD II Trouble Code Chart: Common Examples*

While understanding the DTC structure is helpful, a comprehensive OBD II trouble code chart provides the specific meaning of each code. Here are some common examples from the P0100 to P0605 ranges, focusing on Powertrain codes:

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)

* Note that not all codes are applicable to every vehicle model. Always consult your vehicle’s repair manual for specific code details.

By understanding the structure of Obd2 Diagnostic Trouble Codes and utilizing a comprehensive code chart, both car enthusiasts and professional mechanics can efficiently diagnose vehicle problems, leading to quicker and more effective repairs. This system empowers individuals to take a proactive approach to vehicle maintenance and understand the health of their car.

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