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Hi All,
I have a Astra G 1.8 and have been getting a random check engine light coming on. Put the scanner on it and got P0140 O2 SENSOR CIRCUIT NO ACTIVITY - DETECTED BANK 1 SENSOR 2. I assumed sensor #1 is the one before the cat and sensor #2 is the one further back under the car?
The odd thing is that when I checked the live data sensor #1 was showing odd numbers, jumping between 0.135mV then 0.800mV then 0.095mV. Whereas sensor #2 was showing anywhere between 0.410mV to 0.425mV on the 10 minute test drive. Why would I get a fault code on sensor #2 when its reading seems in spec and no fault code for sensor #1 which seems well out of spec?
Could anyone provide some help with this before I spend money on a replacement O2 sensor.
 

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Hi
You are correct with your o2 sensor locations. The B1S1 appears to be working correctly switching from lean to rich quickly, as this is used to control the fuel mixture. The B1S2 monitors the catalytic converter efficiency and does not switch rich to lean quickly. The 02 sensors can become lazy with time and I have had it happen on a car I owned, and changing the sensor solved it. It could be the cat converter failing though
 
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