Just to confirm, it works with the replacement files!

I think these files have resolved the problem!

That was here: https://github.com/espressif/arduino-esp32/issues/912
See what 'stickbreaker' writes.

I've noticed this but I didn't want to mess about with the core.

I think for what it's worth, it's now time for me to give it a try.

Let's see what happens.

Since using these library files, my ...

Here is what I'm seeing on my old CRT oscilloscope when transmitting from the ESP32 . . .
I2C trace.jpg

The upper trace is the SCL clock, which is high before the oscilloscope trigger. The small blip at the start of the clock signal is deceptive: it's from capacitive coupling within the cable ...

That was here: https://github.com/espressif/arduino-esp32/issues/912
See what 'stickbreaker' writes.

I've noticed this but I didn't want to mess about with the core.

I think for what it's worth, it's now time for me to give it a try.

Let's see what happens.

Things will look nice and sharp on your logic analyser because it only detects 'low' and 'high', nothing in between.

With my 5m of cable and oscilloscope, I am seeing clearly the shape of the rising and falling edges, at least master-to-slave signals. However, because of all the data flying around ...

Alright, I'm going to do some more investigations today.

I believe my logic analyser can detect rising and falling edges as it's 24MHz. It's more of a scope than a logic analyser. I have a scope too but I prefer the logic analyser for this sort of thing.

Changing the clock rate and/or the resistor ...

This is ridiculous...

It's as if this thing is completely random. This thing seems to randomly flit between working and not working on a whim.

I can reset the device and whether it works or not is just potluck!

I'm going to try 1k now.

Should I put pullups on the light sensor too as well as the ...

I know Ethernet cable such as CAT5e is often recommended for I²C over in the Arduino forum but, as you are not using balanced transmission, I would expect use of a screened cable to be much better (some Ethernet cables are screened). I've found these multi-core screened cables for example:
https ...

Ok, I reset the ESP32... I2C started working fully.

I added the logic analyser and the moment I plugged it in to the laptop, I2C instantly failed. I tried again and it eventually worked. With the analyser, I can see that when I2C works, the pulse train is crystal clear, even with the 3.9k. With the ...