My 1996 Volvo 850 (74K miles) has been providing OBD Error codes (P137, P131)that indicate "O2 Sensor Circuit, Low Voltage, Bank 1 Sensor 1" and "O2 Sensor Circuit, Low Voltage, Bank 1 Sensor 2." A couple of weeks ago, based on the mileage, I thought it prudent to renew the O2 sensor. I checked the voltage at the battery (it was fine: 12.34 VDC with engine off and 14.45 VDC with the engine at idle). Of course, this model has two such sensors. I replaced the one upstream from the Catylitic Converter, and the performance was good however, now I am getting the two codes (above). My question is: Do I have to replace both O2 sensors at the same time? Are the latest OBD II codes a result of simply replacing ONE of the TWO O2 sensors?
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