I am copying the following test from the 98 Contour Service CD. It helps explain how the computer checks the O2 sensors.
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The HO2S Monitor is an on-board strategy designed to monitor the HO2S sensors for a malfunction or deterioration which can affect emissions. The fuel control or upstream HO2S is checked for proper output voltage and response rate (the time it takes to switch from lean to rich and vice versa). Downstream HO2S used for Catalyst Monitor are also monitored for proper output voltage. The following illustration shows that input is required from the ECT, IAT, MAF 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.
1. 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), 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.
2. Once the HO2S Monitor is enabled, the upstream HO2S 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. A HO2S heater circuit fault is determined by turning the heater on and off and looking for a corresponding change in the Output State Monitor (OSM) and by measuring the current going through the heater circuit. The HO2S Monitor DTCs can be categorized as follows:
The DTCs associated with HO2S lack of switching are DTCs P1130, P1131, P1132, P1150, P1151 and P1152.
The DTCs associated with HO2S slow response rate are DTCs P0133 and P0153.
The DTCs associated with HO2S signal circuit malfunction are DTCs P0131, P0136, P0151 and P0156.
The DTCs associated with a HO2S heater circuit malfunction are DTCs P0135, P0141, P0155 and P0161.
The DTC associated with the downstream HO2S not running in on-demand is DTC P1127.
The DTCs associated with swapped HO2S connectors are DTCs P1128 and P1129.
3. The MIL is activated after a fault is detected on two consecutive drive cycles.
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You will notice that the P0171 and P0174 DTCs are not related to sensor failure. Those codes are generated when the adaptive limit has been reached. That means that the computer has tried to enrich the cylinders in question, but even after the limit of enrichment range has been reached, the sensor still shows the mixture as too lean.
Here is the diagnostic procedure for a P0174:
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41 DTCs P0172, P0171, P0174 AND P0175: FUEL SYSTEM AT THE CORRECTED FUEL TRIM
Diagnostic Trouble Codes (DTCs) P0171 bank 1 (Cylinder 1) and DTC P0174 bank 2 indicate the fuel/air ratio is too lean. The fuel trim system is at the rich correction limit.
DTC P0172 bank 1 and DTC P0175 bank 2 indicate the fuel/air ratio is too rich. The fuel trim system is at the lean correction limit.
DTCs HO2S Reference list:
-- HO2S-11 = DTCs P0171 and P0172
-- HO2S-21 = DTCs P0174 and P0175
Possible causes:
Fuel system
-- Excessive fuel pressure.
-- Leaking fuel injector(s).
-- Leaking fuel pressure regulator.
-- Low fuel pressure or running out of fuel.
-- Contaminated fuel injector(s).
-- Vapor management valve (VMV) leaking.
Induction system
-- Air leaks after the MAF.
-- Vacuum leaks (vacuum lines and gaskets).
-- Restricted air inlet.
-- PCV system.
-- Fuel purge system.
-- Improperly seated engine oil dipstick.
EGR System
-- leaking gasket.
-- Stuck open EGR valve.
-- Leaking diaphragm.
Base engine
-- Oil overfill.
-- Cam timing.
-- Cylinder compression.
-- Exhaust leaks before or near the HO2S(s).
l Check intake air system for leaks, obstructions and damage.
l Check air cleaner element, air cleaner housing for blockage.
l Verify integrity of the PCV system.
l Check for vacuum leaks.
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That ought to give you some information on what to do.
Steve
