Poweredge R620 No Post

Hello, I have a Dell Server - Poweredge R620 that powers on, then after about 30 seconds or so, cuts out and the indicator light begins flashing orange.

During the initial boot up (immediately after power is supplied to the server), all fans loudly kick into life and spin for a brief second then stop and only fan 4 continues to spin. Also the power button briefly lights up green and the screen also seems to get a connection but it then both cut out and pressing the power button yields no result.

After the first 30 seconds or so of activity, fan 4 then cuts out and the orange indicator light at the rear right hand corner of the unit flashses on and off continually. The motherboard light just below this stays lit green and I can hear power supply 1 whining but neither PSU 1 or 2's fans are on.

Troubleshooting completed -

Cutting the power and resuming the power has been tried multiple times.

Tried holding the power button in for 15 seconds with the power to the server turned off and on.

CMOS jumpers checked.

Bios battery removed whilst powered off and reinserted.
both Power Supply's checked and swapped, power cables also swapped along with plugs that they plug into. Both LED's on the the power supplys are constantly lit green.

2 memory modules available, 1 processor in use. Have tried with a single module in memory slot 1 (both available memory sticks tried) but both result in the issue above.

Checked all fans and they are securely connected however only fan 4 appears to spin after the initial power up however all fans do spin for a second when power is connected.

Thanks!