D8 is on the heat bed is not reading 12.6 volts

I read it was best to start a new thread if the problem is very old.
I own a reprap prusia i3. It is from Sintron who put the kit parts together. It prints really good btw.
Ok I narrowed it down the negative side on the ramps board D8 is not allowing power to go through.
The test I ran not in any order.
Power supply 12.6 volts.
D8 positive Green block negative. 12.6 volts
Heat bed hooked up the power supply outputs 12.6 volts.
No led light comes on D8.

Conclusions:
Bed works,
D8 positive works.
D8 negative does not.

I should add it prints good with a cold bed, but not anything long cause it starts to lift. I just use extra hold hair spray.

Is it software now? It is a brand new board I put in and it can't be the mosfets I am guessing.

It could be the firmware is not configured for a heat bed. For RAMPS, select the appropriate EFB define and set the bed thermistor.

The mosfet control of the bed is via the -ve rail.

So either the mosfet is not being told to turn on (firmware or damaged mpu) or it is dead and just not turning on.