This product works as a data bridge between a car's OBD-II port and Arduino with dedicated Arduino library provided. It provides fast OBD-II data access, as well as 6-axis MEMS motion sensor module and voltmeter for measuring car battery voltage. The adapter draws power from OBD-II port and convert it to 5V (up to 2.1A) for powering itself and attached device (Arduino board, shield or other components). 


  • Access to all standard OBD-II PIDs with extended ELM327 AT command-set
  • Reading and clearing vehicle diagnostic trouble codes (engine & powertrain only)
  • Measuring car battery voltage
  • Built-in MPU-6050 6-DOF motion sensor (accelerometer, gyroscope & temperature)
  • Supplying power for host device (5V up to 2.1A)
  • Serial UART data interface compatible with both 3.3V and 5V micro-controllers
  • Micro USB port for computer/tablet OBD-II access and firmware upgrade
  • Arduino library and example sketches available
  • Low power mode @6mA


The adapter plugs into the OBD port usually located under the steering column or slightly to the left of it. To check if your vehicle is OBD-II certified, open your hood and find the sticker that looks like this:


Vehicles using following vehicle protocols are supported.

  • CAN 500Kbps/29bit
  • CAN 500Kbps/11bit
  • CAN 250Kbps/29bit
  • CAN 250Kbps/11bit
  • KWP2000 Fast
  • KWP2000 5Kbps
  • ISO9141-2

The OBD-II Adapter is compatible with all Arduino range including Arduino UNO, Arduino Leonardo, Arduino Micro, Arduino Nano, Arduino Mini, Arduino Pro Mini, Arduino MEGA 1280/2560/ADK.



The adapter stays plugged into the OBD port usually located under the steering column or slightly to the left of it.  A cable comes out from the adapter and splits into one 4-pin connector two 2-pin connectors, including power lines (VCC/GND) and data lines (Rx/Tx). They can be connected to Arduino with onboard breakout pins or breakout shield. Your Arduino device will look tidy in car with only one cable.

Power Connector (2-pin 2.54 Dupont connector)

  • Red: VCC (connecting to Arduino’s VCC)
  • Black: GND (connecting to Arduino’s GND)

Serial UART Data Connector (2-pin 2.54 Dupont connector)

  • White: Rx (connected to Arduino’s serial Tx)
  • Green: Tx (connected to Arduino’s serial Rx)

USB Port

  • For connection with computer or tablet via micro USB cable
  • For power output (5V/2A)


Extended ELM327 Command Set

We have extended standard ELM327 command set for following purposes:

  • Multiple (up to 8) OBD-II PIDs reading from a single query
  • Accessing built-in MEMS module (MPU-6050)
  • Accessing built-in voltmeter (measuring battery voltage)

Following are the added commands:


  • Function: reading accelerometer data
  • Response: X/Y/Z values from accelerometer in format of X,Y,Z


  • Function: reading gyroscope data
  • Response: X/Y/Z values in the format X,Y,Z


  • Function: reading temperature data
  • Response: temperature (raw) data


  • Function: reading car battery voltage
  • Response: voltage value

The Arduino library has API implemented with all the extended commands.



A dedicated Arduino library is provided for easy access to all the features with any type of Arduino.

Some commonly used APIs are like following:

  • setBaudRate - set adapter serial baudrate
  • readPID - read specified OBD-II PID and return parsed value
  • clearDTC - clear diagnostic trouble code
  • getVoltage - measure car battery voltage
  • getVIN - retrieve Vehicle Identification Number
  • getTemperature - get device temperature
  • readAccel - read accelerometer X/Y/Z values
  • readGyro - read gyroscope X/Y/Z values

There are more extensive APIs experts. Please refer to the library header file for more information.

Here is an example Arduino sketch of a simplest engine RPM indicator, which uses the pin 13 LED (built in on Arduino board) to indicate whether the engine is above 3000 rpm.

#include <OBD2UART.h>

COBD obd;

void setup()
  // we'll use the debug LED as output
  pinMode(13, OUTPUT);
  // start communication with OBD-II adapter
  // initiate OBD-II connection until success
  while (!obd.init());

void loop()
  int value;
  // save engine RPM in variable 'value', return true on success
  if (obd.readPID(PID_RPM, value)) {
    // light on LED on Arduino board when the RPM exceeds 3000
    digitalWrite(13, value > 3000 ? HIGH : LOW);

A more comprehensive example sketch can be found here.

Some commonly used PIDs are defined in OBD library as following.


  • PID_RPM – Engine RPM (rpm)
  • PID_ENGINE_LOAD – Calculated engine load (%)
  • PID_COOLANT_TEMP – Engine coolant temperature (°C)
  • PID_ENGINE_LOAD – Calculated Engine load (%)
  • PID_ABSOLUTE_ENGINE_LOAD – Absolute Engine load (%)
  • PID_TIMING_ADVANCE – Ignition timing advance (°)
  • PID_ENGINE_OIL_TEMP – Engine oil temperature (°C)
  • PID_ENGINE_TORQUE_PERCENTAGE – Engine torque percentage (%)
  • PID_ENGINE_REF_TORQUE – Engine reference torque (Nm)


  • PID_INTAKE_TEMP – Intake temperature (°C)
  • PID_INTAKE_PRESSURE – Intake manifold absolute pressure (kPa)
  • PID_MAF_FLOW – MAF flow pressure (grams/s)
  • PID_BAROMETRIC – Barometric pressure (kPa)


  • PID_SPEED – Vehicle speed (km/h)
  • PID_RUNTIME – Engine running time (second)
  • PID_DISTANCE – Vehicle running distance (km)


  • PID_THROTTLE – Throttle position (%)
  • PID_AMBIENT_TEMP – Ambient temperature (°C)

Electric Systems

  • PID_CONTROL_MODULE_VOLTAGE – vehicle control module voltage (V)
  • PID_HYBRID_BATTERY_PERCENTAGE – Hybrid battery pack remaining life (%)

Additional defines can be added to access all OBD-II PIDs which the car’s ECU provides.



The following table lists the differences among different Freematics OBD-II adapter models.

Features \ Models OBD-II I2C Adapter OBD-II UART Adapter V1 OBD-II UART Adapter V2
Connection Interface I2C Serial UART Serial UART
Additional Interface N/A N/A microUSB
MEMS Sensor MPU-6050 N/A MPU-6050
Voltmeter Yes Yes Yes
Max. Output Power 2A 2A 2.1A
Standby Mode Power 10mA 5mA 6mA



Q: What is this product used for?
A: The most straight-forward use of this product is for making Arduino possible to access vehicle data easily. The OBD-II data, together with other data from GPS or all kinds sensors, can be logged and stored on SD/TF card with Arduino and that makes an open-source vehicle data logger (check out the data logger kits). More extensively, many interesting interaction applications requiring car data can be made.

Q: How is the adapter powered?
A: The adapter gets power from the 12V DC output from the OBD-II port.

Q: Does my Arduino needs power from somewhere in the car?
A: The adapter provides regulated 5V output for powering Arduino and other devices, so no extra power cord is needed.

Q: Do I need a CAN bus shield to use with the adapter?
A: Definitely no. The adapter retrieves data from CAN bus, like a CAN bus shield does and convert the more complicated CAN bus interface to simple serial UART interface which Arduino and most embedded systems are easy to access. The data connection is provided by adapter’s data connector (Rx and Tx).

Q: How do I connect the adapter with my Arduino?
A: The adapter works with all models of Arduino with the dedicated Arduino library and is connected with Arduino by connecting adapter’s Tx to Arduino’s Rx (D0) and adapter’s Rx to Arduino’s Tx (D1). If you want to connect and disconnect the adapter with your Arduino effortlessly, it’s recommended to use a common I/O breakout shield or use an Arduino board with breakout pins for Rx/Tx/VCC/GND.

Q: Is the power provided by the adapter always available in car?
A: This depends on whether the OBD-II port of your car still has power after ignition is off. Actually it is so with most cars.

Q: What’s the maximum frequency of data polling?
A: The OBD-II PIDs are polled one after another. The time for a polling depends on the speed of car’s ECU computer and how busy the computer is in different status. With a typical modern car with CAN bus, the time can be as low as 10ms. In other word, up to 100 times of data polling can be done in one second.