Q.Why does my sago palm have a rotting smell?
We have a large sago palm in the backyard. I started to notice a bad smell a few weeks ago and finally narrowed it down to the sago palm. It smells like something is rotting. Is it root rot? It is in well drained soil but the soil never is bone dry. Also, the lower branches are starting to yellow. This is a large sago palm, about 6 feet tall, so we cannot just repot it or easily check the roots. Help!

Yes, it does sound as if your Sago Palm is suffering from root rot.
Can you pull branches out easily from the center?
You should not be able to.
The rotting smell is your best indicator. It may be past the point of saving it.
Sago Palms need to have well draining soil and cannot sit in wet soil.
Here is a link to proper care of your Sago Palm.
https://www.gardeningknowhow.com/ornamental/trees/sago-palm/how-to-care-for-sago-palms.htm