Troubleshooting ELM327 OBD2 AT Commands: Getting Past the “?”

For automotive enthusiasts and DIY mechanics, the ELM327 OBD2 adapter is an invaluable tool. It opens the door to vehicle diagnostics and performance monitoring, allowing you to communicate directly with your car’s systems. However, getting started with ELM327 and AT commands can sometimes be frustrating, especially when you encounter unexpected responses like the dreaded “?”. This article aims to shed light on common issues faced when using ELM327 OBD2 adapters and provides guidance on navigating AT command syntax for successful vehicle communication.

Understanding the AT Command Syntax for ELM327 OBD2

ELM327 adapters communicate using a set of “AT” commands, short for “Attention”. These commands are the language you use to instruct the adapter to perform specific actions, from initializing communication to requesting diagnostic data. The basic structure of an AT command is straightforward: it begins with “AT”, followed by a command word, and often parameters, and crucially, terminated with a carriage return character r. This carriage return is essential; it signals to the ELM327 adapter that the command is complete and ready to be processed.

When programming in environments like Arduino, you need to ensure this carriage return is correctly included in your command strings. For example, sending the command to reset the ELM327 would look like "ATZr". Without the r, the adapter might not recognize the command, leading to errors or no response.

Decoding the “?” Response: Common AT Command Errors

Encountering a “?” response from your ELM327 adapter typically indicates an error in the command you sent. This is a general error indicator, and pinpointing the exact cause requires a closer look at your command syntax and the communication sequence. Several factors can lead to this error:

• Syntax Errors: The most common cause is a simple typo in the command itself. ELM327 commands are case-insensitive, but the spelling and structure must be precise. Double-check the command word and any parameters against the ELM327 command set documentation.
• Incorrect Command Order: Certain AT commands need to be executed in a specific sequence. For example, before requesting data, you usually need to initialize the adapter, set the communication protocol, and potentially configure header information. Sending data requests before proper initialization can result in a “?” response.
• Incorrect Header Settings (ATSH): The ATSH command, which stands for “AT Set Header,” is critical for specifying the message header used in CAN (Controller Area Network) communication, a common protocol in modern vehicles. If you are working with CAN and using an incorrect header, data requests will likely fail and return “?”.

The ATSH Command and Header Settings for OBD2

The ATSH command is particularly relevant when you need to request specific Parameter IDs (PIDs) or diagnostic services from your vehicle. The header essentially tells the car’s ECU (Engine Control Unit) who is asking for information and what type of information is being requested.

The command ATSH797 is often used to set the header for diagnostic requests. “797” in hexadecimal is a common address used for diagnostic communication in many OBD2 compliant vehicles. However, the correct header can vary depending on the specific vehicle and the ECU you are trying to communicate with. If ATSH797 is not working for you and you are getting “?”, it could indicate that this header is not appropriate for your vehicle or the specific request you are making.

Step-by-Step AT Command Sequence for Data Retrieval

To successfully retrieve data like vehicle speed or throttle position using an ELM327 OBD2 adapter, a typical sequence of AT commands might look like this:

1. ATZr: Reset the ELM327 adapter to its default settings. This is a good starting point to ensure a clean state.
2. ATIr: Request adapter identification. This command makes the adapter return its identification string, confirming it is working and responding.
3. ATL0r: Turn off linefeeds. This simplifies the responses from the adapter, making parsing easier.
4. ATE0r: Turn off echo. This prevents the adapter from echoing back the commands you send, again simplifying the communication.
5. ATSP Autor or ATSP6r: Set protocol to automatic or explicitly set protocol 6 (CAN 11-bit 500kbps), a common OBD2 protocol. Choosing the correct protocol is essential for communication. You might need to experiment with different protocols if “Auto” doesn’t work.
6. ATSH7E0r: Set the header. 7E0 is a commonly used header for ECU requests. Note: While 797 was mentioned in the original post, 7E0 is often the standard request header. You might need to try different headers like 7E0, 7E8 depending on the specific ECU and vehicle.
7. ATWM 81 11 F1r: Set the wake-up message. 81 11 F1 is a typical wake-up pattern for initiating communication.
8. ATR1r: Attempt to receive responses, enabling reception.
9. ATCF 7E8r: Set CAN filter to 7E8. This command configures the adapter to only receive messages with the header 7E8, which is often the response header from the ECU when using request header 7E0.
10. ATMAr: Monitor all CAN traffic. Alternatively, to request specific PIDs (like vehicle speed – PID 0D or throttle position – PID 11 or 45), you would use commands like 010Dr (for vehicle speed using OBD-II service 01) or 0111r or 0145r (for throttle position – PIDs might vary based on vehicle).

Important Considerations:

• Vehicle Specifics: OBD2 implementation can vary slightly between manufacturers and models. The “standard” commands and PIDs are generally consistent, but you might encounter exceptions. Consult vehicle-specific documentation if available.
• Protocol Variations: Modern vehicles use different OBD2 protocols. Ensure you are using the correct protocol setting (ATSP) for your vehicle.
• Troubleshooting: If you continue to encounter issues, use a CAN bus sniffer tool if possible to observe the actual communication on the CAN bus. This can provide valuable insights into whether the commands are being sent correctly and what responses (if any) are being returned by the vehicle.

By understanding the AT command syntax, the importance of correct command sequencing, and the role of header settings, you can effectively troubleshoot issues with your ELM327 OBD2 adapter and unlock the wealth of diagnostic and performance data your vehicle has to offer. Remember to always refer to the ELM327 documentation and OBD2 standards for detailed information and command specifics.