I have had my Enviro Urban for just over a year now and it was working fine until a couple of weeks ago. I noticed readings had stopped so re-charged the batteries, but now it is back on line the temperature reading show as -40 deg C !! Other readings are coming through although I am not sure pressure and humidity are correct. I tried installing the latest Pimoroni firmware for the Enviro but this has not had any effect on the incorrect readings. Has the BME280 stopped working?
Found this thread about a similar problem with the Enviro+ Envio+ BME280 temperature and humidity fault.
It looks like there may be a problem with using the BME280 outdoors, although my Enviro Urban was in the recommended enclosure.
I have got round the built-in BME280 not working by adding an external BME280 breakout, the only problem I discovered was that the built-in BME280 of the Enviro Urban is on address 0x77, not on 0x76