Heated Oxygen Sensor No. 1

Heated Oxygen Sensor No. 1 (HO2S1) is mounted in exhaust manifold in front of Warm-Up Three-Way Catalyst (WU-TWC). HO2S1 produces 100-900 mV under normal operating temperatures greater than 600°F (316°C). When HO2S1 is cold, normal voltage is 1275 mV (1250 mV on Prizm). PCM uses HO2S1 voltage to determine exhaust gas oxygen concentration during engine operation. Low voltage indicates a lean exhaust mixture and a higher voltage indicates a rich exhaust mixture. HO2S1 works similar to a non-heated sensor, except HO2S1 is heated to operating temperatures during engine operation.