The On-Board Diagnostics system, or OBD2, is an invaluable tool for identifying issues in your car. When malfunctions or emission problems arise, your vehicle’s computer logs alphanumeric codes, known as OBD2 fault codes, to pinpoint the source of the trouble. But how do you decipher these codes and use an OBD2 fault code table effectively?
First, to access these error codes stored in your vehicle’s memory, you’ll need to connect a diagnostic scanner to the OBD2 port. This port is typically located beneath the dashboard on the driver’s side of most vehicles.
While there are many affordable and feature-rich OBD2 scanners available for personal use, for in-depth diagnostics and accurate interpretations, it’s always recommended to consult with qualified professionals using professional-grade diagnostic equipment.
Why Does Your Check Engine Light Illuminate?
The check engine light, also known as the malfunction indicator lamp (MIL), illuminates because your vehicle’s electronic control system, managed by one or more Electronic Control Units (ECUs), has detected a fault. These ECUs constantly monitor various sensors and systems throughout your car, and when a reading falls outside of the acceptable parameters, a fault code is triggered and stored, and the check engine light comes on.
Decoding OBD2 Fault Codes: Understanding the Structure
OBD2 fault codes are structured in a standardized five-character format, which provides crucial information about the nature and location of the problem. Understanding this structure is key to effectively using an OBD2 fault code table. Let’s break down each digit:
First Digit: Category of the Fault
The first character of an OBD2 code is a letter that indicates the system where the fault has occurred:
- P (Powertrain): This is the most common prefix and relates to issues within the engine, transmission, and associated drivetrain components. This category covers systems that manage power generation and delivery.
- B (Body): “B” codes refer to problems within the vehicle’s body systems. This includes components like power windows, power seats, air conditioning, airbags, and central locking systems.
- C (Chassis): “C” codes indicate issues with the chassis systems of the vehicle. This encompasses braking systems (ABS), steering, and suspension.
- U (Network or Undefined): “U” codes point to communication issues within the vehicle’s network. Modern cars use complex communication networks to allow different modules to interact. “U” codes can indicate problems with these communication buses, or issues that are not easily categorized within the other systems.
Second Digit: Code Type
The second digit in the OBD2 code is a number that specifies whether the code is generic or manufacturer-specific:
- P0xxx, B0xxx, C0xxx, U0xxx: These codes are standardized and defined by the Society of Automotive Engineers (SAE). They are considered “generic” or “standard” OBD2 codes, meaning they are the same across all makes and models of vehicles that comply with OBD2 regulations. An OBD2 fault code table will primarily list these generic codes.
- P1xxx, B1xxx, C1xxx, U1xxx, P2xxx, P3xxx, etc.: Codes starting with “1” or higher after the system letter are typically manufacturer-specific codes. These codes are defined by the vehicle manufacturer and often relate to more specific or advanced systems within a particular make or model. While a generic OBD2 fault code table might not include these in detail, understanding the prefix helps to know that the issue might require more specialized diagnostic information from the vehicle manufacturer.
Third Digit: Subsystem
The third digit of an OBD2 code provides further detail by indicating the specific subsystem within the broader category identified by the first digit. For “P” codes (Powertrain), the third digit typically represents these subsystems:
- Px1xx & Px2xx: Fuel and Air Metering: These codes relate to issues with the systems that control the air-fuel mixture in the engine. This can include sensors like the mass air flow (MAF) sensor, oxygen sensors, fuel injectors, and related components.
- Px3xx: Ignition System or Misfire: Codes in this range are associated with the engine’s ignition system, including spark plugs, ignition coils, and related circuitry. Misfire codes, indicating incomplete combustion in one or more cylinders, also fall into this category.
- Px4xx: Auxiliary Emission Controls: These codes pertain to components that are designed to further reduce emissions, such as the evaporative emissions (EVAP) system, secondary air injection system, and exhaust gas recirculation (EGR) system.
- Px5xx: Vehicle Speed Controls and Idle Control System: Codes in this range relate to systems that manage vehicle speed and engine idle speed. This can include the idle air control (IAC) valve, vehicle speed sensor (VSS), and cruise control systems.
- Px6xx: Computer Output Circuit: These codes indicate problems with the output circuits of the vehicle’s computer (ECU or PCM). This means there might be issues with the signals the computer is sending to control various actuators or systems.
- Px7xx & Px8xx: Transmission: These codes are specific to automatic transmission issues. This includes problems with transmission sensors, solenoids, and the transmission control module (TCM).
- Px9xx: Input/Output Circuit: Similar to Px6xx but broader, these codes can indicate problems with input or output circuits of various control modules, not just the main computer. This can include sensor signals, actuator controls, and communication lines.
Fourth and Fifth Digits: Specific Fault
The fourth and fifth digits, the final two characters of the OBD2 code, are numerical and provide the most specific identification of the component or system that is malfunctioning. These digits, in combination with the preceding characters, pinpoint the exact nature of the fault. For example, within the P01xx range (Fuel and Air Metering), P0101 specifically refers to a “Mass Air Flow Circuit Range/Performance Problem”.
Common OBD2 Fault Codes: Examples from the P0010 – P0849 Range
To illustrate how an OBD2 fault code table works, let’s examine some common codes within the P0010 to P0849 range, as listed in the original article. This is just a small subset of the thousands of possible OBD2 codes, but it gives a good overview. Remember, this is not an exhaustive OBD2 fault code table, but rather a selection for illustrative purposes. For a complete OBD2 fault code table, you would need to consult dedicated resources.
| OBD2 Code | Description | Possible Issue |
|---|---|---|
| P0010 | Camshaft Position Actuator A Circuit (Bank 1) | Wiring issue to the camshaft actuator, faulty actuator, PCM issue. |
| P0011 | Camshaft Position A – Timing Over-Advanced or System Performance (Bank 1) | Low oil pressure, faulty camshaft actuator, timing chain/belt issue, incorrect engine timing. |
| P0012 | Camshaft Position A – Timing Over-Retarded (Bank 1) | Low oil pressure, faulty camshaft actuator, timing chain/belt issue, incorrect engine timing. |
| P0013 | Camshaft Position Actuator B Circuit (Bank 1) | Wiring issue to the camshaft actuator, faulty actuator, PCM issue. |
| P0014 | Camshaft Position B – Timing Over-Advanced or System Performance (Bank 1) | Low oil pressure, faulty camshaft actuator, timing chain/belt issue, incorrect engine timing. |
| P0015 | Camshaft Position B – Timing Over-Retarded (Bank 1) | Low oil pressure, faulty camshaft actuator, timing chain/belt issue, incorrect engine timing. |
| P0030 | HO2S Heater Control Circuit (Bank 1 Sensor 1) | Faulty oxygen sensor heater, wiring issue, relay problem, PCM issue. |
| P0100 | Mass Air Flow Circuit Malfunction | Dirty or faulty MAF sensor, wiring issue, vacuum leak, intake restriction. |
| P0101 | Mass Air Flow Circuit Range/Performance Problem | Dirty or faulty MAF sensor, vacuum leak, intake restriction, incorrect MAF sensor for vehicle. |
| P0102 | Mass Air Flow Circuit Low Input | Wiring issue (short to ground), faulty MAF sensor, intake leak after MAF sensor. |
| P0103 | Mass Air Flow Circuit High Input | Wiring issue (short to voltage), faulty MAF sensor, PCM issue. |
| P0110 | Intake Air Temperature Circuit Malfunction | Faulty IAT sensor, wiring issue, PCM issue. |
| P0111 | Intake Air Temperature Circuit Range/Performance Problem | Faulty IAT sensor, sensor installed incorrectly, PCM issue. |
| P0112 | Intake Air Temperature Circuit Low Input | Wiring issue (short to ground), faulty IAT sensor. |
| P0113 | Intake Air Temperature Circuit High Input | Wiring issue (short to voltage), faulty IAT sensor. |
| P0115 | Engine Coolant Temperature Circuit Malfunction | Faulty ECT sensor, wiring issue, thermostat problem, cooling system issue, PCM issue. |
| P0116 | Engine Coolant Temperature Circuit Range/Performance Problem | Faulty ECT sensor, thermostat problem, cooling system issue, incorrect coolant, air pockets in cooling system. |
| P0117 | Engine Coolant Temperature Circuit Low Input | Wiring issue (short to ground), faulty ECT sensor. |
| P0118 | Engine Coolant Temperature Circuit High Input | Wiring issue (short to voltage), faulty ECT sensor. |
| P0120 | Throttle/Pedal Position Sensor/Switch A Circuit Malfunction | Faulty TPS, wiring issue, PCM issue. |
| P0121 | Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem | Faulty TPS, throttle body issue, wiring issue, PCM issue. |
| P0122 | Throttle/Pedal Position Sensor/Switch A Circuit Low Input | Wiring issue (short to ground), faulty TPS. |
| P0123 | Throttle/Pedal Position Sensor/Switch A Circuit High Input | Wiring issue (short to voltage), faulty TPS. |
| P0130 | O2 Sensor Circuit Malfunction (Bank 1 Sensor 1) | Faulty oxygen sensor, wiring issue, exhaust leak, intake leak, fuel pressure issue, PCM issue. |
| P0131 | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) | Vacuum leak, exhaust leak, lean air-fuel mixture, faulty oxygen sensor, fuel pressure issue. |
| P0132 | O2 Sensor Circuit High Voltage (Bank 1 Sensor 1) | Rich air-fuel mixture, faulty oxygen sensor, fuel pressure issue, injector leak. |
| P0133 | O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) | Aging oxygen sensor, exhaust leak, intake leak, contaminated oxygen sensor. |
| P0134 | O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1) | Faulty oxygen sensor, wiring issue, exhaust leak, PCM issue. |
| P0135 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) | Faulty oxygen sensor heater, wiring issue, relay problem, PCM issue. |
| P0170 | Fuel Trim Malfunction (Bank 1) | Vacuum leak, intake leak, exhaust leak, faulty oxygen sensor, MAF sensor issue, fuel pressure problem. |
| P0171 | System Too Lean (Bank 1) | Vacuum leak, intake leak, low fuel pressure, faulty MAF sensor, faulty oxygen sensor. |
| P0172 | System Too Rich (Bank 1) | Fuel injector leak, high fuel pressure, faulty oxygen sensor, faulty MAF sensor. |
| P0300 | Random/Multiple Cylinder Misfire Detected | Spark plug issue, ignition coil problem, fuel injector problem, vacuum leak, low compression, timing issue. |
| P0301 | Cylinder 1 Misfire Detected | Spark plug issue in cylinder 1, ignition coil problem in cylinder 1, fuel injector problem in cylinder 1, compression issue in cylinder 1. |
| P0400 | Exhaust Gas Recirculation Flow Malfunction | EGR valve stuck open or closed, EGR system blockage, vacuum line issue, EGR sensor problem. |
| P0401 | Exhaust Gas Recirculation Flow Insufficient Detected | EGR valve stuck closed or partially closed, EGR system blockage, vacuum line issue. |
| P0402 | Exhaust Gas Recirculation Flow Excessive Detected | EGR valve stuck open, EGR system issue, EGR sensor problem. |
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) | Faulty catalytic converter, exhaust leak, faulty oxygen sensors, engine misfire. |
| P0440 | Evaporative Emission Control System Malfunction | EVAP system leak, faulty gas cap, purge valve problem, vent valve problem, charcoal canister issue. |
| P0500 | Vehicle Speed Sensor Malfunction | Faulty VSS, wiring issue, ABS system problem, PCM issue. |
Note: This is a simplified OBD2 fault code table. For accurate diagnosis, always refer to a comprehensive OBD2 fault code table and repair information specific to your vehicle’s make and model.
Using an OBD2 Fault Code Table Effectively
An OBD2 fault code table is a valuable starting point when your check engine light comes on. Here’s how to use it effectively:
- Retrieve the Code: Use an OBD2 scanner to read the fault code(s) stored in your vehicle’s computer. Write down all the codes present.
- Consult the Table: Locate an OBD2 fault code table. You can find these tables online, in repair manuals, or within many OBD2 scanner devices themselves.
- Look Up the Code: Find your specific code in the table. Note the description provided for the code.
- Understand the Description: The description in the OBD2 fault code table gives you a general idea of the problem area. For example, “P0171 – System Too Lean (Bank 1)” indicates a lean running condition in engine bank 1.
- Consider Possible Causes: The table description is a starting point. Think about the “Possible Issues” column in the example table above, or similar information in your chosen OBD2 fault code table. These are potential causes, but not definitive diagnoses. For example, P0171 (System Too Lean) could be caused by a vacuum leak, a faulty MAF sensor, or low fuel pressure, among other things.
- Further Diagnosis is Necessary: Crucially, an OBD2 fault code table provides information, not a definitive diagnosis. It tells you where to start looking. Further diagnostic steps are required to pinpoint the exact cause of the problem. This might involve:
- Visual Inspection: Check for obvious issues like loose gas caps, broken wires, vacuum leaks, or damaged components.
- Sensor Testing: Use a multimeter or scan tool to test the sensors related to the fault code.
- System Testing: Perform tests on related systems, such as a smoke test for vacuum leaks, or fuel pressure test.
- Consulting Repair Information: Refer to your vehicle’s repair manual or online databases for detailed diagnostic procedures specific to your code and vehicle model.
Beyond the Table: Troubleshooting and Repair
Once you’ve used an OBD2 fault code table to understand the potential problem and performed further diagnosis, you can move towards troubleshooting and repair.
- Simple Fixes: Sometimes, the issue might be a simple fix you can do yourself, such as tightening a gas cap or replacing a faulty sensor that you’ve confirmed through testing.
- Professional Help: For more complex issues, or if you are not comfortable working on your car, it’s best to seek professional help from a qualified mechanic. Provide them with the OBD2 codes you retrieved, and the diagnostic steps you’ve taken. This will help them efficiently diagnose and repair your vehicle.
- Clearing Codes: After repairing the issue, you can use your OBD2 scanner to clear the fault codes and turn off the check engine light. However, it’s crucial to actually fix the underlying problem, not just clear the code. If the problem persists, the check engine light will likely return, and the code will reappear.
Conclusion
Understanding OBD2 fault codes and utilizing an OBD2 fault code table is a significant step in modern car maintenance and repair. It empowers car owners with initial insights into potential vehicle problems and guides mechanics towards efficient diagnoses. While an OBD2 fault code table is an essential tool, remember that it’s a starting point. Accurate diagnosis and repair often require further investigation, testing, and expertise. By using this information responsibly and seeking professional help when needed, you can keep your vehicle running smoothly and address issues effectively.
