Abstract: A method comprises inputting used oil analysis data to a pre-trained predictive model, the used oil analysis data including values quantifying a plurality of chemical components measured in a sample of used oil taken from an engine under analysis, determining a probability of at least one fail code with the pre-trained predictive model in response to the used oil analysis data, the at least one fail code corresponding to one of a plurality of predetermined engine failure types, providing the at least one fail code and the probability of the at least one fail code to an expert system, performing with the expert system a root cause analysis of the at least one fail code determine a root cause indicating a preventative maintenance action, and performing the predictive maintenance action on the engine under analysis.