UPDATE:
i took the car to the muffler shop and they located two exhaust leaks in between the oxygen sensors on both banks. i never realized how quiet this car is. fingers crossed that this was the issue causing all the check engine lights.

 

Good luck!

 

UPDATE:
lights came back on. code p0174 came back. im trying to find ways to keep it, but slowly thinking the car doesnt want to cooperate. i appreciate all the help so far.

 

Have you tried spraying the whole engine with brake cleaner? Try it! If the rpms rise then that may help you spot an air leak in the lines

 

See post #2 if you can't locate an air leak.

My fix was the downstream O2 sensor...I'm assuming yours are around 15 years old and have never been replaced.

 

do you have a part number for the downstream oxygen sensor?

 

I have a 2007 GS350 RWD. I purchased the Denso PN 234-4521 from RockAuto. This PN works for both passenger and driver's side on the RWD. I understand a AWD uses the 234-4521 on the passenger side, but uses a 234-4519 on the driver's side. These are the downstream O2 sensors, not the upstream sensors.

I also needed an O2 sensor removal socket specific for the Toyota, Schley Products, PN 66750B, bought at ToolDiscounter online.

My repeating failure was P0420 (passenger side). No exhaust leaks. The only way to keep the code off was by replacing the downstream O2 sensor.

You may also want to look at your upstream O2 sensor for that bank.

Good luck, hope this fixes your problem.