It all depends on the kind of sensor (mechanism) being used and the valid input ranges defined
- both the RPM sensor and the fuel flow sensor count pulses no pulses just means no RPM or no fuel flow (count revolutions of the little propellor inside the red block), no pulse just indicates no RPM or no fuel flow, but the electronics can't 'see' that the wire is not connected to the pulse source.
- the MAP sensor has 3 wires, power, ground and signal, but it's signal is defined to have a value range for the full sensor output range and hence it will not indicate an error if not connected, you can check that the sensor works as before engine start the sensor should give an output indicating air pressure. In this case it presumably means no input value (0 Volt) corresponds to a pressure of 2.8 hg.
- the fuel pressure and oil pressure are 3 wire sensors power ground and sense, the sensor will give an output value (sense) that is only defined for a part of the possible output values, say it is a 5 Volt sensor and the valid outputs are only 1-3 volt for the valid pressure range, if you disconnect the cable then the voltage value that the EMS measures is not in the range 1-3 Volt and hence the EMS 'knows' that there is no proper sensor connected and it will indicate that by displaying a red cross on the screen.
- SAme is for the temperature sensor although this is only a 2 wire sensor (I think), again only a part of the possible resistance range matches valid temperatures and if a resistance is measured outside this range (infinite resistance if you disconnect) if will report an error
