Amperometric mode of electrochemical gas sensor requires several systematic approaches to optimize compositions of electrodes and electrolyte materials when both the electrodes are exposed to same gas environment. Using Gd-doped ceria electrolyte, Ni2+ doped CuO active electrode and fixed inactive electrode La0.5Sr0.5CoO3 electrochemical sensors were investigated for 3–40 ppm ammonia in base gas (5%O2 + N2). Highest sensitivity of 225 μA/decade at 550 °C was obtained for sensor with 15 mol% of Gd-doped ceria (GDC 15) and 2 mol% of Ni2+ doped CuO as electrolyte and active electrode, respectively, when studied in the range 300–650 °C. Among all the electrolyte compositions, GDC 15 showed highest electrical conductivity (0.84 × 10–2 S/cm at 550 °C), and thus, highest sensitivity was correlated at a particular temperature. The vacancy trap mechanism was found to influence electrical conductivity of electrolyte. The same sensor configuration showed highest potentiometric sensitivity of 116 mV/decade at 550 °C. High sensitivity to ppm level NH3, selectivity, short-term and long-term stability are some of the promising results of the developed sensor for exhaust gas applications. Graphical abstract: [Figure not available: see fulltext.] © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.