Sequential firing of the fuel injectors according to the engine firing order is the most accurate and desirable method of regulating multi-port injection. However, it is also the most complex and expensive to design and manufacture. In this system, the injectors are controlled individually. Each cylinder receives one charge every two revolutions just before the intake valve opens. This means that the mixture is never static in the intake manifold. Along with the mixture adjustments that can be made almost simultaneously between the firing of one injector and the next. A camshaft signal sensor or a special distributor reference pulse informs the ECM when the No. 1 cylinder is on the compression stroke. If the sensor fails or the distributor reference pulse is interrupted in any way, some injector systems shut down, while others revert to pulsing the injectors simultaneously.