Raspberry PI 4 wont boot with Expander PI fitted

My Raspberry PI 4 is booting OK and working fine when no HAT is attached. If I attach the Expander PI HAT and go to boot the PI nothing happens. The red LED illuminates and thats it, no futher activity. As soon as I remove the Expander PI, the PI boots OK. Any help on this issue?

If the Raspberry Pi is failing to boot with the Expander Pi connected that normally means there is a short circuit somewhere on the Expander Pi.

Can you check that there are no solder bridges between the pins on the 40-pin GPIO connector? If you have a multimeter try checking the resistance between the 5V pin and ground and the 3.3V pin and ground. If the resistance is close to 0 ohms that would indicate a short circuit.

If you can not find any problems with the soldering on the GPIO connector there could be a problem with the Expander Pi itself in which case I will send you a replacement.

OK, thanks for the responce. Checked 3.3v to Gnd and open circuit. Checked 5v to Gnd and measured 10K ohms. I have desoldered the GPIO pins as much as I can so pretty sure there are no shorts now, the resitance 5v to Gnd increased to 0.4M ohms. The PI is booting up now with the Expander PI connected. Should I be concerned about the 5v to Gnd not being an open circuit?

There should be some resistance between 5V and ground as the components on the Expander Pi will have some internal resistance. If the Raspberry Pi is now booting and connecting to the Expander Pi that means it should all be working correctly.