Heated Oxygen Sensor


The logic module uses the oxygen sensor as part of the standard closed-loop mixture control system required on all gasoline engines equipped with a catalytic converter.  The sensor generates a voltage between 0V and 1V when it is heated to at least 662^F.  It does this by passing oxygen molecules from the outside air into the exhaust stream, so the less oxygen present in the exhaust, the higher the voltage it will produce.  A rich air/fuel mixture will result in very little oxygen in the exhaust (higher sensor output voltage), while a lean air/fuel mixture will result in a greater amount of oxygen in the exhaust (lower sensor output voltage).  Since the sensor output is very sensitive to changes in oxygen content, it has a very limited range.  Therefore, it is used as an indication of a "lean" or "rich" condition, and not as a linear indication of the air/fuel mixture.

Prior to 1987, the sensor was heated only by the exhaust stream (1-wire sensor).  The 1987 through 1988 models received the 3-wire sensor which had a PTC (positive temperature coefficient) heating element that regulated the sensor temperature at 932^F to 1292^F.  This prolonged sensor life by repelling contaminants and brought the sensor up to operating temperature more quickly.  At 77^F, the element should 5 to 10 ohms of resistance.  1989 and later models received the 4-wire sensor, which had an additional ground for the sensor element itself.  Previous designs grounded the sensor through the case on the exhaust manifold or turbo.  This proves to be a poor ground due to corrosion and the high electrical loads on the manifolds from the fuel injectors, ignition, etc.  The sensor element ground was connected to the common sensor ground wire (usually black with a blue tracer).  If you want to get the full 0 - 1V output range of the sensor (for an A/F gauge), it is a good idea to switch to the 4-wire unit and ground it to the battery (not the common sensor ground or the chassis).

The logic module interprets the oxygen sensor output in the following way.  A voltage less than 0.352V (0.391V for 1984 models) is considered "lean", while the voltage greater than 0.547V is considered "rich".  The voltage range in between these values is called the "neutral region" and is not considered to be useful information by the logic module.  The closed-loop "circuit" is part of the logic module's programming, and not a discreet electronic circuit on the circuit board.  The "circuit" simply reacts to the current "value" of the oxygen sensor (rich or lean) by taking the opposite action.  So if the sensor indicates "lean", the circuit will add fuel and vice-versa.  This happens at a rate of about once per second, depending on the condition of the sensor and the engine speed.  Things like leaded fuel, using non-"sensor safe" RTV (silicone compounds), octane booster, and burning oil can contaminate the sensor surface and slow down the reaction time of the circuit.  The circuit is said to be in closed-loop when it is given control over the engine air/fuel mixture in this way by the logic module.  The logic module will take control away from this circuit (open-loop) during cold-start and wide-open throttle (WOT).  The logic module runs the engine rich at WOT to keep combustion temperatures down and to suppress detonation.  The engine will run rich at cold start because it needs the extra fuel when cold (fuel does not atomize well in cold air) and the oxygen sensor needs time to come up to temperature.

Fault Codes

During a cold engine startup, the logic module will run the engine rich until the coolant sensor indicates a temperature greater than 89.6^F for 1984 models, 80.6^F for Turbo II models, and 95.0^F for all other models.  Then, the logic module will wait for 7.39 seconds before entering closed-loop mode.  After another delay of 59 seconds for 1984 models or 718 seconds (11 minutes, 58 seconds) for all other models, the logic module will begin monitoring the oxygen sensor output's neutral region.  If the sensor output stays in the neutral region for more than 21 seconds, a fault code 21 is stored.  The circuit stays in closed-loop and the logic module will still monitor the oxygen sensor signal and allow the engine to run rich, but prevent it from running lean.

Once in closed loop, if the oxygen sensor output indicates "lean" (below the neutral region) or "rich" (above then neutral region) for more than 121 seconds (2 minutes, 1 second), a fault code 51 or 52 is stored for 1985 models, respectively, or a fault code 51 for both conditions for 1984 models.  For all other models, if the oxygen sensor output indicates "lean" or "rich" for more than 715 seconds (11 minutes, 55 seconds), a fault code 51 or 52 is stored, respectively.  The circuit stays in closed-loop and the logic module will still monitor the oxygen sensor signal.  If the condition persists for four times what it took to set the code, the logic module will allow the engine to run rich, but prevent it from running lean.

Return to the Engine Control Unit page

Updated 12/19/2003.

Copyright © 1996-2003 Russ W. Knize.