CHECK ENGINELIGHT CODE Look up a code
THE OBD-II FIELD GUIDE

Decode your
check engine
light.

Enter the code from your scanner to see what it means and what to check first. If you haven’t scanned the car yet, start with the warning light below.

TRYP0420P0300P0171P0455
Front corner of a car with its hood open in a dark workshop
Code libraryFind the meaning of a DTC Warning lightSee what the light is telling you Try a sample scanCapture codes and freeze frame
01ON THE DASH

Steady or
flashing?

Pay attention to what the light is doing and how the car feels. A flashing light calls for a different response from one that stays on.

Understand the warning light
DASHBOARD SIMULATORDEMO / MIL
STEADY LIGHTGet the code soon.

Check your owner’s manual and arrange a scan. If the car runs badly, pull over safely.

02IN THE DRIVER’S SEAT

Read your first scan.

Try a sample scan and see what to write down before you turn the key off.

SCAN WALKTHROUGH
Sample readings for demonstration.

Start under the dash.

Park first. Look below the steering wheel for the wide, 16-slot socket. Follow your scanner’s instructions for the ignition position.

Find the OBD-II port

Copy the code and its status.

This reader shows P0171 as stored. Save every code the reader lists, including any marked pending. Even one different digit can point to a different fault.

See what P0171 means

Keep the freeze frame.

The sample shows a warm engine idling at 720 rpm. If your reader shows a freeze frame, take a picture of it before you clear anything. It gives the shop a better shot at recreating the conditions.

How to read a freeze frame
03UNDER THE HOOD

Which system set the code?

Pick a system to trace its path and see an example code from that part of the car.

SYSTEM MAP
Simplified layout. Parts sit in different places on different vehicles.

A cylinder missed a beat.

P0300 means the computer detected a random misfire or misfires across multiple cylinders. If the light is flashing while you drive, ease off and pull over safely.

Read the P0300 guide How misfire codes work

The computer had to add fuel.

With P0171, the computer added more fuel than expected on bank 1. A shop can compare fuel corrections at idle and at higher airflow to help narrow down the cause.

Read the P0171 guide How fuel trim works

The catalyst monitor saw low efficiency.

P0420 points to the catalyst on bank 1. Save any misfire or mixture codes too. They can change what a technician checks before testing the converter itself.

Read the P0420 guide How catalyst codes work

The fuel vapor system failed a leak test.

P0456 points to a very small EVAP leak. Start at the filler if the light came on after a fill-up. If the filler seals, a shop can test the rest of the vapor system.

Read the P0456 guide How the EVAP system works
A mechanic holding a handheld scan tool at the driver’s door
WORKSHOP / SCAN TOOL
04GET A SCAN

No code reader?

A parts store may be able to scan the car for free. Ask for the exact codes and take a picture of the report. You’ll have something useful to bring to the shop if the light comes back.

Find a free code scan Find the port
05THE CODE LIBRARY

Common codes to look up.

Browse all codes
06FIELD GUIDES

Get more from a scan.

Explore the field guides
01 / Getting started

How to read OBD-II codes

The job is to capture the code, its status and the conditions when it set. Do that before choosing any erase or reset option.

02 / Scan data

What freeze-frame data tells you

A freeze frame is a snapshot of some operating data captured near a fault event. Read it before erasing codes.

03 / Inspection

OBD-II readiness monitors

Readiness tells you which onboard emissions self-tests have finished since the vehicle’s OBD information was cleared.

07BEFORE YOU RESET

Questions after a scan.

Should I clear the code before I take the car in?

Save the report first. Clearing codes can erase the freeze frame and reset emissions readiness. A shop can use the original scan to see what happened. What clearing a code does

The light went off. Can I still find the code?

You might. Check for stored, pending or history codes, and write down what you were doing when the light came on. What those code statuses mean

What if the reader shows no codes?

Check which light is on and which system the reader scanned. If the tool says it couldn’t connect, save that message too. What to check on an empty scan