How to Diagnose Check Engine Light with Smartphone

Turning your phone into an OBD2 scanner is easy. In less than five minutes, you can troubleshoot your car check the engine light yourself. You can read, clear, reset fault codes or monitor your engine performance with your smartphone with the help of an ELM327 Bluetooth OBD-II adapter.

In this article, we will show you how to use your iPhone or Android phone as a diagnostic scanner. If your check engine light is on, you will be able to know right away what the DTC code is, what it means, and even reset it in seconds.

There are many other benefits, such as checking to see if the vehicle is ready to pass the emission test.

What you will need

  • Free OBD2 software
  • Bluetooth OBD2 Adapter
    A Bluetooth ELM327 adapter is probably the only piece you are missing. These OBD2 Bluetooth adapters have flooded the market, and unfortunately, many of them are fake that fail to communicate with all vehicles.


These instructions work on all vehicles 1996 and newer, including domestic makes such as Ford, Chrysler, GM; Asian makes such as Honda, Toyota, Subaru; Europen models such as Jaguar, Land Rover, BMW, Porche, VW, BMW, and Mercedes-Benz, etc.

Plug Bluetooth Adapter into the OBD2 port

car diagnostic using your phone and a obd2 bluetooth adapter

The OBD2 port is located on the driver’s side under the dashboard. Typically it is near the hood release. Here is a couple of pictures of how the OBD2 port looks.

Turn on ignition

check engine light phone app

Turn the ignition until all the dashboard lights come on. This is position II. Don’t turn on the car yet.

Pair Bluetooth Adapter with your phone.

Check engine light with your phone

On your smartphone, go to Settings > Bluetooth and turn it on. Next, search for new devices. You will see the name of the Bluetooth adapter. It may show OBDII, Vlink, Bluetooth ELM 327, BAFX, Carly, Kobra, BlueDriver, or Scan Tool, depending on the Bluetooth device purchased.

Click on the name of the OBDII adapter and enter the pairing code. The device manufacturer should have provided the pairing code.

Enter the pairing code.

car code reader app torque plus bluetooth adapter

If they didn’t provide you with a code, try 1234 or 0000.

Scan vehicle for codes

check emission smog test status using car diagnostic app for android or iphone

Once the pairing is complete, allow the phone to connect to your vehicle’s ECU. Click on read fault codes.

If your check engine light is on, you will get a Diagnostic Trouble Code (often referred ad DTC or fault code) and a description of what it means.

You can also monitor the readiness status to see if your vehicle is ready to pass the emission/smog test.

As you can see in this picture, this vehicle is not ready for an emission test. Once you all the test change to Complete, you will take your car for emission testing and pass it.

Using your phone to diagnose your car’s Check Engine Light (CEL) and fault codes are easy and can save you money in the long run. An OBD2 Bluetooth adapter will also allow you to monitor your car sensor live and test to see if it is ready for emission testing.

Don’t expect to diagnose ABS, Airbag, or ESP light with a cheap OBD2 Bluetooth adapter. If you are buying an ELM327 adapter, you won’t connect to any other system except the engine, which is fine if you are looking to monitor the engine and clear that check engine light.

As Bluetooth technology evolves, we hope to see Bluetooth adapters be able to scan multiple vehicle systems. We are already seeing Bluetooth tools such as BlueDriver OBD-II Scan Tool being able to read and clear codes from ABS, Airbag, and transmission, but this feature works only on GM, Ford, Chrysler, Toyota, BMW, and Mini.

List of Check Engine Light Apps

There are so many Bluetooth OBD car apps for both iPhone and Android devices. We wanted to share with you Top 5 most popular auto diagnostic OBD2 apps.

OBD Car Doctor

  • Can display real-time sensor data. Can store diagnostics parameters from ECU for any OBD-II compliant car. Support data reading PID’s from cars with 2 and more ECU.

Olivia Drive | OBD2 – ELM327

  • This obd2 APP is great for monitoring the ECU. Shows the basic required parameters.

ScanMaster for ELM327

  • For all vehicle diagnostics to OBD-2/EOBD standards. Basically anything 1996 and newer.


  • Allow you to set up a dashboard, data monitor, setup gauges, check for fault codes, and reset—over 1 million installs and 4+ star rating.

Torque OBD2 Diagnostic App

  • We have to admit this is the OBD2 app our staff uses. The Lite version is free and allows you to save logs, monitor emission readiness, read, erase clear check engine light, and a lot more.

Benefits of using a Bluetooth adapter to diagnose your car?

  • Check Engine Light On 
    If your check engine lights came on, you don’t need to go to your dealer (our mechanic) and pay over $100 for a simple diagnostic that you can easily perform yourself. Even if you don’t plan on repairing yourself, it is to your benefit to know the cause of the problem. You can research the problem before you choose the repair shop that is going to fix your car. If the problem is just a loose gas cap, you can fix it yourself for free.
  • Display live sensor data
    This is a list of sensor data that you can monitor live as your engine is running.


    • Engine RPMs
    • Engine Coolant Temperature
    • Fuel System Monitoring
    • Show Vehicle Speed on your phone in case the speedometer is broken.
    • Long and Short Term Fuel Trim
    • Intake Manifold Pressure
    • Intake Air Temperature / Air Flow Rate
    • Timing Advance
    • Throttle Position
    • Oxygen O2 Sensor
  • Check Emission Readiness Status
    You don’t need to drive all the way to your inspection facility to know if your car will pass a smog test or not. You can use your phone to know exactly when to take your car for an inspection.
  • Vehicle Information
    You can retrieve the VIN stored in the ECU using your phone. Just in case it doesn’t match with the one printed on the vehicle sticker.

We hope you find the How to Diagnose Check Engine Light with Smartphone guide helpful. Check these troubleshooting and repair guides for more help on your vehicle.


  • Rushit Hila • ASE Certified

    Rushit Hila, an ASE-certified engineer (G1 Automotive Maintenance and Repair), brings over two decades of hands-on experience in the automotive world to his writing. With a strong educational background, including a Master of Science in Engineering and a Bachelor of Science in Engineering, he has honed his skills and expertise through years of practical work. As a respected authority in the field, Mr. Hila is dedicated to offering insightful and valuable content that resonates with both vehicle owners and mechanics.

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