Lambda Probe, check engine light

["John D. Fox" <jdfox@xxxxxxxxxxxxxxxxx>]

Alan Lambert had some funny stuff with his O2 sensor - and wondered why it is no longer turning on his CHECK ENGINE light

- - essentially the criteria that the motronic computer uses to decide if the Lamdba probe is working has to do with the frequency response ( magnitude and phase) of the sensor voltage with respect to little test modulations of the injector duty cycles. What the computer is looking for is a LEAN going sensor voltage as it dithers the mixture leaner, and a rich going sensor voltage as it dithers the mixture richer. The system has two modes of testing - a gross "do we see ANY change of response" search - and a continous dithering of the mixture as the car runs closed loop in normal operation. As the sensors age, they get clogged up with stuff which reduces the active area of the electrode - which increases the source impedance. This slows down the time response of the system, which is continously adapted to via the dithering process.


I'd guess that the new probe you put on either had a lousy connection in it ( crummy ground connection, or intermittent signal or heater connections) that has somehow improved with vibration and heat , or the sensor got inadvertently contaminated with anti-seize during installation ( or contaminated in storage, or in operation due to some crud coating it in the exhaust system). With time, it looks like it is burning off the contamination, and the error light is less frequent.


If you look with a VOM, you could check that the heater terminals show continuity at some low ( 10 ohms or whatever level), and you could look in operation that the heater terminals have power. Also look at the sensor voltage while running - in closed loop mode you should see the system dithering the mixture around the 600 - 800 mV stoichiometric point. Sometimes the frequency of this dithering is called the "crossing counts/second"  by the test systems or diagnostic manuals.


If you pull out the trouble codes from the Motronic - you can see what IT was complaining about.

                                                 
                                           John Fox

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End of alfa-digest V7 #30
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