Is using GPIO16 of ESP8266 for I2C somehow possible (Sonoff Pow R2)

Hello together,

I recently bought a Sonoff Pow R2 and thought: Hey ... would be nice to add I2C support to it, so I can attach additional sensors.

As GPIOs 5,13 and 14 are being used to connect the HLW8012 or CSE7759 current-sensor,
and GPIO 12 triggers the relay I do not have to much left so I soldered a cable directly to the Pins of the ESP8266 Chip on GPIO4 and GPIO16, as I do not need deep-sleep.

Now some thing is strange: I am sure that I did solder the things correctly and I can use DigitalWrite(..) to pull each PIN high/low so there is no problem with connectivity.

However: If I try to use wire.begin(4, 16) - nothing happens. I do not get responses from Wire.beginnTransmission/Wire.endTransmission

Does anyone have a clue on how to use GPIO16 as SDA/SCL Line ?

I also tried on my nodeMCU and whenever I use the standard GPIOS 4+5 (D2, D1) everything works fine.
As soon as I switch to GPIO16 (D0) - nothing works.

Any ideas?

GPIO16 is the RTC GPIO pin. It is different than the other GPIO pins, and has some limitations.

You'll have to use a different pin.