Carrier comfort pro help

Hi guys I'm new here and I need help with my comfortpro apu.
Recently I bought truck with no working apu unit. First of all it doesn't have any light on driver panel either on apu controller.
Where should I start from?
Thank you

 

Connections at batteries clean and tight.

 

Yes, cleaned and tight

 

Search the sight s manual has been uploaded for the carrier. I can’t remember how to upload the manual

 

My Comfort Pro is exhibiting strange behavior.
It's supposed to go;
STARTING APU...
engines starts
ENGINE RUN CHECK
for about 2 secs and it's running and returns to normal display.
Now it goes
STARTING APU
engine starts but screen still says
STARTING APU
for 2 sec and it shuts off

To complicate matters it's now going
MAIN ENGINE INTERLOCK
as it sound do when the main truck engine is running and you try to start it when it had the interlock wire attached to a key on hot circuit in the fuse box, however that wire is disconnected. The above problem can be recreated with low battery voltage auto start. Any suggestions? Are these related? The first issue makes me think perhaps the RPM sensor is gone.

Help!