elm327_obdii

A Python library for reading vehicle diagnostic data from ELM327 Bluetooth Low Energy (BLE) OBD-II adapters.

This library is used by the ELM327 OBD-II BLE integration in Home Assistant.

Features

• Connect to ELM327 BLE OBD-II adapters via Bluetooth
• Read standard OBD-II Mode 01 PIDs (engine speed, vehicle speed, coolant temperature, fuel level, etc.)
• Read vehicle-specific custom PIDs from OBDb and WiCAN community profiles
• Voltage-gated polling state machine to protect the vehicle's 12V auxiliary battery
• Structured fmt formula representation for signal decoding (bit extraction, linear scaling, enumerations)
• CAN context management (ATSH/ATCRA header/filter switching for multi-ECU vehicles)

Data Sources

The library fetches vehicle profile data at runtime from:

• OBDb — community-maintained database of 160+ vehicle models with 11,000+ signal definitions. Licensed under CC-BY-SA 4.0.
• WiCAN — meatpiHQ/wican-fw repository with 77 vehicle profiles. Licensed under GPL-3.0.

The fetched profile data is subject to those licenses. The library code itself is Apache 2.0 and does not distribute any profile data.

Installation

pip install elm327_obdii

Usage

import asyncio
from elm327_obdii import Poller, PollerConfig, ProfileConfig

async def main():
    config = PollerConfig(
        profile=ProfileConfig(),
        atrv_supported=True,
        voltage_check_enabled=True,
        voltage_on=13.1,
        voltage_off=13.0,
        grace_seconds=30,
    )
    poller = Poller(config)

    # Connect to the adapter (provide a BLEDevice from bleak)
    ble_device = await bleak.BleakScanner.find_device_by_address("AA:BB:CC:DD:EE:FF")
    poller.connect(ble_device, asyncio.get_event_loop(), uuid_write, uuid_read)

    # Poll once
    result = poller.poll_once()
    print(f"State: {result.state}, Voltage: {result.voltage}")
    print(f"Data: {result.data}")

asyncio.run(main())

CLI Usage

The library ships a standalone CLI tool for testing it directly against a BLE OBD-II adapter — useful for development and debugging on any BLE-capable machine. Run it as a module:

python -m elm327_obdii

First run — setup wizard

On the first run (or when ~/.elm327_obdii_config.json doesn't exist), the CLI launches an interactive setup wizard:

1. Scan for BLE devices whose name advertises "OBD" or "ELM327".
2. Pick an adapter from a numbered list.
3. Pick a vehicle profile:
   • None — standard Mode 01 PIDs only (RECOMMENDED_DEFAULTS).
   • WiCAN — fetches the upstream vehicle_profiles.json and lets you pick a car model.
   • OBDb — fetches the OBDb matrix, then walks you through make → model → year selection.
4. Probe the adapter to confirm AT RV works and to discover the BLE GATT UUIDs (falls back to dynamic discovery if the defaults don't match).
5. Save the config (address, UUIDs, voltage thresholds, profile) to ~/.elm327_obdii_config.json so subsequent runs skip the wizard.

Subsequent runs — polling loop

Once a config exists, the CLI loads it, connects to the adapter, and polls in a loop. Each poll prints the state-machine state, battery voltage, and the decoded value of every PID in the query plan:

--- poll #1 ---
state:    car_on
voltage:  14.23 V
success:  True
data:
  ENGINE_SPEED: 1500.0
  FUEL_LEVEL: 87.5
(sleeping 10.0s until next poll)

The poll interval adapts to the state machine: ~10s while the car is on, ~5 min once the voltage gate declares the car off (to protect the 12V auxiliary battery). Press Ctrl+C to disconnect and exit.

Command-line flags

Examples

# First run: launch the setup wizard
python -m elm327_obdii

# Poll once and exit (smoke test)
python -m elm327_obdii --once

# Verbose logging for debugging the state machine / query plan
python -m elm327_obdii --debug

# List the PIDs that would be polled, without connecting
python -m elm327_obdii --list-pids

# Poll every 2 seconds regardless of state
python -m elm327_obdii --interval 2

# Re-run the setup wizard (e.g. to switch adapters or vehicles)
python -m elm327_obdii --reconfigure

# Use a non-default config path
python -m elm327_obdii --config /tmp/elm327_test.json

Config file format

The config file is plain JSON and may be edited by hand:

{
  "address": "AA:BB:CC:DD:EE:FF",
  "uuid_read": "0000fff1-0000-1000-8000-00805f9b34fb",
  "uuid_write": "0000fff2-0000-1000-8000-00805f9b34fb",
  "atrv_supported": true,
  "voltage_check": true,
  "voltage_on": 13.1,
  "voltage_off": 13.0,
  "grace_seconds": 30,
  "profile": {
    "standard_pids": ["FUEL_LEVEL"],
    "custom_pids": [
      {
        "id": "egolf-soc-bms",
        "name": "SOC BMS",
        "mode": "22",
        "query": "028C1",
        "fmt": {"bix": 32, "len": 8, "div": 2.5},
        "can_header": "7E5",
        "can_filter": "7ED",
        "unit": "%",
        "device_class": "battery",
        "state_class": "measurement"
      }
    ]
  }
}

The profile block is the serialized form of ProfileConfig.to_dict() — the same shape the library uses for Home Assistant config entries — so profiles built via the wizard or imported from WiCAN/OBDb are stored verbatim and don't need to be re-fetched on subsequent runs.

Development

python3 -m venv .venv
source .venv/bin/activate
# for Windows CMD:
# .venv\Scripts\activate.bat
# for Windows PowerShell:
# .venv\Scripts\Activate.ps1

pip install -e ".[test]"
pytest

python -m pip install pre-commit
pre-commit install
pre-commit run --all-files

Live tests

Some tests require network access to fetch live vehicle profiles from OBDb and WiCAN:

pytest --run-network

License

Apache License 2.0 — see LICENSE for details.

The test fixtures in tests/fixtures/obdb_egolf/ are sourced from the OBDb Volkswagen e-Golf repository and are licensed under CC-BY-SA 4.0. See tests/fixtures/obdb_egolf/README.md for details.