Car & driving · Free
Read check engine light codes for free
Check engine light on? Read the fault codes yourself for free – with an OBD2 adapter and this page, no app needed. Every code comes with a plain explanation and its usual causes, and you can clear it after the repair.
- No account
- Runs on your device
- Phone & desktop
Read your fault codes
Runs on your device
Connect your OBD2 adapter
- Plug the adapter into the OBD2 port – usually under the steering wheel, near the pedals.
- Switch the ignition on, or start the engine.
- Bluetooth adapter? Turn Bluetooth on. Nothing else to set up.
This browser cannot talk to OBD adapters. Use Chrome or Edge on Android, Windows or Mac – on iPhone, the free Bluefy browser works with Bluetooth LE adapters. The demo car works everywhere.
Wi-Fi adapters cannot work in a browser; Bluetooth LE, classic Bluetooth (on a computer) and USB adapters can.
Your car data stays on your device. The page talks to the adapter directly; codes, VIN and values are never uploaded.
Never use it while you drive. Set things up before you set off, or let a passenger handle the phone. Standard OBD2 reads engine and emissions data; ABS, airbag and coding need workshop equipment.
Read check engine light codes in three steps
Adapter in, ignition on
Plug an OBD2 adapter into the port under the dashboard and switch the ignition on.
Connect and scan
Tap Connect, pick your adapter, and OBD Studio reads stored, pending and permanent codes on its own.
Understand and act
Each code shows what it means, which system it belongs to and what usually causes it – so you know whether to drive on, fix it yourself or see a workshop.
Which OBD2 adapters work
Any ELM327-compatible adapter works, as long as the browser can reach it:
| Adapter | Phone | Computer |
|---|---|---|
| Bluetooth LE (e.g. Vgate iCar Pro BLE, vLinker MC+, OBDLink CX, many ELM327 BLE) | Chrome or Edge on Android; Bluefy on iPhone | Chrome or Edge on Windows, Mac, Linux |
| Classic Bluetooth (the common ELM327 with PIN 1234) | not possible in any browser | Chrome or Edge, after pairing it in the system settings |
| USB (ELM327 USB, OBDLink SX) | – | Chrome or Edge |
| Wi-Fi | not possible in a browser | not possible in a browser |
Buying one? A Bluetooth LE adapter works with phones and computers alike. The demo car lets you try everything before you buy.
Where is the OBD2 port?
Almost always within reach of the driver’s seat: under the steering column, above the pedals or behind a flap in the footwell; in some cars under the ashtray or behind the centre console. Every petrol car sold in the EU since 2001 and every diesel since 2004 has one, as do all cars sold in the US since 1996.
What the check engine light is telling you
A steady light means the engine control unit has stored a confirmed fault – drive on carefully and read the code soon. A flashing light means misfires that can damage the catalytic converter: reduce speed and have it checked. Pending codes show faults seen once; they turn into stored codes if they come back.
Common fault codes in plain words
- P0420 / P0430 – catalytic converter efficiency below threshold. Often the converter, but check the rear O2 sensor and exhaust leaks first.
- P0300–P0312 – misfires (random or per cylinder): spark plugs, ignition coils or injectors.
- P0171 / P0174 – mixture too lean: vacuum leaks, a dirty air flow sensor or weak fuel pressure.
- P0128 – engine warms up too slowly, usually a thermostat stuck open.
- P0442 / P0455 / P0457 – fuel vapour leak; start with the fuel cap.
- P0562 – system voltage low: battery or alternator.
OBD Studio explains every generic code it finds and gives the usual causes for the frequent ones.
Every fault code guide
- P0420Catalyst efficiency below threshold
- P0171System too lean
- P0300Random or multiple misfire
- P0455EVAP system large leak
- P0442EVAP system small leak
- P0128Engine warms up too slowly
- P0172System too rich
- P0401EGR flow too low
- P0101Air flow sensor implausible
- P0135Front O2 sensor heater
- P0016Crankshaft and camshaft out of step
- P0011Intake camshaft timing over-advanced
- P0141Rear O2 sensor heater
- P0113Intake air temperature sensor high
- P0457Fuel cap loose or missing
- P0299Turbo underboost
- P0700Transmission control system fault
- P0087Fuel rail pressure too low
- P0562System voltage low
- U0100Lost communication with engine computer
Should you clear fault codes?
Clearing turns the light off and erases the freeze frame, but it does not repair anything: if the fault remains, the code returns. It also restarts all readiness self-tests, so clear codes after a repair – not right before an inspection.
Readiness before an emissions inspection
Testers (MOT, smog check, TÜV/AU, contrôle technique) read the readiness monitors: the car’s own self-tests for catalyst, O2 sensors, EVAP and more. After codes were cleared or the battery was disconnected, they need a few drives to complete. The Inspection tab shows each monitor and whether the car looks ready.
Live data that helps you find faults
Fuel trims show whether the engine runs lean or rich, coolant temperature reveals a lazy thermostat, battery voltage tells you if the alternator charges, and O2 sensor graphs show a tired sensor. Record any values as a CSV file or measure 0–100 km/h and fuel use on the way.
What standard OBD2 can and cannot read
OBD2 is the legally required emissions interface: engine and emissions faults, live engine values, readiness and vehicle data are open to every scanner. Modules like ABS, airbags or comfort electronics use manufacturer protocols that need brand-specific workshop tools; OBD Studio does not write to any control unit.
More OBD Studio pages
Frequently asked questions
How do I read check engine light codes without a scanner?
You still need a small OBD2 adapter (Bluetooth LE or USB), but no scan tool and no app: this page turns your phone or computer into the scanner and explains every code.
What does a P0 code mean?
P0 codes are generic powertrain codes with the same meaning in every car. P1 codes are manufacturer codes whose meaning depends on the brand; OBD Studio says which kind you have.
Is OBD Studio really free?
Yes. Every function – fault codes, clearing, live data, inspection readiness, self-tests, timer, trip computer, logging and the report – is free, with no account, no subscription and no ads inside the tool. You only need an adapter.
Which OBD2 adapter do I need?
An ELM327-compatible adapter with Bluetooth LE works on Android phones and computers; classic Bluetooth and USB adapters work on computers. Wi-Fi adapters cannot be used from a browser.
Does it work on iPhone?
Safari cannot talk to Bluetooth devices. On iPhone, open OBD Studio in the free Bluefy browser and use a Bluetooth LE adapter. The demo car works in every browser.
Can I turn off the check engine light?
Yes: Clear codes removes stored and pending codes and switches the light off. If the fault is still there, the light comes back – and the readiness tests start over.
Is it safe for my car?
OBD Studio only reads standard OBD2 data. The one thing it changes is clearing codes, and only when you confirm it. It never writes or codes control units.
Which cars are supported?
Every car with an OBD2 port: petrol cars from 2001 and diesels from 2004 in the EU, all cars from 1996 in the US, and most cars worldwide since. The protocol is found automatically.
Do I need internet in the car?
Only to load the page. The connection to the adapter is Bluetooth or USB, so mobile data is enough and the scan keeps working if the signal drops.
