Early this spring, I put down new sod. The grass looked incredibly
healthy for several months. But now, although it's still very thick,
the grass is starting to look yellowish. It's not underwatering, as I
know the gray-green tinge underwatering causes.


Just for grins, try three things:
1) Get down and look at the cut blades closely. Are the tips torn-looking
or cut?

Sharp lawnmower blades leave a nice "cut" looking edge, while dull ones
look ragged. The ragged edge often gives a yellowish or brownish cast
to the lawn
http://www.floridagardener.com/misc/...nyourblade.htm

Also examine the grass blades for brown or yellow or reddish spots or
or pustules (a sign of fungal diseases)

2) Lift a piece of sod in the yellowest portion -- not a big piece, or very
deep... what you're looking for is to see if the roots are penetrating
the soil underneath, if the roots look healthy, and if you see
signs of insect activity. Also check to see if the native soil under
the sod is as damp as the sod. You can also check for excessive thatch
buildup (though I'd be surprised if you have much in this new a lawn).
At the same time you do this, collect some of the native soil underneath
the sod for soil analysis if you hadn't done that before sodding.
At a minimum, I'd like to see soil pH, P, K and Fe.

3) Apply some fertilizer to a small section; something with fairly
high ratios of N to P and K. Watch the area for fairly immediate
greening up. Be prepared to live with that spot a little greener
than the rest of the lawn for a year or three if you do this
---------

What species is/are in the lawn? What have the temperatures been like,
day and night?

What do the local golf course fairways look like?

My bets, without more information, are on bad mower blades, heat stress,
and pH.