@mfalkvidd I've just started to look into this sensor and the main page is still quite confusing as already stated by @skywatch

It seems the choice at present is between the BMP280 or BME280. the latter also includes humidity.
Maybe the title could say Temperature/Pressure/Humidity? I just ordered a replacement Si7021, not realising there was another all in one sensor available.
It is clear that most ebay sellers are also confused and are wrongly listing BMP devices as BME.
I found a good summary to the confusion here https://goughlui.com/2018/08/05/note-bosch-sensortec-bmp280-vs-bme280-sensor-confusion/
Hope this helps someone - Think I'll be going for a BME280 here as it as some positive reviews https://www.ebay.co.uk/itm/Breakout-Temperature-Humidity-Barometric-Pressure-BME280-Digital-Sensor-Module/401513062540

Breakout Temperature Humidity Barometric Pressure BME280 Digital Sensor Module

$3.54

238 available

Having spent some more time fault finding, it seemed that the logic level converter chip had been soldered on wonky and one of the pins was not connected.
Ive unfortunately damaged the PCB in an attempt to fix so a new one has been ordered from another chinese seller.

Having now had time to read futher about the GY-21 is appears it will work with both 3.3 and 5V microcontrollers.

Interesting to see there is a GY-21P board that includes a pressure sensor. I would be interested to hear from anyone who has used this.

Edit 9/10/19 Just an update on this.
I now have new sensors on order Si7021 (both 3.3 and 5v boards) and also a BME280 which has temp, pressure and humidity.

@Yveaux I have just tested on a second board. This time a 5V arduino uno. Same result - as you say at "while (not sensor.begin())" call.
This board has SCL and SDA pins as well as A4 and A5. I have tried both.
Looks like a duff sensor then

@Yveaux That link seems to take me to the DHT22 guide....but yes I followed the Si7021 guide.

I have hooked up a GY-21 module as per the instructions to a pro mini 3.3V (Previously working with DHT22 sensor.

I just get "not recognized" message in the serial monitor. I have metered out A4 and A5 pins on my clone board and they are connected to the main IC.

This page http://www.guillier.org/blog/2015/07/sht21htu21d-humidity-and-temperature-sensor/ seems indicate that this board can be used on both 3.3 and 5V arduinos.

Am I missing something?

Thanks.

@electrik Yes, I drilled a 6.5mm hole, just big enough for the SMA connector to fit through and then screwed the antenna back on. Because the plate sits inbetween the radio and antenna, and touches the outside of the SMA, it is connected to ground.

Finally fixed as detailed here https://forum.mysensors.org/topic/10138/repeater-node-causing-problems-solved/25

So after some considerable time and loss of sanity I now appear to have a stable setup. I'd like to share what I have done in the hope it may save others a lot of time and frustration.

By enabling debug in the sketches, I eventually realised that I could not get any node to communicate reliably with the gateway. I was getting lots of NACKS even at short distances.

I was sure that the problems were radio related but after trying many NRF24 modules I was about to give up.

Then I read a post somewhere about ground planes.

As a last resort I added a piece of aluminium between my gateway antenna and the radio. (PA/LNA Version). Eureka!! This has pretty much solved the NACK issues.

Following this one or two nodes still intermittently lost contact after a few hours or days. What is strange is that even if moved close to the gateway, errors continue until the gateway is power cycled? I still don't know why this is.

I solved this problem by converting the Node PCB antennas into a dipole antenna as detailed at https://www.instructables.com/id/Enhanced-NRF24L01/.
Now all nodes stay connected.
Now after a couple more weeks of testing all is still good.

I am using Domoticz with the MQTT output passed onto Node Red and now have some really cool things going on.

A big thanks to all that replied and pointed me in the right direction.

@sundberg84 Thanks. I will aim to get both sets of logs. What I am asking is if this can be done with Myscontroller alone, or if I need to connect up my FTDI?

I am using ethernet gateway and can connect to it with Myscontroller.
Can I view debug logs of the remote nodes using this method or do I need to carry on debugging each one with usb/serial converter?
To debug the gateway with MyScontroller I guess I need to enable dubug in the gateway sketch?
I do not use OTA updates. Still trying to get reliable communication between nodes and gateway without NACKS.