Non-Nernstian electrochemical planar sensors with a Pt electrode and a sensing electrode made using Nb2O5 powders were fabricated. Two cell configurations were compared, one with and the other without Pt paste under the metal oxide electrode. The sensors were tested both in laboratory and in situ (i.e., downstream of a three-way catalytic converter of a FIAT fire 1242 c.c. spark ignition engine). The best sensing performance, both in the laboratory and at the engine bench, was obtained for the yttria-stabilized zirconia Pt parallel to YSZ parallel to Nb2O5 cell, without Pt below the oxide electrode. For this sensor configuration, very large electromotive force response values were obtained for hydrocarbon exposure both in the laboratory and in field tests, even at temperatures as large as 700 degrees C.