On-board diagnostics tests are incredibly useful when it comes to �listening� to your car. Typically, a full on-board diagnostic test of your vehicle can provide vital information about the following things: Engine management issues Transmission problems Any faults with your ignition coils such as firing failure or low/high rpm levels Throttle performance Exhaust quality Brake health Engine coolant levels How well the fuel injection system is performing Ignition timing And so much more While on-board diagnostic tests are primarily geared towards the engine, depending on the vehicle, information about additional features such as the lights, tyres, and comfort controls can be gathered as well.