I'm not too sure what the air pressure sensor is, but as far as MyECU is concerned, it has three wires. One connects to ground, one connects to 5 volts and the other gives a voltage that is connected directly to one of the microcontrollers analog input channels.
The air pressure is sampled continuously and used to do a fine tuning of the amount of fuel that is delivered to the engine. As the air pressure increases, its density increases, necessitating a richer mixture.
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