Possible causes

– Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 fuse
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 harness is open or shorted
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit poor electrical connection
– Faulty Engine Control Module (ECM)

When is the code detected?

The P0038 is set when the ECM detects an excessively High voltage through the heated oxygen sensor heater circuit

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0038 TOYOTA Description

Oxygen Sensors (O2S) or Heated Oxygen Sensors (HO2S) need to reach a minimum operating temperature of 750 degrees F to produce an accurate voltage signal. The faster the heated oxygen sensor reaches that temperature the faster the sensor will start sending an accurate signal to the Engine Control Module (ECM).
In order to achieve the require temperature, a heater element is included inside the heated oxygen sensor. The ECM controls the heated oxygen sensor heater element based on signals from the engine coolant temperature and engine load. The ECM controls the heater element circuit by allowing current flow to ground.
The ECM monitors the voltage signal received through the heater element circuit and determines the state of the circuit by comparing the voltage detected with the factory specifications.

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TOYOTA Technical Service Bulletins for P0038 code