Heater works fine as long as car is moving. Once at idle, either in gear or on park. Heater blows cold air, even when set on HOT setting.
Target vehicle is the 2000 LS 400 with about 150k miles on it. Only had for about 6 months so not entirely familiar with the vehicle.
Possible solutions found were air gap in the coolant and bad thermostat.
Opening up the hood I found at least two things that pointed to air gap in the cooling system.
1. The overflow reservoir was only half full. I thought it was fine, but if you look closely at the tank the 'low' and 'full' marks are about 1/2 inch apart at the very top of the tank. The reservoir needs to be completely full with red coolant to operate correctly. I was not aware of this.
2. When squeezing the top radiator hose with my hands... there was no feel of fluid in the hose and air was easily pushed back into the reservoir. I could see and hear the air bubbles in the reservoir.
Using Toyota coolant I filled the reservoir to the top level. I then powered up the car and turned the heater on full 'hot' and the defrosters on. It was about 20 degrees outside at the time and slightly warmer in the garage with the door open. I then proceeded to squeeze the upper radiator hose to get the air out of the system. Then I'd refill the reservoir and repeat. I did this until the car got to operating temperature and could easily feel liquid in the upper hose as opposed to air. Took about 1/2 gallon of fluid.
When I finished, with the car never being driven and only idling in the garage, the heater was blowing hot air. I then replaced the cap on the reservoir and all things look good a few days later.
Hope you find this helpful.