Sienna Check Engine/OBD II Code Question

The sensor heater circuit code would concern me more than the other. Might just need a new sensor as long as it has the proper voltage to it for the heater. And yes, one code can cause another

The air/Fuel ratio sensor (A/F) is is referred to as a wide range or wide ratio sensor because of it ability to detect air/fuel ratios over a wide range.
The avantage of using the A/F sensore is that the ECM can more accurately meter the fuel reducing emissions. To accomplish this, the A/F sensor:
-Operates at approximately 650*c, much hotter than the oxygen sensor 400*c
- Change its curent output in relation to the amount of oxygen in the exhaust stream.
The A/F sensor voltage output is opposite of what happens in the narrow range oxygen sensor.Voltage output through the detection circuit increases as the mixture gets leaner.
This is brief description of the A/F sensor.
As A/F sensor and oxygen sensor work together, a lazy oxygen sensor taking longer time to switch from rich to lean or vice versa can affect emissions .
check the oxygen sensor itself, and you will need a diagnostic chart to do the rest of the test.
check the oxygen sensor heater circuit .
the heater element resistance can be checked with a DVOM.
mesure the voltage signal of the A/F using OBII
The A?F sensor curent output cannot be accurately mesured directly.
good luck.

Thanks for the prompt responses, but I'm afraid I have one more possibly stupid question:

Are we talking about two separate sensors here, or are these just two codes referring to one possible malfunctioning sensor? Looks to me like there are three oxygen sensors - two in the exhaust manifold and one near the CC. Which one(s) am I talking about here?

Thanks again!

you have 2 oxygen sensors( bank 2 sensor 1) and ( bank 1 sensor 1)
and you have the heated oxygen sensor ( bank 1 sensor 2)
On V- types engines one sesor will be referred to as Bank I sensor1 and the other as Bank @ sensor 1.
The oxygen sensor after the catalytic converter is used by the ECM primarily to determine catalytic converter efficiency.
This sensor is referred to as sensor 2.
With two catalytic converters, one sensor will be Bank 1 sensor and the other as Bank 2 sensor 2.
check the two sensors.
for the heated Oxygen sensor (bank 1 sensor 2)
Use OBD II to mesure the voltage signal on the last one.
If OBD 11 scan tool is used, refer to the repair manual for conversion.
The A/F is designed so that at stoichiometry, there is no curent flow and the voltage put out by the detection circuit is 3.3 volts.Arich mixture, which leaves very little oxygen in the exhaust stream ,produces a negative current flow.The detection circuit will produce a voltage below 3.3 volts.A Lean mixture, which has more oxygen in the exhaust stream ,produces a positive curent flow.The detection circuit will now produice a voltage signal above 3.3 volt.
*check for vacuum leak,and EGR leak, Excessive fuel pressure, check both the sensors( Contamination or corrosion of the platinum electrodes or Zirconia elements will reduice the voltage signal output,contamination from the negine coolant,excessive oil consuption, additive used in sealants, and the wrong additives in gasoline,For the Oxygen Sensor Heater check the engine coolant temperature and engine load( determined from the MAF or MAP sensor signal) this heater circuit uses approximatly 2 amperes using a DVOM.

:) good luck