ELM327 Review & About ELM 327 OBD2 Interface
Posted by Alex E. on 01 October 2014 04:14 PM


Presentation of ELM interfaces

There are several types of interface. The most common use a chip made by ELM Electronics.

ELM327 is the most widely used and practical, supports all OBD protocols. IE: KWP, PWM, VPN and CAN.

The other interfaces only support one protocol. For example, ELM320 only supports PWM. And ELM322 supports VPN and the ELM323 KWP.

Right now, there are 4 types of ELM327 interface:

  • ELM327 RS232 (RS or Series): This type of output is gradually disappearing from modern PCs. It is the cheapest.
  • ELM327 USB: A bit more expensive that the RS and requires installation of a USB driver (FTDI or Silicon Labs). Advantages: all PCs are equipped with a USB port.
  • ELM327 Bluetooth: It has the advantage of being wireless and can be used with a computer or a smartphone.
  • ELM327 WiFi: Its wireless connection means it can be used with a computer or a smartphone. Only the WiFi interfaces can be used with the iPhone/iPad. It's the most expensive one.


Below for illustration, is a photo of the 4 types of interface using this integrated circuit (electronic chip):

4 types of ELM327
From left to right: a metal box USB interface, a plastic box, a Bluetooth version and the latest WiFi version


elm327 interface compare comparison


Despite appearances, these 4 interfaces are practically identical, at least electronically. Only their external aspect and the connection type differ. At their heart is an ELM327 circuit.

Another thing which may be confusing is the "firmware" version. That may vary.

The official versions currently in circulation are 1.3a and 1.4b, 2.1

Online, you will also find common version 1.5. We have bought one and can confirm that it doesn't exist! It is only a copy of version 1.2 masquarading as 1.5. Check out tips and advice section for how to know which ELM version you have.


Where can I buy an ELM interface?


Get genuine ELM327 USB / Bluetooth / WIFI from our store.

Instructions for the ELM327 interfaces

1. Installing the drivers:

2. Description of LED indicators:

  • Power (red LED): Indicates that the interface is powered on. This indicator must be fully lit for the interface to be correctly powered.
  • Tx OBD (yellow LED): Flashes when data are sent from the interface to the vehicle.
  • Rx OBD (green LED): Flashes when data are sent from the vehicle to the interface.
  • Tx USB (yellow LED): Flashes when data are sent from the PC to the interface.
  • Rx USB (green LED): Flashes when data are sent from the interface to the PC.

Tip: On some interfaces, there are no labels on the indicators. In this case, when you power on, the red LED lights first followed briefly by: 1. Tx OBD, 2. Rx OBD, 3. Tx PC, 4. Rx PC.


Operating principle of the ELM327 interface


The electronic unit contains 3 main sub-assemblies to operate the interface:

  1. Electrical voltage adaptors: On-board networks in cars have voltage levels that require specific drivers. For an ELM327, as the device supports several protocols, several drivers are necessary. These protocols can be classified into 3 families, CAN networks, K/L lines and PWM/VPN. For more details about these protocols, see our page communication standard.

  2. The ELM327 chip: The integrated circuit, which gives its name to the whole device, is the brains of the interface. It selects the protocol and converts it into one understood by modern PCs. This small circuit (speed 4Mhz) acts as a bridge between the protocols.

  3. Voltage adaptors for the PC: The electronic chip is not in itself able to communication with a computer; it needs to adapt the electrical levels before sending data streams.


The diagram below shows the main function of an interface.

Overview ELM327 operation


After this brief presentation, the term "interface" should mean more. The role of the ELM327 is to act as an interface between 2 different environments: the car, governed by standards with a high level of reliability and a low price, and the IT world more focussed on modularity.


Tips and advice for using ELM327 interfaces

How do I know which software version it is?

The interfaces do not always have the same level of quality. As we mentioned above, the software version displayed is not always reliable.

Some versions of the circuit are on sale that are not even listed by the manufacturer (reminder: ELM Electronics). User returns have taught us that some versions, that do not officially exist, are in circulation.

To clarify the situation:

  • EXIST: versions 1.0 1.1 1.2 1.3 1.3a 1.4 and 1.4b, 2.1
  • DO NOT EXIST: versions 1.2a 1.4a 1.5 and 1.5a (and in general all those not listed in "EXIST")

We do not need to go into the details as to why these non-official versions exist. What is important as a user is to be able to qualify your interface. IE: be able to verify its capacity.

To do so, we recommend you do the following:

  • Connect to your vehicle using our software.
  • Use the terminal tab to enter the commands described in the table below:
  • If the interface replies "OK", the command is supported. Otherwise a "?" will appear meaning the command is unknown.
  • Important: After this manipulation, remember to "reset" the connection if you want to continue to communicate with your vehicle.

Compatibility Command Expected reply
version 1.2 and + ATKW0 OK
version 1.3 and + ATS0 OK
version 1.3a and + ATFE OK
version 1.4 and + ATSS OK
version 1.4a and + ATCSM1 OK
version 2.0 ATAMT 40 OK


This little test will enable you to determine what level of function is installed in your interface.

Even if this is only a simplified version of our complete procedure, it is effective and fast.

TOAD software is 100% operational with interfaces from version 1.2, even though it uses some commands from version 1.3, there is no impact on function.


ELM circuit ELM for electronics technicians

If your ELM interface breaks down or if you want to create your own circuit, the diagram below will help you.

It shows the circuit of an ELM327 RS232 interface.

The basic circuit is always the same whatever the connection mode (RS232, USB or Bluetooth).

The difference lies mainly in the components connected to pins 17 and 18 of U1 (ELM327 integrated circuit).

For USB interfaces, a VCP (Virtual COM Port) component is used to "convert" an RS link into USB like USB adaptor cables <-> RS232.

There are currently two families of components in the ELM327.

The manufacturer FTDI with the component FT232 (more details on the FTDI site) and Silicon Labs with the component CP210x (more details on the Silicon Labs website).

Bluetooth interfaces use an RS modem.<->Bluetooth.

WiFi interfaces use an RS modem.<->WiFi allows adhoc wireless communication.


ELM327 Interface diagram ELM327 wiring diagram

Click on the images to enlarge

In the event of malfunction:

ELM Electronics has created a very good and practical application note with the voltage levels and a test procedure. This document can be found at the following address: http://www.elmelectronics.com/AppNotes/AppNote02.pdf.



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