B3000 SE Regular Cab 4WD V6-3.0L OHV (1998)
Heated Oxygen Sensor (HO2S) Monitor Description
HEATED OXYGEN SENSOR (H025) MONITOR DESCRIPTION
The Heated Oxygen Sensor (H02S) Monitor detects deterioration of oxygen sensor performance. The oxygen sensor creates a specific voltage (between
zero and one volt) in accordance with the current Air/Fuel ratio by sensing the oxygen content in the exhaust. Lean of stoichiometric, the HO2S will
generate a voltage between 0.00 and 0.45 volt. Rich of stoichiometric, the HO2S will generate a voltage between 0.45 and 1.00 volt. This function
deteriorates as time and mileage increase. The HO2S Monitor determines there is a problem if sensor response is slow, absent or fixed. The Fuel System
Monitor and the Misfire Monitor must pass before the HO2S Monitor will run.
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The PCM monitors and compares both the front and rear oxygen sensors.The front oxygen sensor (upstream) is used for fuel injection control. The
rear oxygen sensor (downstream) is used to monitor the condition of the Catalytic Converter.
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The Heated Oxygen Sensor Monitor evaluates the upstream and downstream oxygen sensors for proper function. After both the Misfire and Fuel
System Monitors have run and passed, the Heated Oxygen Sensor Monitor will run. This monitor checks the peak voltage and the frequency of
voltage change. An oxygen sensor will fail this monitor if the voltage is above a programmed threshold (too high volts) or if the sensor is lazy with
too slow a response frequency.
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Oxygen sensors do not provide a usable signal until they reach 600 degrees F (316 degrees C). It is necessary to heat oxygen sensors so they can
provide a reliable feedback signal as quickly as possible after a cold start.
