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Car Heater won't blow and Thermostat Going Up and Down

Hi! My car (Lexus LS 430 2011) stopped blowing hot air when the heat is on and at the same time I noticed this break, the thermostat would jump to H while driving, then almost immediately back down to safe zone b/w H and C. Happens every 5-10 minutes or so with heat off and air either off or on-doesn’t seem to matter. Any suggestions? A broken fan?

When the engine is cold, remove the pressure cap from the coolant recovery tank.

Start the engine, and while the engine idles watch the coolant in the tank.

If bubbles begin to form in the coolant, that’s an indication of a blown head gasket.


Thank you for the reply! Would it still be a possible head gasket issue if the engine is not actually overheating? Just registering hot for a few seconds on the thermostat then back to normal? Thx!

The coolant level is very low. What year is your car? The last year for the LS 430 was 2006.

Yes, it is possible for there to be a problem with the head gaskets (you have two), even if the engine isn’t overheating.

This problem really needs the hands-on attention of a qualified mechanic, because we are all just giving our best guesses via cyberspace. When you go to that mechanic, just so that you don’t confuse the issue, it would be a good idea to use correct terminology. What you see “jumping to H” while you are driving is the temperature gauge, not the thermostat.

The thermostat is inside the engine’s cooling system, and it controls the flow of coolant. If you tell a mechanic that you have a problem with your thermostat, you just might confuse the issue, thereby delaying his diagnosis and increasing your bill.

Yes. Engine combustion gasses under pressure coming into the cooling system can do havoc there. There could also be coolant being lost out the exhaust. When the engine has cooled, check inside the radiator and at the overflow tank and top up if needed. A bad HG should be fixed sooner not later for best engine life.

Sorry! Correction: it’s a Lexus LS 460 (not 430) and it’s a 2009.

Good tip-thank you for the help with terminology. Since there is a themostat elsewhere in the car, I need to make sure I use the right wording when I take it in.

Refill the cooling system, if the coolant is 3 quarts low the heater won’t work on that car.

If the coolant keeps running low and you can’t see any leaks the engine valley plate might be leaking, that is a common problem on that engine.