The technology is now available for an onboard vehicular diagnostic system. Such a system has immediate economic benefit when applied to heavy long-haul trucks, in terms of reduced incidence of catastrophic downtime, reduced maintenance cost, and improved fuel economy. A design philosophy for a modular organization of the parameters measured is described. Attention is focused on a basic set of parameters useful in all heavy trucks. The state of the art of sensors available to implement such a basic module is described in detail. Fuel flow-rate sensing in a return-flow diesel injector system is noted as a special problem.