Thermo sensor and check engine light
 

On a 1990 RX NA, will a faulty thermo sensor always throw a CEL code? Will it most of the time?

A faulty thermo sensor will give you a check engine light. Either check if the connector is plugged in or replace it.

I got a faulty thermosensor code and faulty O2 code when I unplugged my relief solenoid valve for diagnostic of a problem I was having.

From my experience, on S5's anyway, is that if anything is part of the ECU input loop for a particular function is lost, it will throw codes for all input devices associated with that input system. For example, part of the fuel mapping at idle is provided by the MAP, AFM and thermo. When I unplug the relief solenoid, the port air and split air no longer function at all, so the ECU sees voltage at the oxygen sensor during idle due to the lack of port air flowing. The ECU then throws an O2 sensor code, starts ignoring the thermosensor, and runs a base map just from the AFM and MAP. It has been a while since I did the test, so I may be a bit off, but that is basically how it worked/works.

Do you have any other codes showing? The thermosensor typically doesn't just die, but rather become somewhat inaccurate. I believe the ECU will only throw a code when the thermosensor is either shorted or open circuit.