Is this a sign of a faulty Mobo?
It very well could be, or a problem with one of the slots and how it is soldered to the motherboard.
As for RAM slots, you stated one as not working, you did not specify which one this is. The RAM slots which are marked on the motherboard as the examples I am providing here slot a or slot 1
or DIMM 1
etc (Motherboard manufacturers have different ways of naming the RAM slots.) If your motherboard has two slots and slot #1 is defective, some motherboards do not like having slot #1 empty, but slot #2 in use. If slot #1 is the defective slot, you can run into more problems if you are only using slot #2.
Make sure there isn't anything in the slot preventing a good electrical contact on one of the gold colored strips on the RAM and its mating surface in the slot, such as a tiny dust particle.
Check the RAM slots base where it rests on the main-board for a faulty solder joint, this will require the board to be out of the computer case, so you can observe the underside of the printed circuit board. Gently moving the RAM slot might reveal a moving lug, indicating inadequate solder on this connection. If this is found, it can be repaired by using a 15 Watt solder pencil and refluxing the joint.
I have seen this happen on many electronic components, you have to keep a keen eye on the joints because some faulty joints are hard to spot at first, but if you can see the leg or lug of a part moving, it is an indicator that the joint needs re-flowing. ( A word meaning reheating with a solder pencil. Warning: Do not use a solder gun, or any soldering device with a higher than 15 Watt rating, too much heat can destroy electronic components, using anything higher than 15 Watts is asking for trouble!)
Any questions we're here to answer them.