2001 Ford Escort ZX2 service engine soon light on and not shifting out of gear?

This happened about 3 weeks ago. The service engine soon light came on and it was like it was stuck in lower gear and not changing over to a higher gear to go faster. Took it to the garage, they pulled an error code (they never said what it specifically was, but briefly mentioned a speed sensor that was an error due to the shifting issue that the computer recognized was odd) and then reset it. They did change the transmission fluid and it seemed to drive fine to them. They said they didn't experience any issues when test driving it. It drove fine for a few weeks with no service engine soon light or trouble shifting. Today it is doing the same thing again. Transmission was rebuilt just a couple of years ago by a very reputable place that does quality work.

What could it be? The light isn't blinking, just solid. All info on the light meaning just states that it is "related to the emission control computer and should be serviced when convenient." The car just passed emissions maybe a couple of months ago at most, if not sooner, so I don't think there's suddenly a catalytic converter issue.

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