Line replaceable units (LRUs) of auxiliary power units (APUs) are parts which can be quickly swapped while in between flights. Some of the LRUs on the Honeywell 131-9B APU, particularly the startergenerator, cause a lot of operational problems and unscheduled removals of the APU, which is why their condition needs to be monitored. By monitoring certain flight parameters, most importantly the exhaust gas temperature, EGT, and the start time, the condition of the starter-generator can be evaluated, which can be used to improve the maintenance, repair and overhaul (MRO) process, reduce downtime and extend the overall life of the APU. The data from one engine start is used as a baseline for a healthy APU, after which a gas path model is used to estimate the reduction in power during startup of a degraded APU. With the power reduction of each component known, the increase in start time can be attributed to a degraded starter-generator and/or a degraded turbine. An increase in EGT of roughly 150 °C could potentially indicate the APU is unable to start due to a severely degraded turbine. Additionally, it has been noticed that trends in other engine parameters are linked to the condition of LRUs (for example a sudden spike in oil temperature is likely caused by a faulty temperature control valve).