DTC 21- Oxygen Sensor Circuit

This code sets when the main oxygen sensor signal voltage is reduced between .35-.70 volt for 60 seconds under the following conditions:

Engine coolant temperature at 176°F (80°C) or greater.
Engine speed at 1500 RPM or greater.
Engine load.
Main oxygen sensor signal voltage alternating at greater than or less than .45 volt.

Fig 1: DTC 21 - Schematic

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Fig 2: DTC 21 - Detection Driving Pattern

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Fig 3: DTC 21 - Inspection Using Oscilloscope

Fig 4: DTC 21 - Diagnostic Flowchart