The present invention pertains to methods for monitoring a thermoelectric heating and cooling device (103, 203, 303) of a system (100, 200, 300) for cycling liquid reaction mixtures through a series of temperature excursions, comprising applying a first quantity selected from an electric current and an electric voltage to, in a first case, said heating and cooling device (103, 203) or, in a second case, to a portion (314) of said heating and cooling device (303), and measuring a second quantity selected from the non-selected first quantity and temperature to obtain a first test value; applying the selected first quantity to, in the first case, another thermoelectric heating and cooling device (104, 204) or, in the second case, another portion (315) of the heating and cooling device (303) and measuring the second quantity to obtain a second test value; determining a monitoring value based on a comparison of said first and second test values; and comparing said monitoring value with a pre-defined threshold value for said monitoring value to obtain a monitoring result. The present invention further relates to systems for cycling liquid reaction mixtures through a series of temperature excursions, adapted to perform such method.