OBD-II Fault Codes
Pick your code for a mechanic-grade explanation — causes ranked by probability, whether you can keep driving, and repair costs.
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Emissions System
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)P0430 — Catalyst System Efficiency Below Threshold (Bank 2)P0135 — O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)P0141 — O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)P0130 — O2 Sensor Circuit Malfunction (Bank 1, Sensor 1)P2195 — O2 Sensor Signal Stuck Lean (Bank 1, Sensor 1)P0131 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)P0137 — O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)P0133 — O2 Sensor Slow Response (Bank 1, Sensor 1)P0155 — O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)P0160 — O2 Sensor Circuit No Activity Detected (Bank 2, Sensor 2)
Fuel & Air System
P0171 — System Too Lean (Bank 1)P0174 — System Too Lean (Bank 2)P0101 — Mass Air Flow Sensor Range/PerformanceP2187 — System Too Lean at Idle (Bank 1)P2189 — System Too Lean at Idle (Bank 2)P2293 — Fuel Pressure Regulator 2 PerformanceP0113 — Intake Air Temperature Sensor 1 Circuit High (Bank 1)P0506 — Idle Air Control System RPM Lower Than ExpectedP0507 — Idle Air Control System RPM Higher Than ExpectedP0172 — System Too Rich (Bank 1)P0102 — Mass Air Flow (MAF) Circuit Low InputP0299 — Turbocharger / Supercharger "A" UnderboostP0087 — Fuel Rail / System Pressure Too LowP0103 — Mass Air Flow (MAF) Circuit High InputP0112 — Intake Air Temperature Sensor 1 Circuit Low (Bank 1)P0234 — Turbocharger / Supercharger "A" Overboost ConditionP0106 — MAP / Barometric Pressure Circuit Range/PerformanceP0107 — MAP / Barometric Pressure Circuit Low InputP0108 — MAP / Barometric Pressure Circuit High InputP0121 — Throttle / Pedal Position Sensor "A" Circuit Range/PerformanceP0122 — Throttle / Pedal Position Sensor "A" Circuit Low InputP0123 — Throttle / Pedal Position Sensor "A" Circuit High InputP0175 — System Too Rich (Bank 2)
Ignition System
P0300 — Random/Multiple Cylinder Misfire DetectedP0301 — Cylinder 1 Misfire DetectedP0340 — Camshaft Position Sensor Circuit MalfunctionP0335 — Crankshaft Position Sensor Circuit MalfunctionP0011 — "A" Camshaft Position — Timing Over-Advanced (Bank 1)P0014 — "B" Camshaft Position — Timing Over-Advanced (Bank 1)P0325 — Knock Sensor Circuit Malfunction (Bank 1)P0021 — "A" Camshaft Position — Timing Over-Advanced (Bank 2)P0012 — "A" Camshaft Position — Timing Over-Retarded (Bank 1)P0341 — Camshaft Position Sensor "A" Circuit Range/Performance (Bank 1)P0302 — Cylinder 2 Misfire DetectedP0303 — Cylinder 3 Misfire DetectedP0304 — Cylinder 4 Misfire DetectedP0327 — Knock Sensor 1 Circuit Low (Bank 1)
EVAP System
P0455 — EVAP System Leak Detected (Large Leak)P0442 — EVAP System Leak Detected (Small Leak)P0440 — EVAP System Malfunction (General)P0456 — EVAP System Leak Detected (Very Small Leak)P0446 — EVAP System Vent Control Circuit MalfunctionP0449 — EVAP System Vent Valve / Solenoid Circuit MalfunctionP0443 — EVAP Purge Control Valve Circuit MalfunctionP0444 — EVAP Purge Control Valve Circuit Open
EGR System
Cooling System
P0128 — Coolant Thermostat (Below Regulating Temperature)P0118 — Engine Coolant Temperature Sensor Circuit HighP0117 — Engine Coolant Temperature Sensor Circuit LowP0116 — Engine Coolant Temperature Sensor Circuit Range/PerformanceP0125 — Insufficient Coolant Temperature for Closed-Loop Fuel Control
Transmission
Network & Communication
Engine
Charging & Electrical
Engine Control Module
Emissions System (EVAP)
Ignition / Crankshaft Sensor
More codes are published regularly, prioritized by real-world fault frequency.