Since I have both the 400 and 430 I visit both forums and found this thread on the 430 side very interesting:
https://www.clublexus.com/forums/ls4...-cleaning.html

I checked on my 400 and found we have the same type of sensor for the climate control system. While it may not serve the exact same purpose as the 430 it is there in about the same place, but about twice the size.
I followed the instructions in the thread and popped mine apart...and it definitely needed cleaning. Not really sure how it was working the filter was so filthy...
Just thought I'd pass on this little known maintenance tip....
Enjoy the ride....

Update: Found on 1998 - 2000 models and you can adjust the sensitivity of the sensor also.

 

In addition... you can also change the sensitivity...and it truly is a smog/exhaust gas sensor..

See below: (from the online Lexus Manual)

Each time you push the air intake control button, the
mode changes in order from the RECIRCULATED AIR
mode to AUTOMATIC mode to OUTSIDE AIR mode,
then back to the RECIRCULATED AIR mode. Each
indicator light shows which mode is now selected.
1 RECIRCULATED mode – If quick circulation of
cooled air is desired, select this mode.
2 EXHAUST GAS AUTOMATIC mode – If the
outside air contains harmful substances like
exhaust gas (i.e., when the vehicle running just in
front of your vehicle accelerates rapidly), the
system changes automatically from the OUTSIDE
AIR mode (with indicators 2 and 3 on) to the
RECIRCULATED mode (with indicators 1 and
2 on).
3 OUTSIDE AIR mode – The system will take fresh
outside air into the vehicle.
Turning off the air conditioning manually will cancel the
EXHAUST GAS AUTOMATIC mode. If you select the
“AUTO” mode during fan operating, the air conditioning
turns on and the EXHAUST GAS AUTOMATIC mode
will operate.
For normal use, it is best to keep the air intake control
button set to OUTSIDE AIR mode. If recirculated air is
used during heating, the windows will fog up more easily.
To change the sensitivity of the sensor switching
between the RECIRCULATED and OUTSIDE AIR in
the AUTOMATIC mode, push the air intake control
button until you hear a beep. The display changes
as illustrated below.
The sensitivity of the sensor can be adjusted only while
the vehicle has stopped.
When you push either side of the ”TEMP” button several
times, the sensitivity changes from ”+3” down to ”–3”. If
you do not operate any button for a few seconds, the
previous display returns.
If manual on–off of the air conditioning is desired –
Push the ”A/C” button to turn the air conditioning on and
push it again to turn the air conditioning off. If you turn
off the air conditioning manually, the EXHAUST GAS
AUTOMATIC mode will be cancelled.
If the outside temperature is lowered down to around
0 C (32 F), the air conditioning will turn off
automatically. At this time, the indicator on the A/C
button is off and the EXHAUST GAS AUTOMATIC
indicator light remains on. However, the EXHAUST
GAS AUTOMATIC mode does not operate. When the
air conditioning turns on as with the raised temperature,
the EXHAUST GAS AUTOMATIC mode resumes.
If the system is used for ventilation, heating in dry
weather or removing frost or exterior fog on the
windshield, turn the air conditioning off once it is no
longer required. This will improve fuel economy. The air
conditioning can be used for year–round automatic
temperature control including cooling and dehumidifying
operation.

 

good to know! where is the like button? haha

for what year lexus was the above sensitivity instructions for?

 

that was for the model year 2000... but pretty sure the 98-2000 would be the same.. not sure about the priors...