Applying the solid oxide electrolysis cell (SOEC) for CO2 electrolysis has great potential for efficient large-scale on-site CO production. However, ppb-levels of impurities, e.g. sulphur compounds, can be detrimental for a SOEC operated with CO2. In this work, we tested and analyzed different SOECs in CO2/CO mixtures, applying different gas qualities at industrially relevant conditions and compared with similar operation in steam electrolysis. While stable long-term operation is achieved for H2O/H2 test conditions, detrimental degradation occurs for the CO2/CO tested cells. We demonstrate that different cells show similar high degradation rates when tested under conditions where part of the produced CO is recycled and re-used as feed along with fresh CO2. This imply that it is not impurities in the CO, but in the CO2 that initiate the observed degradation. The degradation can exclusively be attributed to an increase in polarization resistance from the Ni/yttria-stabilized zirconia based fuel electrodes. Post-mortem analyses of cells reveal carbon deposits in the fuel electrodes, despite operating thermodynamically far from carbon-inducing conditions. Finally, effect of cleaning the inlet CO2 gas stream to mitigate the detrimental fuel electrode degradation is reported.