"A 5.1 V 3A supply" does not really answer the question I asked. This board is full of descriptions of problems caused by 'power supplies' which are really battery chargers and are unable to maintain their rated voltage and current simultaneously. I strongly suggest that you try getting and using the 'Official' Pi 4 power supply.MrDitchy wrote: ↑Sat Jul 27, 2019 4:54 amI have just tried another TV this time a 1080p model I get a no signal although my monitor on HDMI 1 appeared with just the trash can suggesting that it may a lack of power from the HDMI output. I will try and get a second computer monitor and see if that cures the problem. Again on boot both screens had the rainbow square.
So I booted the PI and then plugged the second screen(my 720p TV) into the HDMI 1 and now it works as the second screen. It may hang when booting but at least it works.
My first Pi setup (3B) was bought "from an approved re-seller as part of a kit", and, although it initially seemed to be OK, the power supply started getting flaky after a few weeks. I do have one other non-official power supply that seems to be reliable, but my two official ones have never given me a moment's concern. My new Pi4 (when it is delivered - sigh!!) will have an official supply.
It seems highly improbable (but not impossible, I grant) that the make or specs of the system SD card is the cause of this. If I wanted to test this, I would clone the 'non-booting' (on the Pi4) system to the 'booting' SD card. My prediction is that the clone of the 'non-booting' system to the 'booting' SD card would not boot with two monitors connected, and that there is a system software difference between the two. Both being up to date does not rule this out.