Possible causes

– Faulty Air-Fuel Ratio (A/F) Sensor Bank 1 Sensor 1
– Air-Fuel Ratio (A/F) Sensor Bank 1 Sensor 1 harness is open shorted to ground
– Air-Fuel Ratio (A/F) Sensor Bank 1 Sensor 1 circuit poor electrical connection
– Air-Fuel Ratio (A/F) Sensor Bank 1 Sensor 1 circuit fuse
– Faulty Engine Control Module (ECM)

Tech notes

There is a factory service bulletin for the following Toyota Models:

2004 Toyota Sienna
2004 Toyota Sienna Factory Service Bulletin OBDII Code P0031

2004 Toyota Highlander
2004 Toyota Highlander Factory Service Bulletin OBDII Code P0031

2005 Toyota Avalon
2007 Toyota Avalon
2007 Toyota Camry
2008 Toyota Camry
2005 and 2007 Toyota Avalon, 2007 and 2008 Toyota Camry Service Bulletin OBDII Code P0031

When is the code detected?

The P0031 is set when the ECM detects an excessively low voltage through the heated oxygen sensor heater circuit

Symptoms

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

P0031 TOYOTA Description

Air-Fuel Ratio (A/F)sensor need to reach a minimum operating temperature of 1200 degrees F to produce an accurate voltage signal. The faster the air-fuel ratio 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 air-fuel ratio sensor. The ECM controls the air-fuel ratio 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.

http://cardiytool.com/images/2083.gif

TOYOTA Technical Service Bulletins for P0031 code