Fabrication of a highly sensitive amperometric electrochemical gas sensor would require optimisation of the electrode materials and rigorous testing of device performance under different processing conditions. An ammonia sensor based on lanthanum gallate electrolyte having the composition La0.8Sr0.2Ga0.8Mg0.1Ni0.1O3 (LSGMN) has been investigated for its detecting at ppm level. A single phase solid solution of 30 mol% Zr4+ in CeO2 (CZ73) as an active (sensing) electrode yielded optimum results. Similarly, La0.5Sr0.5Mn0.8Ni0.2O3 (LSMN5582) was found to be the best composition for the inactive (reference) electrode. All the devices were found to have maximum sensitivity at 400 °C and a typical response and recovery times of 40 s and 110 s respectively. Moreover, they exhibited better efficiency in amperometric mode as compared to potentiometric mode and were stable up to several cycles of operation. It was established through mechanistic studies that even though both the electrodes were simultaneously exposed to both the gases, higher sensitivity was obtained when CZ73 was biased at +1 V with respect to LSMN5582 as against −1 V, implying that NH3 oxidation took place preferentially at CZ73 whereas oxygen reduction took place at the inactive electrode. Step by step sensitivity at 400 °C improved from 28.2 to 35 μA/decade. Further, the electrolyte thickness was decreased to 0.4 mm in view of reducing the internal resistance and a prototype device was fabricated that yielded enhanced sensitivity of 49 μA/decade under optimized conditions. © 2017 Elsevier B.V.