An approach based on Electrochemical Impedance Spectroscopy (EIS) was formulated and investigated to understand the most efficient fuel conditions for suitable long term operation of a solid oxide fuel cell stack under power generation conditions. The procedure helped to reflect the individual effects of CO/H₂ volumetric ratio, S/C ratio and fuel utilization under the presence of a simulated alternative fuel at 0.4 A cm^-2. Signature impedance plots were obtained with variation of each parameters and subsequent circuit modelling was used to calculate the electrochemical parameters associated with the plots. It was seen that the impedance plots show an additional arc at lower frequencies attributed to the diffusion of CO during the water gas shift reaction near the anode (CO + H₂O → H₂ + CO). Tests with various CO/H₂ ratio helped to narrow down the operating regimes where CO electro-oxidation was expected to take an important part with H₂ oxidation. S/C ratios less than 2 were tested to understand the limits of non-carbon interaction regions with the anode, without reducing system efficiency. Optimum fuel utilization was obtained for cell operation, highlighting regions having a negative effect on performance. Finally, 8 different combinations of the optimized three parameters were tested long term with the stack, and the most efficient blend was determined in terms of degradation of power in 500 h.