For the oxygen sensor to deliver accurate voltage signals quickly, the sensor needs to be heated. A PTC element inside the oxygen sensor heats up as current passes through it. The ECM turns on the circuit based on engine coolant temperature and engine load (determined from the MAF or MAP sensor signal). This heater circuit uses approximately 2 amperes.
The heater element resistance can be checked with a DVOM. The higher the temperature of the heater, the greater the resistance.
The oxygen sensor heater circuit is monitored by the ECM for proper operation. If a malfunction is detected, the circuit is turned off. When this happens, the oxygen sensor will produce little or no voltage, and possible set DTC P0125.
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