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?