Econoline E350 1 Ton V8-5.4L Prop SOHC VIN Z (1998)
The H025 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 H025 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.
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The HO2S Monitor evaluates both the upstream (Fuel Control) and downstream (Catalyst Monitor) HO2S for proper function.
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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.
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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.
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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.
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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:
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HO2S signal circuit malfunction - P0131, P0136, P0151, P0156
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HO2S slow response rate - P0133, P0153
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HO2S heater circuit malfunction - P0135, P0141, P0155, P0161
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Downstream HO2S not running in on-demand self test - P1127
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Swapped H025 connectors - P1128 and P1129
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H025 lack of switching - P1130, P1131, P1132, P1150, P1151, P1152
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HO2S lack of switching (sensor indicates lean) - P1137
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H025 lack of switching (sensor indicates rich) - P1138
