Gaseous emission measurements of a diesel engine including hydrocarbons (HC), carbon monoxide (CO), and oxide of nitrogen (NOx) are made in accordance with the procedures specified in the Federal Register. However, it is very difficult to maintain constantly the accuracy of these emission measurements due to failure of the emission analyzer. The authors have thus developed the Pre-Alarm Diagnostic System for a Diesel Emission Analyzer.Firstly, the authors have carried out analysis of all failure modes that are classified into initial failure, random failure and wearing-out failure, plus tolerance level for prediction of failures and method of predicting failures.Next, the authors have developed the Pre-Alarm Diagnostic System that is able to easily discover these failures before the exhaust emission test. In this system, 40 sensors, such as pressure, temperature, voltage etc, are laid in each pipe line of connection between exhaust emission sampling pump and analyzer. The signals of these sensors are analysed and various failures are predicted by computer.This paper describes the points to which very careful consideration was given during the development of the Pre-Alarm Diagnostic System.