Read Your Check Engine Light With Your Phone and an OBD2 Adapter
A small plug-in adapter and a free app let your phone read the code behind your check engine light. Here is what you need, what it costs, and how to read codes safely.
The check engine light comes on, and suddenly you don't know whether it's a loose fuel cap or something serious. A garage can read the reason in a minute, but you can often do it yourself in your own driveway. You need two things: your phone and a small OBD2 adapter that plugs into the car.
To be clear from the start, a phone cannot talk to your car on its own. The adapter is what connects the two. This page explains what OBD2 is, which adapter to buy and what it roughly costs; the second page shows you how to read the code, what it means, and when to clear it.
Open the step-by-step code guide
What the check engine light is really telling you
Your car's engine computer constantly checks sensors on the engine, fuel system and exhaust. When a reading stays outside its expected range, the computer stores a diagnostic trouble code and switches on the light. The code is a short label, such as P0455, that points to the system where the problem was detected.
How the light behaves matters. A steady light usually means a fault that should be checked soon but is not an emergency. A flashing light usually means the engine is misfiring badly, which can overheat and damage the catalytic converter, so ease off and get it looked at as soon as you can.
What OBD2 is, and whether your car has it
OBD2 (on-board diagnostics, second generation) is the standard way cars report their health. It defines a 16-pin socket in the cabin and a shared set of codes, so any compatible tool can read them.
- United States: cars and light trucks from the 1996 model year onward use OBD2.
- Europe: the equivalent standard, EOBD, applies to petrol cars from about 2001 and diesel cars from about 2004.
- Other regions: rules differ, so check your owner's manual or search for your model and "OBD2".
The socket is usually under the dashboard on the driver's side, near the steering column. On some cars it sits behind a small cover or near the centre console.
The adapter you need, and what it roughly costs
An OBD2 adapter is a small plug that fits into the socket and sends data to your phone. Most work with apps that expect an ELM327-compatible adapter, and they connect in one of three ways: classic Bluetooth, Bluetooth Low Energy (LE) or Wi-Fi.
Prices vary by store, country and brand, so treat these as rough ranges. Basic generic adapters are often sold for well under $20, while better-known brands usually cost more, from around $20 to over $100 for premium models with extra features.
If you use an iPhone, or might switch to one later, choose a Bluetooth LE or Wi-Fi adapter. Classic Bluetooth OBD2 adapters generally don't work with iPhones.
What your phone can and cannot tell you
With an adapter and an app, your phone can show the stored code, a plain description of it, and live sensor readings. That is often enough to tell a loose fuel cap from a failing part, and to walk into a garage knowing what to ask.
It cannot tell you exactly which part to replace. A code says where the computer noticed a problem, not always why. The same misfire code, for example, can come from a worn spark plug, a failing coil or a fuel problem. The code is the start of a diagnosis, not the end of one.
On the next page, you will connect the adapter and read your code step by step.