Signals on the CAN BUS are two way, so not only is it possible to read the signals, like battery voltage or which door is open. You can also control features of the car over the CAN BUS like the air conditioning functions. The A/C is a common feature to control over the android headunit interface to the CAN BUS because the large screen often necessitate the removed access to these controls.
Steering wheel controls and the CAN BUS are the most common interface required. We have a post covering ‘What is the CAN BUS BOX with an Android Head Unit?’ that helps with explaining the CAN BUS box often supplied with a custom fit android headunit.
Not always CAN
Not all cars are CAN compatible. Early models could have propitiatory BUS systems that connect the various computers computer circuits together. You can also find more than one BUS in a single car.
The question arises, is it possible to connect an older, non-can compatible car to a CAN BUS of the android headunit?
There are some custom interfaces available that can convert non-can signals, like VAN or SSM to the standard CAN BUS that is then compatible with your android headunit CAN BOX.
PSA VAN BUS
morcibacsi has a github detailing the system he designed to interface the VAN bus in a year 2000 PSA model Citroen/Peugeot models. Often they are found with the RD45 stereo.The PSA VAN-CAN protocol bridge is a good place to start if you have a VAN based BUS and want to convert the signals to CAN.
The Factory Five Forum has details of the Subaru SSM to Can-Bus Convertor DIY project by Ajzride. The post explains the hardware required and links in the githib code that converts the Subaru SSM to CAN.
Thankfully modern cars are pretty much standard CAN BUS now. So if there is a custom fit android headunit available for your model, it will (or at least should) come with the correct CAN BUS box for easy connection to the car network. Having to build CAN converter circuits from scratch could be viewed as a fun project for some, very keen and motivated individuals. It is great to see the clever minds at work cooking up projects to solve problems. For everyone else, its either a CAN BUS box or nothing.