This page is dedicated to helping you understand and troubleshoot OBD2 fault codes!
If you’re unfamiliar with OBD2, it stands for On-Board Diagnostics and is a system installed in modern cars to monitor their performance and detect any issues.
When your car’s OBD2 system detects a problem, it will generate a fault code, which is a unique identifier that corresponds to a specific issue. These codes can be read using an OBD2 scanner or code reader, which can help you diagnose the problem and find a solution.
Below I’ve compiled a list of common OBD2 fault codes that you may encounter. For each code, I’ve provided a brief outline of what it means.
The fault codes are accessed by connecting a handheld fault code reader to the onboard computer through the OBD port, which is usually located under the driver’s side dashboard.
If you need a crash course on reading and clearing fault codes using an OBD2 code reader, check out this link – How to use a fault code reader (video)
If you need to buy a code reader, check out this link – Fault code reader I recommend for DIYers
Common OBD2 Fault Codes
List of diagnostic trouble codes (DTCs) that mechanics and car owners frequently encounter.
- P0101 – Mass or Volume Air flow Circuit Range/Performance Problem
- P0113 – Intake Air Temperature (IAT) Sensor Circuit High Input
- P0128 – Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
- P0133 – Oxygen Sensor Slow Response Bank 1 Sensor 1
- P0135 – Oxygen Sensor (HO2S) Heater Circuit Fault Bank 1 Sensor 1
- P0141 – Oxygen Sensor Heater (H02S) Heater Circuit Fault Bank 1 Sensor 2
- P0171 – Fuel System Too Lean Bank 1
- P0172 – Fuel System Too Rich Bank 1
- P0174 – Fuel System Too Lean Bank 2
- P0175 – Fuel System Too Rich Bank 2
- P0300 – Engine Misfire Detected (random misfire)
- P0301 – Cylinder 1 Misfire Detected
- P0302 – Cylinder 2 Misfire Detected
- P0303 – Cylinder 3 Misfire Detected
- P0304 – Cylinder 4 Misfire Detected
- P0325 – Knock Sensor
- P0401 – Exhaust gas recirculation (EGR) flow is insufficient
- P0420 – Catalyst System Low-Efficiency Bank 1
- P0430 – Catalyst System Low-Efficiency Bank 2
- P0440 – Evaporative Emission System Fault
- P0442 – Evaporative Emission (EVAP) system – Generic/leak detected/small leak detected
- P0455 – Evaporate Emission System Leak
- P0446 – EVAP Vent Solenoid Valve Control System
- P0506 – Idle Control System RPM Lower Than Expected
- P0507 – Idle Control System RPM Higher Than Expected
- P0700 – Transmission Control System Malfunction
- P0741 – Torque Converter Clutch Circuit Performance or Stuck Off
It’s important to note that while fault codes are helpful, they can sometimes be misleading, as when one system goes down, it can cause other tangentially related fault codes to set. When multiple codes are set in this way, it can lead to misdiagnosis.
Successful diagnoses will require a working knowledge of the system under diagnosis, an OBD2 code reader, and, at times, may also require a Voltmeter for circuit testing.
Fault Code FAQs
- OBD won’t connect
- OBD won’t clear codes
- OBD scanner won’t turn on
- Check engine light after oil change
- Are OBD scanners universal?
- What scanner for Duramax?
You may find the following links helpful:
- How to use a fault code reader (video)
- Fault code reader I recommend for DIYers
- DIY troubleshooting and repair page
- Beginner DIY maintenance page
- Recommended mechanics tools
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.