OBD2 Codes Lookup — Decode Every Check Engine Light Code
Look up obd2 codes to learn what they mean, what usually causes them, and what to check next — free, in plain English.
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Popular OBD2 Codes
These check engine light codes turn up in repair shops every single day. Here is what each one means — and what usually causes it.
P0420
Catalyst efficiency below threshold. Your car’s computer thinks the catalytic converter is not cleaning exhaust gases properly. Usual suspects: a worn-out catalytic converter or a failing oxygen sensor.
P0171
System too lean (Bank 1). There is too much air — or too little fuel — in the mixture on bank 1. Common causes: a vacuum leak or a dirty mass airflow sensor.
P0300
Random or multiple cylinder misfire. The engine is misfiring on different cylinders instead of just one. Often worn spark plugs, failing ignition coils, or low fuel pressure.
P0456
EVAP system very small leak. A tiny leak in the fuel-vapor (evaporative emissions) system. Frequently it is just a loose, cracked, or missing gas cap.
Browse OBD2 Codes by System
Every obd2 code belongs to a vehicle system. Pick a system to narrow down where your fault lives.
Fuel & Air
Fuel delivery and air-metering faults, like lean or rich mixture codes.
Misfire & Ignition
Misfire and ignition faults, from single-cylinder to random misfires.
Emissions
Catalytic converter, oxygen sensor, and EVAP system codes.
Idle & Speed
Idle-control and vehicle speed sensor codes.
Computer / ECM
Internal computer faults and communication-circuit codes.
Transmission
Gearbox, shift-control, and torque-converter codes.
Diesel & DPF
Diesel particulate filter and diesel-emissions codes.
How OBD2 Codes Work
OBD2 (On-Board Diagnostics II) is your car’s built-in self-diagnosis system — standard on cars sold since 1996. It watches engine and emissions sensors around the clock and reports problems as diagnostic trouble codes. Here is the whole process in three steps.
1. Your car watches itself
The engine control unit (ECU) — the car’s computer — constantly monitors its sensors: fuel mix, airflow, RPM, and temperature.
2. A fault becomes a code
When a sensor reading drifts outside its normal range, the ECU stores a five-character trouble code and switches on the check engine light.
3. You read it with a scanner
Plug an obd2 scanner into the 16-pin port under the driver’s-side dash to pull the code — then look it up here in plain English.
How to read a 5-character code
Every diagnostic trouble code has five characters, and each one pinpoints the problem. The first letter names the system: P for powertrain (engine and transmission), B for body, C for chassis, U for network communication. The second digit tells you whether the code is generic (0 — the same for every brand) or manufacturer-specific (1). The third digit narrows down the subsystem, such as fuel and air, ignition and misfires, or emissions. The last two digits identify the exact fault.
Frequently Asked Questions
What is OBD2?
OBD2 (On-Board Diagnostics II) is your car’s built-in self-diagnosis system, standard on cars sold since 1996. It monitors engine and emissions sensors and reports problems as five-character diagnostic trouble codes.
How do I read my check engine light codes?
Plug an obd2 scanner into the 16-pin diagnostic port under the driver’s-side dashboard. The scanner displays the code — and many auto parts stores will read it for free.
What does a flashing check engine light mean?
A steady light usually means a non-urgent issue to check soon. A flashing light signals a severe misfire that can damage the catalytic converter — ease off the gas and get it checked right away.
Can an OBD2 scanner turn the check engine light off?
Yes. After the repair, a scanner can clear the code and reset the light. If the fault is still present, the light will come back.
Does the code tell me exactly which part to replace?
Not exactly. A code points to the troubled system — for example, P0420 flags catalytic-converter efficiency, but the real cause is often the oxygen sensor. Always diagnose before replacing parts.