How do I Troubleshoot my Heater in my Lexus SC300

First of all, since l is a Lexus, bring it to a dealer becaues they will know the car better then anyone. If u do not want to pay dealership prices, take it to a smaller privatly owned garage & have them diagnose it, it may take them longer because they may not be familar with that product. And if youdo not want to pay thoes prices, tell up the symptoms & do the guess & check method. It will most likly cost ur more in the long run, & cause more strees in ur life for the later. If the car is new, or still under warranty, take it to a dealer & have them diagnose the problem, even if it is not covered by warranty. Anything u do to that car that is not approved by the dealer, will void warranty because the dealer will want to pin that problem on what u have done. Many people will not agree with this, but anything to do with cooling/heating systems r never cheap for repair, & r usually recomended for a garage to deal with it. They r trained for it, & if anything happens, it can cause engine overheating, or electrical shorts/opens/grounds/voltage spikes/ect. The cheapest repair at a garage would be about $100 for a thermostat, radiator cap & top up, & it can lead anywhere up to $2000 or more, all depending on the car, & at my shop rates.

l have a 93 lexus sc 300 and had the same problem. it was the hearter control valve. l was able to jerry rige it and fix it myself.