The engine light/VSC light/trac light have been on since last week. (2008 RX-350 AWD 58000 miles). Went to advanced Auto and borrowed their OBDII. Two code: P0138 and p2195 (bank 1, sensor 1 and bank 1, sensor 2)
I called a local Lexus dealer. They are willing to check it out for me for 30 minutes, free of charge. After that, they will have to charge me for troubleshooting and then parts and labor.
Well, after doing some research, here are what I have done.
1. checked gap cap, make sure it is tight (once you hear a click, it's tight)
2. checked the air filter housing and air intake. did not see anything disconnected
3. Using the OBDII reader, I erased the engine code P0138 and P2195
All light went off but came back after driving about 10 minutes.
4. Replace Bank 1/sensor 1 : bought a Denso air/fuel ratio sensor (234-9042) online for about $172.
This is the sensor between the engine and the firewall. You will have to raise the car to be able to see it. I disconnected the battery negative cable just to be safe (optional; you may not need to do that)
5. Raised the front of the car. Did spray PB Blaster on the bottom of the sensor. Waited for about an hour.
6. Loose up the sensor with a O2 sensor socket ($8 on amazon), then used my hand to turn and get the sensor out.
7. Installed the new sensor.
8. Reconnected the battery
All lights are gone.
I tested the old sensor. The sensor is in pretty bad shape and it is dead! terminal 1 and 2 are not connected any more. Using an ohmmeter, one can measure the resistance between terminals 1 (HT) and 2 (+B). Under 68 F, the ohm reading on a good sensor should be between 1.8 and 3.4 ohms. My old sensor does not give any reading any more.

I am disappointed that a Lexus sensor is already dead after 58000 miles. The sensors on my other two cars (BMW and Honda) are still doing their job after 100K miles. But at least I do not have to buy the bank 1, sensor 2 at this point. It seems to me P0138 code (bank 1 sensor 2) was caused by the dead A/F sensor on bank 1.
Now the question is if the A/F sensor on bank 1 is in a bad shape, should I replace the A/F sensor on Bank2? (it's much easier to replace the one on bank2 as it's between the engine and radiator, you can see it after you open the hood)

 

Since its so easy to access, I would wait until you have an issue with it.

 

This is the A/F sensor terminal diagram. From the male plug, one can test a sensor without actually removing the sensor from a bank.