HEGO HO2S oxygen sensor location and catalyst efficiency diagram

OBD-II HEGO Identification & Location
Earlier systems (pre-'96) used only 1 HEGO immediately before the first cat.

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Before madly ripping out all the emissions systems on your vehicle, read this article.

Upper pane is for 4 HEGO systems (OBD-II). Middle pane is for 1 (EEC-IV) or 2 (inline engine OBD-II). Rare 3-HEGO system (not shown) for early OBD-II uses locations 11, 21, & 12 (single downstream at catalyst exit). Lower pane is Catalyst Monitoring Function.

'96 Bronco HEGO12 is at the outlet of the 2nd cat, just before the pipe hanger & flange for the muffler inlet.

My '93 5.8L uses Bosch HEGO F1UF-9F472-AA (DY-731), 0 258 003 384, 366 16 90, 3 F 16 (probably the date code: 1993 6th month 16th day).

The heated oxygen sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. A high concentration of oxygen (lean air/fuel ratio) in the exhaust produces a low voltage signal less than 0.4 volt. A low concentration of oxygen (rich air/fuel ratio) produces a high voltage signal greater than 0.6 volt. The HO2S provides feedback to the PCM indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The HO2S generates a voltage between 0.0 and 1.1 volts between the HEGO wire (Gy/LB) and SIGRET wire (Gy/R).

Embedded with the sensing element is the HO2S heater. The heating element heats the sensor to temperatures of 800C (1400°F). At approximately 300C (600°F) the engine can enter closed loop operation. The VPWR circuit supplies voltage to the heater and the PCM will complete the ground when the proper conditions occur. For model year 1998 a new HO2S heater and heater control system are installed on some vehicles. The high power heater reaches closed loop fuel control temperatures sooner, regardless of exhaust system temperature. The use of this heater requires that the HO2S heater control be duty cycled, to prevent damage to the heater. The 6 ohm design is not interchangeable with new style 3.3 ohm heater.

Owner caption April 2007 · Photo 1532 of 5000