Fault detection and isolation fuel system lean monitor rationalized with manifold absolute pressure sensor

A lean operation fault detection and isolation (FDI) method for an engine of a vehicle, the engine having a fuel system comprising at least one fuel injector, the lean operation FDI method comprising:

• receiving, by a controller and from an intake manifold absolute pressure (MAP) sensor, a measured MAP indicative of a pressure of air in an intake manifold of the engine, the air having been drawn into the intake manifold through an air intake;
• detecting, by the controller, a lean operation fault where the engine is operating with a lean air/fuel ratio;
• estimating, by the controller and using an observer, (i) an air/fuel ratio of an exhaust gas produced by the engine and (ii) the MAP;
• monitoring, by the controller, first and second residual values indicative of differences between (i) the estimated air/fuel ratio of the exhaust gas and a measured air/fuel ratio of the exhaust gas from an exhaust oxygen (O2) sensor and (ii) the estimated MAP and the measured MAP from the MAP sensor, respectively;
• and
• based on the monitoring of the first and second residual values, determining, by the controller, which of (i) the air intake, (ii) the exhaust O2 sensor, and (iii) the fuel injector is malfunctioning and causing the lean operation fault.