open-loop operation & O2 sensors
Regarding your problem with extended open-loop (closed-loop fault) operation. I found some interesting details in a TSB. The TSB is for the 2000 MY so some details my be different. What I wanted you to see is how complex the O2 system is and what items can prevent "closed loop" operation.
Note, I do not think they apply in your situation as there is no signal from the #21 O2 sensor (signal confirmed from #11 sensor). But this could be a fault in the O2 heater circuit.
>>from 2000 MY service CD
The HO2S Monitor is an on-board strategy designed to monitor the HO2S sensors for a malfunction or deterioration that can affect emissions. Under specific conditions, the fuel control or upstream HO2S sensors (Figures 1 and 3) are checked for proper output voltage and response rate (the time it takes to switch from lean to rich or rich to lean). Downstream HO2S sensors (Figures 1 and 3) used for Catalyst Monitor are also monitored for proper output voltage. Input is required from the ECT or CHT, IAT, MAF, TP and CKP sensors to activate the HO2S Monitor. The Fuel System Monitor and Misfire Detection Monitor must also have completed successfully before the HO2S Monitor is enabled.
The HO2S sensor senses the oxygen content in the exhaust flow and outputs a voltage between zero and 1.0 volt. Lean of stoichiometric (air/fuel ratio of approximately 14.7:1 for gasoline engines), the HO2S will generate a voltage between zero and 0.45 volt. Rich of stoichiometric, the HO2S will generate a voltage between 0.45 and 1.0 volt.
^ The HO2S Monitor evaluates both the upstream (Fuel Control) and downstream (Catalyst Monitor) HO2S for proper function.
^ Once the HO2S Monitor is enabled, the upstream H025 signal voltage amplitude and response frequency are checked. Excessive voltage is determined by comparing the HO2S signal voltage to a maximum calibratable threshold voltage.
^ A fixed frequency closed loop fuel control routine is executed and the upstream HO2S voltage amplitude and output response frequency are observed. A sample of the upstream HO2S signal is evaluated to determine if the sensor is capable of switching or has a slow response rate.
^ An HO2S heater circuit fault is determined by turning the heater on and off and looking for a corresponding change in the OSM and by measuring the current going through the heater circuit.
^ The MIL is activated after a fault is detected on two consecutive OBD II drive cycles.
The HO2S Monitor DTCs can be categorized as follows:
^ HO2S signal circuit malfunction - P0131, P0136, P0151, P0156
^ HO2S slow response rate - P0133, P0153
^ HO2S heater circuit malfunction - P0135, P0141, P0155, P0161
^ Downstream HO2S not running in on-demand self test - P1127
^ Swapped HO25 connectors - P1128 and P1129
^ HO2S lack of switching - P1130, P1131, P1132, P1150, P1151, P1152
^ HO2S lack of switching (sensor indicates lean) - P1137
^ HO2S lack of switching (sensor indicates rich) - P1138
Regarding your problem with extended open-loop (closed-loop fault) operation. I found some interesting details in a TSB. The TSB is for the 2000 MY so some details my be different. What I wanted you to see is how complex the O2 system is and what items can prevent "closed loop" operation.
Note, I do not think they apply in your situation as there is no signal from the #21 O2 sensor (signal confirmed from #11 sensor). But this could be a fault in the O2 heater circuit.
>>from 2000 MY service CD
The HO2S Monitor is an on-board strategy designed to monitor the HO2S sensors for a malfunction or deterioration that can affect emissions. Under specific conditions, the fuel control or upstream HO2S sensors (Figures 1 and 3) are checked for proper output voltage and response rate (the time it takes to switch from lean to rich or rich to lean). Downstream HO2S sensors (Figures 1 and 3) used for Catalyst Monitor are also monitored for proper output voltage. Input is required from the ECT or CHT, IAT, MAF, TP and CKP sensors to activate the HO2S Monitor. The Fuel System Monitor and Misfire Detection Monitor must also have completed successfully before the HO2S Monitor is enabled.
The HO2S sensor senses the oxygen content in the exhaust flow and outputs a voltage between zero and 1.0 volt. Lean of stoichiometric (air/fuel ratio of approximately 14.7:1 for gasoline engines), the HO2S will generate a voltage between zero and 0.45 volt. Rich of stoichiometric, the HO2S will generate a voltage between 0.45 and 1.0 volt.
^ The HO2S Monitor evaluates both the upstream (Fuel Control) and downstream (Catalyst Monitor) HO2S for proper function.
^ Once the HO2S Monitor is enabled, the upstream H025 signal voltage amplitude and response frequency are checked. Excessive voltage is determined by comparing the HO2S signal voltage to a maximum calibratable threshold voltage.
^ A fixed frequency closed loop fuel control routine is executed and the upstream HO2S voltage amplitude and output response frequency are observed. A sample of the upstream HO2S signal is evaluated to determine if the sensor is capable of switching or has a slow response rate.
^ An HO2S heater circuit fault is determined by turning the heater on and off and looking for a corresponding change in the OSM and by measuring the current going through the heater circuit.
^ The MIL is activated after a fault is detected on two consecutive OBD II drive cycles.
The HO2S Monitor DTCs can be categorized as follows:
^ HO2S signal circuit malfunction - P0131, P0136, P0151, P0156
^ HO2S slow response rate - P0133, P0153
^ HO2S heater circuit malfunction - P0135, P0141, P0155, P0161
^ Downstream HO2S not running in on-demand self test - P1127
^ Swapped HO25 connectors - P1128 and P1129
^ HO2S lack of switching - P1130, P1131, P1132, P1150, P1151, P1152
^ HO2S lack of switching (sensor indicates lean) - P1137
^ HO2S lack of switching (sensor indicates rich) - P1138