
Many manufacturers has built in fault codes to alert of any problems with the EFI sensors that tell the ECU how much fuel to inject at different rpm and load. Most engine sensors emit some noise in their signals and the problem is this value tolerance that the fault code looks for has to be large enough not to trip a false fault code due to noise but also small enough to know when the sensor is causing a problem. To avoid customer complaints generally this tolerance is on the large side so some performance problems can go unnoticed.