RX 300 no heat when temperature outside above 55 degrees

mudpie1963

I have a 2000 Lexus RX300. The heat only comes on when the temperature outside is below 55 degrees. If the temperature outside is above 56 or 60 degrees than only room temperature air blows out.

wwest

You're not quite stating that correctly.

First, the "heat" is ALWAYS ON....!!

There is NOTHING to block the flow of engine coolant to the heater core so the core is ALWAYS heated to, within a close range of 180F.

What you are more likely experiencing is the shear IDIOCY of the Lexus automatic climate control system. The system will automatically switch to "cooling" mode once the passenger cabin rises to within a few degrees of your temperature control setpoint.

That can be, IS, very discomforting on a cold winter night.

With the 2000 the best you can do is manually switch the outlet airflow back into footwell or combined footwell and dash mode.

Later models have a few corrective C-best options to help these matters.

CAUTION...!!

Winter is coming...

Sounds as if you may be new to the 2000 so be FULLY aware of these models to have a high propensity for sudden and unexpected, inadvertent, windshield fogging, especially during the wintertime.

My '01 F/awd RX300 has at least two of the c-best options and I have had the dealer set them so the A/C compressor cooling system can be turned off, indefinitely so, throughout the wintertime season, the second option "unlinks" the A/C from operating automatically in defrost/defog/demist mode.

Due to a climate control design flaw pre-dating Toyota's entire Lexus line operation of the A/C in a cool/cold climate can too easily lead to a very HAZARDOUS driving experience.

Even later models have a c-best option that prevents the system from automatically switching to cooling mode (coolish, DRY airflow from dash outlets via ~20F warmer air from footwell outlets) once the cabin warms up.

The DANGER of sudden windshield fogging rises preciptously if you happen to drive from an area 10-15F above freezing in an area at or below freezing with the A/C compressor system enabled. The early morning, coolish or COLD morning, instances of windshield fogging you will probably soon begin experiencing, if not already, are the result of YESTERDAY'S A/C operation and the accumulation of condensate on the 10,000 square inches of evaporator vane surface area.

Were I you I would not only be QUICK to manually switch the system to footwell mode, or even do so pre-emptively, I would add an electrical switch (as I have done with my '01 996/911 C4) to be sure the A/C compressor does not operate during times actual cooling is NOT needed.

Use of the A/C for purposes of dehumidification is TOTALLY dependent on local climatic conditions, therefore completely random, a WILD CARD, for which the clear majority of us have no "sense", or sensors for detection of the potential for DANGER.

mudpie1963

The heat will not come on even if I operate the heat controls manually if the temperature is above 56 degrees. The heat use to come out just fine when I put the control on hot manually. Is there some kind of sensor that needs to be replaced?

wwest

I would first check the IAT, Interior Air Temperature, sensor. It mounted behind that small "window" just above the driver's right knee.

Even so, if you put the system in max heat position all sensor should be over-ridden. If you have done that and still have no heat you likely have a vapor lock, an air bubble, trapped somehwhere in the heater core or hoses to/from.