Lexus heater blows cold

Check your coolant if it shows full you might want to check your thermostat you would know if the heater core was disconnected. those would be the most obvious answer also is hour heater flap operating

It could be the heater control valve make sure the vacuum line is connected to the valve and that it is working properly. If it is not replace the valve. Hope this helps.

SOMEHOW I BELIEVE YOUR TWO INTAKE & RETURN HEATER HOSES THAT FEED THE HEATER CORE HAVE BEEN REVERSED. EITHER THAT OR a plugged heater core

you may be low on coolant, if not a common tell tale sign of a faulty thermostat is the car staying on the cold indictaor for a very long period of time especially after acceleration., some other issues may be a clogged heater core, however the most likely culprit is low anti freeze or a malfunctioning thermostat.

Check your coolant level and make sure its full, next i would look into replacing your thermostat, if your thermostat is stuck open your engine isnt able to reach operating temperature therefore heater output suffers, your coolant temp gauge should read around halfway when the engine is hot if its not look into replacing the thermostat, if your gauge is reading halfway check the cable going to your heater box when you adjust your temperature on your heater controls its possible that the cable broke or the blend door inside the heater box is stuck into the cold position allowing only cold air to come through the vents.

Follow the given link, maybe you get something useful. 1993 Lexus LS400 Service Repair Manual Download

Sounds like a thermostat problem replace it

Yes, a non-certified person could replace something on the car. The person has to understand what he/she's doing, though.
Best way: open the radiator cap and drain the coolant. Leave the radiator cap off until done.
Place a container under the radiator to catch the coolant.
Replace the thermostat, and refill the radiator.

the heater core could be clogged you might again check your coolant level just to be sure its full

Check your coolant level sounds like it might be low.