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OBD-II Communication Module (ELM327)

[DOCUMENTATION IN DEVELOPMENT]

Warning: This documentation module is currently in the process of creation and technical review. Some flowcharts or PID formulas may be subject to change until the final stable version.

This module handles the real-time telemetry extraction from the vehicle's ECU via a Bluetooth ELM327-compatible dongle. https://es.wikipedia.org/wiki/OBD-II_PID

Architecture & Data Flow​

This is a example of requesting the Engine RPM (PID 0C) and parsing the response. The app must implement a loop to continuously poll desired PIDs while the connection is active.

State Machine (Connection Lifecycle)​

  1. DISCONNECTED: No Bluetooth socket active. No communication with the dongle.

  2. CONNECTING_BT: Establishing SPP/BLE connection with the dongle’s MAC address.

  3. BT_CONNECTED: Bluetooth link established successfully, but no communication with the ELM327 firmware yet.

  4. INITIALIZING_ELM: Sending base AT commands (ATZ, ATE0, ATL0, ATS0, ATH0, ATSPx) to configure the ELM327.

  5. ELM_READY: The ELM327 responded correctly with OK and prompt >. Firmware is responsive and ready.

  6. ECU_CONNECTING: Attempting to establish communication with the vehicle ECU (protocol detection or bus initialization in progress, may show SEARCHING...).

  7. ECU_CONNECTED: Successful ECU communication confirmed (e.g., valid response to 0100 or other test PID).

  8. CONNECTED_IDLE: Fully connected and ready. Waiting for PID requests or queue processing.

  9. POLLING: Actively sending PID requests in a loop and parsing responses.

  10. RETRYING: Temporary communication issue detected (NO DATA, BUS BUSY, STOPPED). The system is retrying the last command with a retry counter.

  11. RECOVERING: Medium-level fault detected (BUFFER FULL, LV RESET, FB ERROR). Performing partial reset, buffer flush, or reinitialization sequence.

  12. ERROR_TIMEOUT: The ECU or dongle stopped responding within the configured timeout window.

  13. ERROR_PROTOCOL: Protocol-level failure (UNABLE TO CONNECT, BUS INIT...ERROR, repeated CAN ERROR). ECU communication could not be established.

  14. ERROR_FATAL: Critical failure state (Bluetooth disconnect, repeated initialization failure, unrecoverable firmware error). Requires full restart of the connection lifecycle.

Standard PIDs Reference & Formulas​

When reading Mode 01 (Live Data), the response will contain data bytes referenced as A, B, C, D depending on the payload length. View more PIDs here: Wikipedia

PID (Hex)DescriptionExpected BytesFormula to DecimalUnit
01 04Engine Load1 (A)A / 2.55%
01 05Coolant Temp1 (A)A - 40°C
01 0CEngine RPM2 (A, B)((A * 256) + B) / 4RPM
01 0DVehicle Speed1 (A)Akm/h
01 10MAF Air Flow Rate2 (A, B)((A * 256) + B) / 100g/s
01 2FFuel Tank Level1 (A)A / 2.55%

Warning: Not all vehicles support all PIDs. The app must first query 01 00 to get a bit-encoded map of supported PIDs for the specific car.

Error Handling​

  • SEARCHING...: The ELM327 is attempting to automatically detect the vehicle’s communication protocol. The application must wait for completion before sending new commands.

  • NO DATA: No response was received from the ECU. The requested PID may not be supported, the engine may be off, or the ECU did not reply within the timeout period.

  • CAN ERROR: A physical or low-level communication error occurred on the CAN bus. This may indicate wiring issues, electrical noise, or a faulty dongle.

  • BUS ERROR: General bus communication failure. Often caused by protocol mismatch or electrical problems.

  • BUS INIT...ERROR: The ELM327 failed while initializing communication with the vehicle’s protocol.

  • BUS BUSY: The communication bus is currently busy or saturated. The application should retry after a short delay.

  • DATA ERROR: Corrupted or malformed data was detected in the received frame.

  • BUFFER FULL: The ELM327's internal buffer is full, likely due to too many queued commands without reading responses. The app should pause sending new commands until the buffer is cleared.

  • ACT ALERT: Activity alert from the ELM327 indicating unusual or problematic bus communication activity.

  • LV RESET: The ELM327 has reset due to low supply voltage. The app should reinitialize the adapter and verify vehicle battery stability.

  • STOPPED: The current operation was interrupted, typically because a new command was issued before the previous one completed or due to a manual interrupt.

  • UNABLE TO CONNECT: The ELM327 failed to establish communication with any ECU in the vehicle.

  • FB ERROR: Firmware-level internal error reported by the ELM327. Common in clone adapters.

  • ?: The ELM327 received an unrecognized or improperly formatted command.