4SNS TCBC - Which Mac Pro sensor is that? Briefly my Mac Pro (4,1 Early 2009 ) stopped booting which I eventually traced to the plastic clip holding the north bridge down failing. Seems to be a common issue.

Replaced the clips and now it will boot again and is stable but refuses to run the 2nd CPU. Kernel boot messages show it's there but disabled.

Apple hardware test gives error of 4SNS/1/40000000: TCBC-252:000 which from various sites on the net I understand to mean


4SNS - Sensor
TCBC
|----- Temperature
 |---- CPU
  |--- CPU B ?
   |-- C ?`


Any idea on the numbers ?

Can anyone help me diagnose here, is it new Heatsink time or new Logic board time. Neither is great but a heatsink is much cheaper than a logic board.Is it something else that's fixable ?


Both CPUs verified good in socket A.  
All RAM verified good in Socket A RAM slots. 
No diagnostic LEDs lit.
New thermal paste everywhere
SMC reset
PRAM reset


EDIT: RAM Clarification
EDIT: Things done
Thanks
I rang Apple in the end, they didn't know either without seeing it. 8-) 

Suggested I take it in for a free diagnosis.

Before I did that I tried taking the heatsink sensors connected off and replacing it.

Bingo, all fixed. 

So the answer must be that this error is the heatsink sensor.