Air Fuel Ratio Sensor Heater

This heater serves the same purpose as the oxygen sensor heater, but there are some very important differences.

Engines using two A/F sensors use a relay, called the A/F Relay, which is turned on simultaneously with the EFI Relay. This heater circuit carries up to 8 amperes (versus 2 amperes for 0, heater) to provide the additional heat needed by the A/F sensor.

This heater circuit is duty ratio controlled pulse width modulator (PMW) circuit. When cold, the duty ratio is high. The circuit is monitored for proper operation. If a malfunction is detected in the circuit, the heater is turned off. When this happens, the A/F sensor will not operate under most conditions and DTC P0125 will set.

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