// Check engine light

Using a code reader on a check engine light

A check engine light is the car asking you to look. A code reader tells you where. Here is how to read it properly, what to do with the code, and the one mistake that makes a small problem harder to find.

Step by step

  1. Find the port. The OBD2 port is a 16-pin socket, usually under the dashboard on the driver's side.
  2. Plug in with the engine off, then turn the ignition on. Handheld readers take power from the port; there's nothing to charge.
  3. Read the codes. Note every code, not just the first.
  4. Save the freeze frame. It's a snapshot of engine conditions at the moment the fault was stored: speed, load, temperature and more. It's the most useful clue you'll get.
  5. Check live data if the reader supports it, for readings that look out of place.
  6. Look the code up, then decide on a fix before clearing anything.
Don't clear first. Clearing turns the light off and erases the code and freeze frame, but the fault is still there. It also resets the emissions readiness monitors, so the car can fail an inspection until they've run again. Read, record, fix, and only then clear.

Reading the code

OBD2 codes have five characters, like P0301.

PartMeaning
First letterP powertrain, B body, C chassis, U network
First digit0 generic (same meaning on every car), 1 manufacturer-specific
Last threeThe system and the specific fault

P0301, for example, means a misfire detected on cylinder 1. A code tells you what the computer noticed, not always what broke. A misfire code can come from a spark plug, a coil, an injector or a mechanical problem, and the freeze frame and live data help narrow it down.

Solid or flashing?

Common first codes

Vacuum and evaporative leaks that won't show themselves are what a smoke machine is for; see the smoke machine guide.

When a code reader isn't enough

A code reader covers engine and emissions. If the warning light on your dash is ABS, airbag, traction control or transmission, a basic reader usually can't reach that module. That's the job of a full-system scanner; see the Bluetooth scanner guide or the buyer's guide. And if you want to command components to test them, see the bidirectional guide.

The two readers in our shop

ReaderWhat it doesPrice
V519 HandheldColour screen; reads and clears codes, live data, I/M readiness€39
ANCEL AD310Backlit monochrome screen; codes, live data, freeze frame, I/M readiness€39

Both plug in and work without a phone, an app or a battery. If you'd rather use your phone, a Bluetooth adapter does the same job; see the Bluetooth guide above.

Questions people actually ask

Can I drive with the check engine light on?
If it's solid and the car drives normally, usually yes for a short time while you find the cause. If it's flashing, or the car runs badly, reduce load and get it checked promptly.
Will the light come back after clearing?
If the fault is still there, yes. Some faults need a few drive cycles to be detected again, so the light may return days later.
Does my car have OBD2?
Petrol cars sold in the US from 1996 and in Europe from 2001, and European diesels from 2004, generally do. Look for the 16-pin port under the dashboard.
Is a code reader enough for an inspection?
It can show whether the readiness monitors are complete, which many inspections check. It won't fix a failed monitor; the car needs to complete its drive cycles.

Code readers in the shop.

V519 and ANCEL AD310, €39 each. Shipped free, 2-year warranty.

See them in the shop → Which tool do I need?