I'd say, before you jump to conclusions that involve costly fixes, try first to see if the problem is a common one....such as
grubs, eating at the roots of the grass.
Sometimes you can see possible clues if you notice any skunks, squirrels, birds even, rabbits.....et al, scratching at the surface. They are undoubtedly listening, smelling, searching for grubs.
Other pests might be if you notice any holes that weren't there before--that could mean voles or moles...they eat the grubs or they themselves are eating the roots. And mice too will eat grass roots.

Too much fertilizer in a section. Too much water causing short roots which, your California sun dries out the grass.
Too little moisture---but that doesn't sound like the reason.
Poor growing conditions.

I would suggest you not adding fertilizer at this time...instead try using a good compost overlay -about 2"..and let the grass grow up and take it down.

If you smother that with 2 inches of compost you'll have a compost lawn for the rest of the year. The grass will die out completely. Compost is applied 1/4 inch at a time. But I would not go to that expense yet.

Where do you live in California? It's a huge state that covers every environmental climate and soil condition. You need to be pretty specific. Some places are too big to characterize with one word. Los Angeles, for example is waaaay to vague especially because nobody on this forum lives in Los Angeles (except that one guy in Baldwin Hills). Where you live gets to the point of whether you are trying to grow a plant that is not suited to the environment.

Also is that a Marathon variety? Marathon has been grown in almost every part of SoCal, but you have to treat it differently in the desert than you do near the coast (sorry, beach). What I'm thinking is you are watering wrong.

When you water for 9 minutes, is any part of the yard soggy with standing water? Do you know how much water your sprinklers put out in 9 minutes? You can test with a tuna or cat food can. This is very good information to have because generally grass needs about 1 inch per week during the hottest heat of summer and only 1 inch per month in the cooler months. In between (like now) you might water once every 3 weeks. As it warms up, once every 2 weeks. The short duration high frequency regimen is only for getting new sod/seed established. Once the roots are knitted into the soil, then you can back off on frequency and move toward one inch all at one time.

How did you prepare the yard for the sod? What was there before, how did you get rid of it, and how did you amend the soil (if you did)? Was the sod put down by professionals or did you do it? If you did it, was the sod put down the same day it arrived to you? Did they roll the new sod after they put it down?

If you had someone do this, I would get them involved immediately. They might have the answer for you. They might even offer to replace the dead spots. If they do not offer, I would get more sod from them (unless you get a real strong feeling of sleazy, or fly-by-night). Then cut out the dead stuff and cut in the new.

I would suggest you not adding fertilizer at this time...instead try using a good compost overlay -about 2"..and let the grass grow up and take it down.

Ignore this post. If you were to do that would kill your sod.