High temperature pressurized bleed air generated in the engine compressor is used for air conditioning in a jetliner. The engine bleed air must be regulated to a target temperature in the bleed air temperature control system using cold ram air before it can be used in the cabin or other low temperature area. The bleed air system is a control system that consists of sensors, and valve actuators. Potential faults associated with these components need to be considered in the design of the control system. This dissertation focuses on fault detection and diagnosis of the bleed air temperature control system. The faults in temperature sensors and valve actuators are detected using two unscented Kalman filters. The source of a fault is identified using the squared residuals. Nonlinear governing equations for the engine bleed air temperature control system are derived in state space form. Convergence analysis of the proposed fault detection and diagnosis methods is conducted. Heat exchanger is another core component in the bleed air temperature control system. A common fault associated with a heat exchanger is fouling which reduces the heat transfer efficiency, introduces additional resistance to flow, and increases fuel consumption. This dissertation presents a heat exchanger fouling detection method based on the valve control command of an engine bleed air temperature control system. Heat exchanger fouling is monitored by estimating the deviation of valve control command. A simulated bleed air temperature control system has been designed and built. Experiments have been conducted for temperature sensor fault, valve actuator fault, and heat exchanger fouling. Computer simulations for each of these cases are conducted and compared to experimental results.