This work describes an in situ method for the testing of mechanical flow meters. The method was developed on diaphragm gas meters, but can be applied to most mechanically-based flow meters. The proposed fingerprint method analyses the behaviour of the meter during a work cycle. The result from the analysis is a ''fingerprint'' of the meter, in which mechanical errors in the meter can be detected. This is explained by the mechanical connection between the sensing mechanism and the mechanical output from the meter. An error analysis of a diaphragm meter was carried out to deduce the type of errors that are likely to occur in such meters. The most severe malfunctions are leakage, internal friction and liquid in the meter. These errors can either lead to measuring errors or can cause the meter to cease functioning. The fingerprint method has been tested on meters with simulated errors and on used meters taken from the gas grid in Lund. The fingerprint method can correctly diagnose errors such as leakage, increased friction and liquid in the meter.