onBLENotify callback never resolves Promise — OBDLink CX adapter, iOS

[Crimi01]

Problem

Hello,
I'm Crimi01. I'm searching for a solution to my problem. I don't know if this topic already exists, but I'm trying to connect via OBD2 on iOS using Capacitor 5.7.8 and the Capacitor Bluetooth LE 3.1.4 module, and I really need some help. Thanks in advance.

I'm connecting to an OBDLink CX BLE adapter (ELM327 protocol) installed on a 2025 BMW M3.

What works

• BLE connection stable ✅
• discoverServices OK ✅
• startNotifications OK ✅
• addListener registered OK ✅
• writeWithoutResponse sends commands OK ✅
• ECU responds — data visible in Xcode logs ✅

What doesn't work

• onBLENotify callback is called BUT my Promise resolver (cmdResolve)
  always times out
• RAW response is always null

Xcode logs show data arriving correctly:

TO JS {"value":"37 45 38 20 30 34 20 34 31 20 30 43 20 31 36 20 36 41 20 0d"}
TO JS {"value":"0d 3e "}

My current onBLENotify implementation:

BC.addListener(notifKey, (data) => {
if (data && data.value) onBLENotify(data.value);
});

function onBLENotify(value) {
if (value && typeof value === 'object' && value.value)
value = value.value;
if (typeof value === 'string') {
const bytes = value.trim().split(' ')
.filter(h => h.length > 0)
.map(h => parseInt(h, 16))
.filter(b => !isNaN(b));
value = new TextDecoder().decode(new Uint8Array(bytes));
} else if (value && value.buffer) {
value = new TextDecoder().decode(new Uint8Array(value.buffer));
}
_scanBuf += value;
if (_scanBuf.includes('>')) {
const r = _scanBuf.replace(/>/g, '').trim();
_scanBuf = '';
if (cmdResolve) { cmdResolve(r); cmdResolve = null; }
}
}

Question

What is the exact format of the data object received in the
addListener callback?

Is it:

• { value: DataView } ?
• { value: string } ?
• { value: Uint8Array } ?

How should I correctly decode the incoming BLE notification data
from OBDLink CX to get the ASCII response string?

Environment

• Capacitor: 5.7.8
• @capacitor-community/bluetooth-le: 3.1.4
• iOS: 18
• iPhone 17 Pro Max
• OBDLink CX (BLE 5.2)
• OBD Service UUID: 0000fff0-0000-1000-8000-00805f9b34fb
• Notify UUID: 0000fff1-0000-1000-8000-00805f9b34fb
• Write UUID: 0000fff2-0000-1000-8000-00805f9b34fb

[peitschie]

Hi @Crimi01

I really can't tell from your code snippet here. You definitely shouldn't be using addListener directly on this library itself, as notifications are expected to be started purely via startNotifications

I'd suggest having a close read of https://github.com/capacitor-community/bluetooth-le#heart-rate-monitor, as this walks through all the important points.

[Crimi01]

Thank you for your answer!

Can you show me the correct implementation for
onBLENotify with startNotifications?

The callback receives a DataView directly?

Like this:
await BC.startNotifications(
deviceId, service, characteristic,
(value: DataView) => {
const text = new TextDecoder().decode(
new Uint8Array(value.buffer)
);
// accumulate in buffer...
}
);

Is this correct?