Guide
How to Read OBD2 Codes
When the check-engine light comes on, an OBD2 scanner tells you why. Here’s how to read the codes yourself — and how the CarsJudge AI app explains them in plain English so you know whether it’s urgent.
What OBD2 is
Every car sold in the US since 1996 (and most worldwide since the early 2000s) has an OBD2 port — a standardized connector that reports engine and emissions data. A scanner or Bluetooth adapter plugs in and reads stored trouble codes.
Where the port is
It’s almost always under the dashboard on the driver’s side, near the steering column. Plug in the scanner with the ignition on.
What the codes mean
Codes start with a letter and four digits: P = powertrain (engine/transmission), B = body, C = chassis, U = network. A leading P0 is a generic code; P1 is manufacturer-specific.
- P0300: random/multiple cylinder misfire.
- P0420: catalytic converter efficiency below threshold.
- P0171: system running too lean.
Clearing codes — carefully
You can clear codes after a repair, but clearing them does not fix the underlying problem — if the fault remains, the light returns. Never clear codes just to pass an inspection. Diagnose first.
Frequently asked questions
Where is the OBD2 port located?
On almost all cars the OBD2 port is under the dashboard on the driver’s side, near the steering column. Turn the ignition on, plug in the scanner, and read the codes.
What does a P0420 code mean?
P0420 means the catalytic converter’s efficiency is below the threshold the car expects. It can be caused by a failing converter, oxygen sensors, or exhaust leaks — diagnose before replacing parts.
Is it safe to clear OBD2 codes?
Clearing a code is safe after you’ve fixed the cause, but it does not repair anything. If the fault is still present, the check-engine light will come back. Diagnose the issue first.