A diagnostic system for an engine includes an error detection module and a diagnostic module. The error detection module selectively detects a fuel control error based on a change in a fuel correction value used to determine a quantity of fuel supplied directly to an exhaust system. The diagnostic module identifies a cause of the fuel control error based on a period since a last refueling event. The diagnostic module identifies the cause as one of variation in an actual heating value of the fuel and faulty operation of a fuel injection system that supplies the fuel. The diagnostic module further identifies the cause based on a first amount of the fuel contained in a fuel tank at a start of the last refueling event and a second amount of the fuel added to the fuel tank during the last refueling event. A control system and method are also provided.