Oxygen sensors work very efficiently when the sensing element temperature is above 750'F (400'C). At warm cruise, it is not difficult to maintain oxygen sensor temperatures at or above this point. However, when the engine is first started or when idling or when driving under very light load, the oxygen sensor can cool down, forcing the fuel system to return to open loop operation.
This allows feedback operation under conditions which might otherwise require open loop fuel control. The ECU monitors the following parameters and cycles the oxygen sensor heater on:
• When intake air flow is below a given point.
The oxygen sensor heater control system maintains sensor accuracy by turning on the heater element whenever intake air volume is low (exhaust temperatures are low under these conditions). By heating the sensor electrically, sensor detection performance is enhanced.
• coolant temperature is above approximately 32'F (O'C).
• specified time has elapsed after starting.
The oxygen sensor heater and ECU are wired as shown in the diagram. Whenever the above mentioned conditions are met, the ECU turns on the driver transistor to supply a ground path for heater current.
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