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 necessitates the removal of 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 explain the CAN BUS box is 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. You can also find more than one BUS in a single car.
Is it possible to connect an older, non-can-compatible car to a CAN BUS of the Android headunit?
Some custom interfaces can convert non-can signals like VAN or SSM to the standard CAN bus 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 the year 2000 PSA model Citroen/Peugeot models. Often they are found with the RD45 stereo. The PSA VAN-CAN protocol bridge is an excellent 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 GitHub code that converts the Subaru SSM to CAN.
Thankfully modern cars are pretty much standard CAN BUS now. So if a custom-fit Android headunit is available for your model, it will (or at least should) come with the correct CAN BUS box for easy connection to the car network. Building CAN converter circuits from scratch could be fun for some keen and motivated individuals. You see clever minds at work cooking up projects to solve problems. For everyone else, it’s either a CAN BUS BOX or nothing.