For efficient operation of the catalytic converter, the mixture ratio must be maintained close to the stoichiometric ratio - 14.7 parts of air to one part of fuel.
Efficiency of combustion can be monitored by measuring the percentage of oxygen in the exhaust gas. A high percentage may mean the mixture entering the cylinder is too lean.
The oxygen sensor in the exhaust manifold, or in the engine pipe, tells the ECU how much oxygen is in the exhaust gas. The ECU then varies injector opening time to achieve a correct mixture ratio.
Feedback also lets fuel settings change as components age. This is called adaptive learning. The ECU memorizes its fuel settings for different operating conditions, and stores them for future use.If a fault occurs in a component or part of a system, a fault code can be displayed on the check engine lamp in front of the driver.
Alternatively, a scanning tool can be connected to a diagnostic terminal, and information on the fault can be analyzed.
Although most injectors operate at the nominal battery voltage of 12 volts, many sensors have a reference voltage of 5 volts. This is done by a voltage regulator in the control unit. It gives more accurate signaling, as fluctuations due to changes in battery voltage do not occur.