When the engine is started cold, the ECM mode is open loop. This uses preset parameters to set injector pulse width, idle air, and ignition advance. The engine will run fine, but uses more fuel, and it will not make max horsepower.

Once the engine warms up, it goes into closed loop. At that point, it takes input from the various sensors, TPS, MAF, O2, temp sensor, oil pressure, knock sensor, ect., and uses that to calculate injector pulse width, Idle air, and ignition advance. This is closed loop. It will make more horsepower, and increase the fuel mileage.

The sensors for the most part operate on resistance. The computer supplies the sensor with a set voltage, and samples the return voltage, and uses the difference to make the calculations. If the return voltage is the same as supplied voltage, it reads that as a malfunction, and holds it in open loop.

When the early installers started retrofitting EFI engines, many of them just wired them for open loop to make them work with stock PROMs in non stock applications. One way to do this was to remove the temp sensor, and twist the wires together. It wasn't right, but it worked.

If you are happy with the way it runs, just leave it. But if you want all that you can get you will probably have to rework the harness for the right pin outs, and replace the PROM, or go to an aftermarket harness.