Sagos are one of the easiest plants to grow, though the growth can be frustratingly painful for some people. Usually a dose of fertilizer or removing the bottom third of the fronds will get the plant pushing out new fronds for you.
This plant has been working away putting out pups at the base last year and it’s getting ready to seed this year.
Yellowing on the outside of older fronds is not a problem and very rarely cause for concern. Just remove older fronds when they turn brown.
Scale can be a problem. Picking scale off a sago is a job for only the most devoted of gardeners. I’d recommend using orange oil. The first symptom of scale is often yellowing leaves. So if you have newer leaves turning yellow, flip them over and check for scale.
If there is no scale and there is yellowing of newer leaves check for fungus. Take a magnifying glass out to your sago and go over the leaves looking for bumps. Fungus can almost always be cured with a fungicide from your local nursery.
If there is no scale and no fungus then you are sitting where I am now. It is most likely a magnesium or nitrogen deficiency. Since this plant is busy making babies and seeds, I’m guessing that is probably the correct answer. I’ll fertilize tomorrow. If a lack of nutrients is the problem, the yellow spots will remain yellow, but new growth will be green.
Sagos do not do well below freezing. The fronds will brown and will not green up in spring. Remove the old fronds once the weather warms up and new fronds will appear in a few months.