Oxygen Sensor Output - Negative Value???
 

Replaced both oxygen sensors in a two sensor system with new Bosch units following fault code displays indicating that both sensors and sensor control units were bad.

Now get a fault code that only one sensor is bad. No control unit fault codes are displayed.

With engine fully warmed, data stream for sensor one is -0.15 to -0.11 volts. Data stream for second sensor is positive and appears normal. With engine off, sensor 1 reads -0.11 v; sensor 2, 0.43 v.

Value for oxygen integrator 1 is stable at approx 1.000075, with essentially no fluctuation. Integrator 2 fluctuates from approx. 0.99xxxx to1.028xxx.


Is this negative voltage reading symptomatic of a faulty oxygen sensor, or does the fault reside elsewhere, e.g., control unit?

First of what model & year car?? Most of the engines with ME fuel systems will read negative O2 sensor voltage, but they will cycle to positive as well.
ALSO depending on year model universal sensors WON'T work properly. Even a 1997 E320 uses different sensors than a 1996.

Vehicle is a '95 BMW 540iA (M60 V-8 motor) which is equipped with version M3.3 Motronic DME. Model-specific oxygen sensors were employed. Both sensors are located downstream of the cats.

Am more interested in the technical aspects of this problem rather than an actual model specific answer.... was unware that lambda voltages could dip into negative values. In this particular case, however, sensor 1's voltages remain negative at all times.

From the link that Arthur has kindly posted, a negative value is indicative of a defective sensor.

HAS the short block been replaced on your car?? BMW used a different type of aluminum alloy on the 1993 thru 1995 M60's & ALL of them will erode the cylinder walls and create all kinds of problems!! Check engine codes for O2 problems show up FIRST!!

Also on that engine there is a plastic crankcase "breather" plate on the rear on the intake that causes simular problems.