Mac fan starts and stops one diagnostics light

Hi

My iMac 2013 will not boot up. The fan starts immediately after being plugged in (power button not pressed) then stops a couple of seconds later, power diagnostics led 1 is on, 2 comes on for a brief moment then goes off. This pattern goes on in a loop every few seconds. Pressing power button has no effect.

I did dismantle the iMac to reprogram the bios chip before this happened. I'm sure i connected everything back up. Have I messed up a connector or cable that could cause this behaviour?

Your help is appreciated.

Thanks