OBD2. On-Board Diagnostics.
Every modern car has it.
It’s the system watching what’s going on under the hood… all the time.
When something’s off, it flags it.
That flag is a fault code.
Each code points to a specific problem area. Not always the exact failed part—but it gets you close.
Plug in a scanner, pull the code, and you’ve got a starting point.
Below is a list of the most common OBD2 codes I see on the roadside.
Plain-English meaning. No fluff.
Plug a handheld scanner into the OBD port.
That port sits under the driver’s side dash on most cars.
Once connected, it pulls the fault codes straight from the car’s computer.
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
The trouble codes you’ll see again and again…….the ones I deal with daily.
- P0101 – Mass or Volume Air flow Circuit
- P0102 – MAF Sensor Low Input
- P0103 -MAF Sensor High Input
- P0106 – MAP Sensor Range or Performance Problem
- Range/Performance Problem
- P0113 – Intake Air Temperature (IAT) Sensor Circuit High Input
- P0118 – Coolant Temp Sensor High Input
- P0121 – TPS Range/Performance
- P0122 – TPS Low Input
- P0128 – Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
- P0130 – O2 Sensor Circuit Bank 1 Sensor 1
- 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
- P0217 – Engine Overtemperature
- P0220 – Throttle/Pedal Position Sensor Circuit
- 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
- P0316 – Misfire on Startup
- P0325 – Knock Sensor
- P0335 – Crankshaft Position Sensor Circuit
- P0340 – Camshaft Position Sensor Circuit
- P0351–P0356 – Ignition Coil Circuit Faults
- 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
- P0441 – EVAP Incorrect Purge Flow
- P0442 – Evaporative Emission (EVAP) system – Generic/leak detected/small leak detected
- P0446 – EVAP Vent Solenoid Valve Control System
- P0455 – Evaporate Emission System Leak
- P0456 – Small EVAP Leak
- P0480 – Cooling Fan 1 Control Circuit
- P0496 – EVAP High Purge Flow
- P0506 – Idle Control System RPM Lower Than Expected
- P0562 – System Voltage Low
- P0507 – Idle Control System RPM Higher Than Expected
- P0700 – Transmission Control System Malfunction
- P0715 – Input Speed Sensor Circuit
- P0720 – Output Speed Sensor
- P0730 – Incorrect Gear Ratio
- P0741 – Torque Converter Clutch Circuit Performance or Stuck Off
- P0750–P0770 – Shift Solenoid Faults
- P0780 – Shift Malfunction
- P2195 / P2197 – O2 Stuck Lean
- P2196 / P2198 – O2 Stuck Rich
Fault codes help.
But they don’t tell the whole story.
One fault can trigger a chain reaction… and suddenly you’ve got a list of codes that look unrelated.
That’s where people go wrong.
They chase the codes instead of the cause.
To get it right, you’ll need:
- A scanner
- A basic understanding of how the system works
- And sometimes a multimeter to test the circuit properly
That’s how you avoid throwing parts at it.
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
Visit our DIY Car Maintenance page and level up your car care skills — or keep the quick-reference version below in your glovebox.
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- About the Author
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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.