Heater problem

Sounds like the car is low on coolant. Meaning that there is a leak somewhere. When the engine is cold, open the hood and take off the radiator cap. If you do not see coolant in there, then there is a leak. You will need to run a cooling system pressure test. Basically, you are going to hook an air pump up to the radiator and apply pressure. If the car is leaking, (1) it will not hold pressure and (2) it will be leaking somewhere. Follow the leak.

You can borrow the pressure tester (pump) from AutoZone, Advance Auto, or O'Reilly auto parts for free with a deposit. Here is a how to guide: https://www.2carpros.com/articles/radiator-pressure-test

Start there and let us know what you find. If you do see a leak, take some pictures and post them here. Then we might be able to identify the offending part.

Full of coolant no leaks. Only heats up when temperature control is turned to warm instead of cold.

Hmm, could be a clogged heater core. You can try flushing it out with a rather unorthodox trick that I use. I am attaching a link to a general guide on replacing heater cores. What you need from that is the picture showing the heater hoses. You would disconnect those hoses at their other ends, hook your garden hose up to one hose, point the other in a direction that it will not get important components wet. And then turn the water on low pressure.

This will push some muddy cruddy looking stuff out. When that stuff comes out, turn the pressure up. Let the water run for a couple of minutes after it is cleared up. Then turn the water off and hook the hoses back up. Refill the cooling system and see how it does now.

Link: https://www.2carpros.com/articles/replace-heater-core

Heater core or possibly deverter valve is my guess, but I put the car in the shop for them to trace down the problem. I am too old to be crawling around working on it myself. I will post what was discovered when they find it. Thanks everyone for your in put.

No problem. Please do let us know what it is. We always want to know, and the information will be there for others in the same situation.

Well shop had car for 2 full days and couldn’t duplicate problem or even get it to get hot so mystery remains

Well I would still suggest flushing the core. If that doesn't do it, then look at the diverter valve.

So far it’s working fine. Yes I agree about flushing heater core and will do that at my first opportunity

Well turns out it continued to happen until I replaced water temperature control switch. Finally appears to be fixed

Hmm interesting. I hope that fixed it for you. Please keep us posted and come back to this site if you need us in the future.

Thanks