At the end of 2001, measurements indicated that the resistance between the conductors had fallen in SKB's (Svensk Kärnbränslehantering AB) electrically heated test canisters. Electrochemical impedance spectroscopy (EIS) was employed together with chemical analysis of gas compositions and corrosion products to investigate the causes of the resistance decrease inside the canister. The measured system is complicated and the evaluation not straightforward. However, it seems that the plug connecting the external electrical power supply to the heaters in the canister is a critical component and that a moisture film around the electrical contacts of the plug may explain the resistance decrease. It is suggested to continue to monitor the EIS spectra with time to gain more information about the cause of the resistance drop.