1991 Ford Taurus engine

The dash gauge is not involved with the Engine Computer. In the early '90s there was nothing to "reset". That is a holdover from mechanics becoming accustomed to disconnecting the batteries to force the computers to relearn data. GM had a lot of trouble with Engine Computers and this was a common practice although it was rarely a permanent fix. This wasn't a problem on Fords or other car brands.

When you DO disconnect the battery, the Engine Computer has to relearn the characteristics of all the sensors and it has to rebuild the fuel trim tables. It does that by comparing many things to each other. When you replace a single sensor, the job of relearning its personality, so to speak, is much easier and faster.

The next step is to connect a scanner to view live data. In particular you'll want to see the percent of idle air control being commanded. If it is very low, look at the other sensor readings to determine why a low idle speed is being calculated. If the percentage is fairly high but you're not getting that response, the main suspect would be the idle air control assembly or the wiring to it. The air passage could be blocked with carbon too, but that is not common on that engine.