Why use a pull-up resistor
If nothing is connected to an ECU pin, the input is "floating" (not pulled to GND or pulled to VCC).
To prevent the above this "unknown state", a pull-up or pull-down resistor will ensure that the input pin is either high or low.
Pull-up on AIN (0-5V) for temperature sensors
In some cases, you want to wire an temperature sensor into a regular 0-5V input, then an external pull-up resistor must be wired from the +5V power supply to the AIN (0-5V) input wire.
Example wiring for external pullup resistor to +5V power supply.
