Abstract Field measurements of NO x , HCs and CO concentrations on an engine test bench were performed on YSZ-based planar sensors. Sensors with two parallel Pt finger electrodes, one coated with WO 3 thick film as sensing electrode, were fabricated. The planar sensor was positioned close to a commercial λ sensor, downstream the three-way catalytic converter of a FIAT fire 1242 cc spark ignition engine coupled to a dynamometer. The performance of the gas sensors was measured at the stoichiometric air/fuel ratio ( A / F ∼ 14.3) and in lean ( A / F ∼ 15) and rich ( A / F ∼ 13) conditions under different engine regimes (rpm and torque). The response of gas sensors was compared to the response of the commercial λ probe and related to the exhaust gas concentrations measured by spectroscopic analytical equipment at the engine exhaust end. Preliminary measurements showed promising results in terms of selectivity, sensitivity, stability, reproducibility, and response time.