HP z600 red light and beeps. 4 beeps 5 times

[Gregprew]

Hi there

Today I took the heat sink off my z600 work station to apply thermal paste. 

I had taken the heat sink on and off before without any problem. 

However, today the machine will no longer boot. 

I get a red light which blinks, and beeps. The sequence is 4 beeps/blinks, and it does this sequence 5 times in a row. 

I cannot find what this particular sequence means, and am at a total loss. I've replaced battery ad pressed CMOS button as well as checked other bits are connected. 

Any help appreciated.  

TIA 

[TheOldMan]

Here is the manual for that PC.

https://images10.newegg.com/User-Manual/User_Manual_83-147-454.pdf

Page 183 has the beep code for that condition - Table 6-8

Activity                                              Possible cause 

Red Power LED blinks four              Power failure (power supply is overloaded)
times, once every second,
followed by a two-second
pause. Four beeps.

Recommended action:

CAUTION: Internal components might be powered even when the
workstation is off. To prevent damage, disconnect the workstation power
cord before you remove a component.
1. Open the access panel and ensure that all power connections (18-
pin Main, 8-pin CPU, 6-pin Memory) are secure on the system board.
2. Check if a device is causing the problem by removing all attached
devices (such as hard, diskette, or optical disk drives, and expansion
cards.) Power on the workstation. If the system enters the POST,
power off and replace one device at a time; repeat this procedure
until failure occurs. Replace the device that is causing the failure.
Continue adding devices one at a time to ensure all devices are
functioning properly.
3. Check the power supply functionality.
a. Disconnect the AC power.
b. Remove power supply from system chassis. (See Removing the power
supply on page 78 for details.)
c. Plug in the AC power.
If the power supply fan spins and the BIST** LED lights, the power supply
is functional. The problem might be on the system board. Contact HP for
assistance.
If the power supply fan does not spin or the BIST ** LED does not light, the
problem might be in the power supply