Apparatus and methods for Fault Detection Diagnostics (FDD) for a Heating, Ventilating, Air Conditioning (HVAC) system with an economizer to detect: 1) air temperature or relative humidity sensors failed/faulted, 2) dampers not modulating, 3) excessive outdoor air, 4) economizing when should not, 5) not economizing when should and 6) inadequate outdoor air. Economizer damper position is measured using at least one inertial Micro-Electro-Mechanical System (MEMS) device: 1) magnetometer, 2) accelerometer, 3) IMU and 4) rotatiometer. The magnetometer detects damper position using a small permanent-magnet attached to a movable damper and a MEMS device attached to the economizer frame. The accelerometer and IMU detect damper position using a MEMS device attached to the damper, and the ratiometer detects angular damper position. The methods determine whether or not the HVAC economizer, sensors, actuator and outdoor air dampers are functioning properly. If economizer faults are detected, then the method provides ele
