Check Engine light in stop and go traffic

My daughter has a 1996 Saturn SL with something over 100K miles on it. It runs generally well, although using a quart of oil every thousand miles or so. It passes the smog check required for her registration. It’s had new spark plugs within the recommended period.

Recently she went on a road trip, and hit stop-and-go traffic for a couple of hours in one large city; it being January in a warm climate, the weather that day was presumably neither very hot nor cold. The Check Engine light came on. Fortunately she had the time to take it to a mechanic recommended by a friend local to the area, who pronounced the engine OK. She proceeded without incident for the rest of her thousand mile trip.

Sounds to me like the emissions crossed some threshold beyond “marginal” during the lengthy idling and running at low speed. Question: if we wanted to avoid such a condition, what kind of repair would you expect to be recommended, if we have someone take another look? I guess it’s worth having an idea in advance, in case the recommendation is expensive, so that we can judge whether to bother or not.

Did they read the codes???

What color is the car and what city did it stall in?

Seriously, knowing the color and city would help about as much as the details provided thus far.
A bunch of things could cause that condition.

As MikeInNH implies, the check engine light means that error codes have been stored in the computer - and yes, it is all emissions related in one way or another.

The codes look like “P1234” and there are hundreds (thousands?) of them. If she took it to a shop the first thing they should have done is read them. Often the codes will get written down on an invoice. If you can’t figure that out, many auto parts stores will read the codes for free. If you can get the code(s), write them down as exact “P” numbers and post them.

Without that no one would know where to start.