All OBD2 Codes — Free Diagnostic Guides
The 100 most common check engine codes. Every guide includes plain-English explanations, causes ranked by frequency, common misdiagnoses, and honest repair cost guidance.
🔴 Tier 1 — High Traffic
25 codes · accounts for most check engine lights
P0171
System Too Lean (Bank 1)
P0420Catalyst Efficiency Below Threshold (Bank 1)
P0300Random/Multiple Cylinder Misfire Detected
P0442Small EVAP System Leak Detected
P0455Large EVAP System Leak Detected
P0301Cylinder 1 Misfire Detected
P0302Cylinder 2 Misfire Detected
P0303Cylinder 3 Misfire Detected
P0304Cylinder 4 Misfire Detected
P0128Coolant Thermostat Below Regulating Temperature
P0430Catalyst Efficiency Below Threshold (Bank 2)
P0172System Too Rich (Bank 1)
P0401EGR Flow Insufficient Detected
P0113Intake Air Temperature Sensor Circuit High Input
P0101Mass Air Flow Sensor Range/Performance
P0340Camshaft Position Sensor Circuit Malfunction
P0335Crankshaft Position Sensor Circuit Malfunction
P0456Very Small EVAP System Leak Detected
P0446EVAP Vent Control Circuit Malfunction
P0411Secondary Air Injection System Incorrect Flow
P0174System Too Lean (Bank 2)
P0175System Too Rich (Bank 2)
P0135O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
P0138O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
P0141O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
🟠 Tier 2 — Sensor & Electrical
25 codes · diagnosis & symptoms focus
P0440
EVAP Emission Control System Malfunction
P0404EGR Circuit Range/Performance
P0305Cylinder 5 Misfire Detected
P0306Cylinder 6 Misfire Detected
P0320Ignition/Distributor Engine Speed Input Circuit
P0325Knock Sensor 1 Circuit Malfunction
P0330Knock Sensor 2 Circuit Malfunction (Bank 2)
P0131O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
P0132O2 Sensor Circuit High Voltage (Bank 1, Sensor 1)
P0133O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
P0136O2 Sensor Circuit Malfunction (Bank 1, Sensor 2)
P0151O2 Sensor Circuit Low Voltage (Bank 2, Sensor 1)
P0152O2 Sensor Circuit High Voltage (Bank 2, Sensor 1)
P0155O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
P0161O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 2)
P0102Mass Air Flow Sensor Circuit Low Input
P0103Mass Air Flow Sensor Circuit High Input
P0111Intake Air Temperature Sensor Circuit Range/Performance
P0112Intake Air Temperature Sensor Circuit Low Input
P0116Engine Coolant Temperature Circuit Range/Performance
P0117Engine Coolant Temperature Circuit Low Input
P0118Engine Coolant Temperature Circuit High Input
P0121Throttle Position Sensor Circuit Range/Performance
P0122Throttle Position Sensor Circuit Low Input
P0123Throttle Position Sensor Circuit High Input
🟡 Tier 3 — Internal & Transmission
30 codes · repair cost & technical focus
P0201
Injector Circuit Malfunction (Cylinder 1)
P0202Injector Circuit Malfunction (Cylinder 2)
P0203Injector Circuit Malfunction (Cylinder 3)
P0204Injector Circuit Malfunction (Cylinder 4)
P0261Cylinder 1 Injector Circuit Low
P0264Cylinder 2 Injector Circuit Low
P0267Cylinder 3 Injector Circuit Low
P0270Cylinder 4 Injector Circuit Low
P0341Camshaft Position Sensor Circuit Range/Performance (Bank 1)
P0345Camshaft Position Sensor Circuit Malfunction (Bank 2)
P0336Crankshaft Position Sensor Circuit Range/Performance
P0351Ignition Coil A Primary/Secondary Circuit Malfunction
P0352Ignition Coil B Primary/Secondary Circuit Malfunction
P0353Ignition Coil C Primary/Secondary Circuit Malfunction
P0354Ignition Coil D Primary/Secondary Circuit Malfunction
P0521Engine Oil Pressure Sensor Range/Performance
P0522Engine Oil Pressure Sensor Circuit Low Voltage
P0562System Voltage Low
P0563System Voltage High
P0601Internal Control Module Memory Checksum Error
P0606ECM/PCM Processor Fault
P0700Transmission Control System Malfunction
P0705Transmission Range Sensor Circuit Malfunction (PRNDL)
P0715Input/Turbine Speed Sensor Circuit Malfunction
P0720Output Speed Sensor Circuit Malfunction
P0731Gear 1 Incorrect Ratio
P0732Gear 2 Incorrect Ratio
P0733Gear 3 Incorrect Ratio
P0740Torque Converter Clutch Circuit Malfunction
P0741Torque Converter Clutch Circuit Performance
🟢 Tier 4 — Actuators & Specialized
20 codes · deep dive guides
P0010
Camshaft Position Actuator Circuit (Bank 1)
P0011Camshaft Position Timing Over-Advanced (Bank 1)
P0012Camshaft Position Timing Over-Retarded (Bank 1)
P0013Exhaust Camshaft Position Actuator Circuit (Bank 1)
P0014Exhaust Camshaft Timing Over-Advanced (Bank 1)
P0016Crankshaft/Camshaft Position Correlation (Bank 1 Intake)
P0017Crankshaft/Camshaft Position Correlation (Bank 1 Exhaust)
P0087Fuel Rail/System Pressure Too Low
P0088Fuel Rail/System Pressure Too High
P0089Fuel Pressure Regulator Performance
P0183Fuel Temperature Sensor Circuit High Input
P0217Engine Coolant Over Temperature Condition
P0218Transmission Over Temperature Condition
P0380Glow Plug/Heater Circuit Malfunction
P0299Turbocharger/Supercharger Underboost Condition
P0403Exhaust Gas Recirculation Circuit Malfunction
P0405EGR Sensor A Circuit Low
P0496EVAP System High Purge Flow
P0505Idle Control System Malfunction
P0507Idle Control System RPM Higher Than Expected