These have been given many names, all identify the same item on the engine:
• Oxygen Sensor (O2) or (O2S) or (O2′s)
• Exhaust Gas Oxygen Sensor (EGO)
• Heated Oxygen Sensor (HO2)
• Heated Exhaust Gas Oxygen Sensor (HEGOS)
Not all Oxygen sensors are heated, so using the word “heated” might not be correct for your vehicle application.

What is a “Rich Mixture,” “Ideal Mixture,” and a “Lean Mixture” all depends on what conext you use it in. The ideal mixture is the amount of fuel needed to make an engine perform as commanded. A rich mixture is when there is more fuel than needed, and lean mixture is not enough fuel.

The HEGO Sensor 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 volts. A low concentration of oxygen (rich air/fuel ratio) produces a high voltage signal greater than 0.6 volts. The 02 provides feedback to the computer (EEC) indicating air/fuel ratio in order to achieve a near stoichiometric air/fuel ratio of 14.7:1 during closed loop engine operation. The 02 generates a voltage between 0.0 and 1.1 volts.

Diagnostic Trouble Code (DTC)	HEGO Location	Fault Definition
172 / 41 KOER	Right or Rear	HEGO sensor indicates system lean.
136 / 91 KOER	Left or Front
176 CM	Left or Front
173 / 42 KOER	Right or Rear	HEGO sensor indicates system rich.
137 / 92 KOER	Left or Front
177 CM	Left or Front
144 / 41 CM	Right or Rear	No HEGO sensor switching detected or disconnected.
139 / 91 CM	Left or Front
171 CM	Right or Rear	Adaptive Fuel Limit reached.
175 CM	Left or Front

All of the following items will cause a HEGO code:
• Check engine oil for fuel contamination.
• Check fuel pressure.
• Check fuel systems ability to hold pressure for 60 seconds.
• Engines general condition.
• Fuel injectors (leaky, stuck, dirty).
• Secondary Air Injection (AIR) system.
• PCV / Hose stuck or blocked.
• Vacuum leaks.
• Fuel Injector O-Rings.
• CANP.
• MAP/MAF sensor.
• Ignition Coil Failure.
• Wiring harness general condition.
• Old HEGO sensors (should be replaced every 60,000 miles).