Electrochemical planar sensors using Nb2O5 powders to fabricate sensing electrodes were tested both in laboratory and “in-situ”, i.e. downstream a three-way catalytic converter of a FIAT fire 1242 c.c. spark ignition engine. Two cell configurations were compared, one with and the other without Pt paste under the metal oxide electrode. The best sensing performance both in laboratory and at the engine bench was obtained for the Pt | YSZ | Nb2O5 cell, without Pt below the oxide electrode. For this sensor configuration, very large EMF response values were obtained for hydrocarbon exposure, even at temperatures as high as 700°C.