There are 4 O2 sensors on newer, 2 on older C5s.

B1S1 (precat)
B2S1 (precat)
B1S2 (aftercat)
B2S2 (aftercar)

The precat O2's are what actually determines the oxygen content as it directly relates to the
combustion process.

The aftercat O2 sensors simply monitor the function of the Cat. They are there to throw a code if the
Catalytic Converter falls out of spec. (ie, doesnt perform its function). The aftercat O2's will also throw
a code if they are faulty or not connected. They are not required for proper engine operation.

Are you getting erro code DTC P0137 ?

If so this is from the service manual:

The powertrain control module (PCM) supplies a voltage of about 450 mV between the HO2S high and
low signal circuits. The oxygen sensor varies the voltage over a range from about 1,000 mV when the
exhaust is rich, down through about 10 mV when the exhaust is lean.

The PCM monitors and stores the heated oxygen sensor (HO2S) voltage information. The PCM
evaluates the HO2S voltage samples in order to determine the amount of time the HO2S voltage was
out of range. The PCM compares the stored HO2S voltage samples taken within each sample period and
determines if majority of the samples are out of the operating range.

The PCM monitors the HO2S voltage for being fixed below a predetermined voltage. If the PCM detects
the voltage is below a predetermined voltage, a DTC sets.

Conditions for Running the DTC
Criteria 1
DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0125, P0200, P0335, P0336, P0351-P0358,
P1120, P1220, P1221, P1258 are not set.
The ignition voltage is more than 9.0 volts.
The fuel system is operating in Closed Loop.
The AIR and the catalyst diagnostics are not active.
The throttle position (TP) angle is between 2 percent and 70 percent.
OR

Criteria 2
DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0125, P0200, P0335, P0336, P0351-P0358,
P1120, P1220, P1221, P1258 are not set.
The ignition voltage is more than 9.0 volts.
The AIR and the catalyst diagnostics are not active.
The Power Enrichment mode is enabled for more than 1.0 second.
Conditions for Setting the DTC
Criteria 1
The HO2S signal voltage remains below 80 mV.
The Criteria 1 conditions are present for 80 seconds.
OR

Criteria 2
The HO2S signal voltage remains below 420 mV.
The Criteria 2 conditions are present for 10.0 seconds during the Power Enrichment mode.
Action Taken When the DTC Sets
The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that
the diagnostic runs and fails.
The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic
fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the
second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure.
The PCM writes the conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the
diagnostic runs and does not fail.
A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any
other emission related diagnostic.
Use a scan tool in order to clear the MIL and the DTC.
Diagnostic Aids

Important
Remove any debris from the PCM\TAC module connector surfaces before servicing the PCM\TAC
module. Inspect the PCM\TAC module connector gaskets when diagnosing/replacing the modules.
Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion into the
PCM\TAC modules.
For any test that requires probing the PCM or a component harness connector, use the Connector Test
Adapter Kit J 35616-A . Using this kit prevents damage to the harness/component terminals. Refer to
Using Connector Test Adapters in Wiring Systems.

The PCM sets this DTC when the vehicle runs out of fuel.
Heated oxygen sensor (HO2S) wire--Sensor pigtail may be in contact with the exhaust manifold or the
exhaust system.
An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provides the
supply of oxygen. Inspect the HO2S wires and connections for breaks or contamination. Refer to Wiring
Repairs in Wiring Systems.
Inspect for an intermittent ground in the signal wire between the connector and the sensor.
Lean injectors--Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 or
Fuel Injector Balance Test with Special Tool .
Fuel contamination--Water near the in-tank fuel pump inlet can be delivered to the injectors. The
water causes a lean exhaust and can set a DTC. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Fuel pressure--System will be lean if fuel pressure is low. Refer to Fuel System Diagnosis .
Exhaust leaks--An exhaust leak near the HO2S can cause a lean condition. Refer to Inspection
Procedure in Engine Exhaust.
Vacuum or crankcase leaks can cause a lean condition.
If the above are OK, the HO2S may be at fault.

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Team ZR-1
Teamzr1.com

[This message has been edited by teamzr1 (edited 02-14-2001).]