OBD-II field guides
Learn what a scanner can show you, what to save before clearing codes and how to read the evidence after a repair.
Meet a trouble code.
Start with the first character. The full five-character code identifies the condition the module reported.
Read the code formatPowertrain codes can cover the engine, transmission, fuel system and emissions equipment.
Pick up where you are.
Flashing check engine light
Ease off the accelerator and pull over safely. A flashing check engine light while driving can signal a misfire severe enough to damage the catalytic converter.
02 / Getting startedHow 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.
03 / Scan dataWhat 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.
04 / Scan dataPending, stored and permanent codes
A code’s status tells you how the vehicle recorded the fault. Write down the status your scanner shows alongside the code.
05 / InspectionOBD-II readiness monitors
Readiness tells you which onboard emissions self-tests have finished since the vehicle’s OBD information was cleared.
06 / Getting startedWhat happens when you clear OBD-II codes?
A clear command can erase diagnostic evidence and reset emissions readiness. Save the complete scan and fix the underlying issue before using it.
07 / Warning lightCheck engine light on with no codes
Start by confirming the warning symbol and which systems your reader actually scanned.
08 / Getting startedHow to read an OBD-II code format
A five-character trouble code points you toward a system and a defined fault condition. Copy every character exactly.
09 / Getting startedWhere is the OBD-II port?
Look under the driver’s side of the dashboard first. The OBD-II port is a wide, slightly tapered socket with two rows of eight slots. It’s where your code reader plugs in.
010 / Getting startedWhere to get a free check engine light scan
AutoZone, O’Reilly Auto Parts and most Advance Auto Parts stores offer free check engine code scans in the US. Call the store with your year, make and model before you go.
011 / Getting startedCan you check engine codes without a scanner?
On most 1996 and newer cars, you need a reader plugged into the OBD-II port to see the actual code. You can still learn something from the dashboard message or the car’s app. A free parts-store scan is the quickest way to get the code itself.
012 / Warning lightCheck engine light came on after getting gas
Check the fuel cap first. On many cars, a cap that’s loose, missing or not clicked shut can turn on a steady check engine light. Expect the light to stay on for a few drives after you fix it.