OBD2 trouble codes explained
Your check engine light is on and your scanner shows a code. These guides explain the most searched generic OBD2 codes in plain English: what they mean, common symptoms and causes, and how to read and clear them with your phone.
Catalyst System Efficiency Below Threshold (Bank 1)
Your engine computer has decided that the catalytic converter on bank 1 is storing and releasing less oxygen than expected, which suggests it is not converting exhaust gases efficiently.
Emissions / catalytic converterP0171System Too Lean (Bank 1)
The engine computer is adding more fuel than it is allowed to on bank 1 and still sees a lean mixture, meaning there is too much air or too little fuel.
Fuel and air meteringP0300Random/Multiple Cylinder Misfire Detected
The engine computer has detected misfires that are not limited to one cylinder, or that move between cylinders.
Ignition / misfireP0456Evaporative Emission System Leak Detected (Very Small Leak)
During a self test of the fuel vapor (EVAP) system, the engine computer found a very small leak, often in the range of a pinhole.
Evaporative emissions (EVAP)P0430Catalyst System Efficiency Below Threshold (Bank 2)
The engine computer has found that the catalytic converter on bank 2 is not storing and releasing oxygen as well as expected.
Emissions / catalytic converterP0455Evaporative Emission System Leak Detected (Large Leak)
The engine computer could not hold pressure or vacuum in the fuel vapor (EVAP) system during its self test, which indicates a large leak or an opening in the system.
Evaporative emissions (EVAP)P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
The engine did not reach its expected operating temperature within the time the engine computer allows, which usually points to the cooling system rather than an emergency.
Engine coolingP0299Turbocharger/Supercharger "A" Underboost Condition
The engine computer requested more boost pressure than the turbocharger or supercharger delivered, for longer than it allows.
Turbocharger / superchargerP0301Cylinder 1 Misfire Detected
The engine computer has detected that cylinder 1 is not firing properly often enough to count as a misfire.
Ignition / misfireP0340Camshaft Position Sensor "A" Circuit (Bank 1 or Single Sensor)
The engine computer is not getting a valid signal from camshaft position sensor A, or the signal is out of the expected range.
Engine timing sensorsHow to read an OBD2 code
First letter: system
P is powertrain (engine and transmission), B is body, C is chassis and U is network communication.
Second digit: who defined it
0 means a generic SAE code with the same meaning on every OBD2 vehicle. 1 usually means a manufacturer specific code.
Last digits: the fault
The third digit points to a subsystem, such as fuel and air metering, ignition or emissions controls, and the last two identify the exact fault.
A code tells you which self test failed, not which part to replace. The same code can have different causes on different vehicles, so check the details before buying parts.
Read your codes with OBD Master Pro
Use your Android phone with a generic ELM327 adapter over Bluetooth, Bluetooth LE, Wi-Fi or USB. Read stored, pending and permanent codes, check freeze frame and live data, and clear the check engine light after a repair.
