I have a 2000 cirrus. I lost heat suddenly on Monday night. The gauge read halfway to H.

Today I had a breath of heat for 30 seconds and it went to cold. I drove around a while to see what would happen. The gauge went all the way to H. It did not overheat though.

Could it be the thermostat? What else could it be?

Thanks.

Maggie

if the gauge went all the way to hot and there is not heat i assume that the vehicle is low/out of coolant...
and it should overheat if thats the case......
first have the coolant level checked....if low add and check for leaks......
<<<<<<don't drive it anymore until your sure it has coolant in it....>>>>>>>.

It still has coolant. there is a slight leak.

Is it water pump or thermostat?

yes it could be the thermostat.......the upper radiator hose should be hot to the touch if the thermostat is open ...if it is hot,check both heater hoses both should be hot if coolant is circulating thru the heater core...
if not suspect thermostat...
you said small leak?? like how small..seepage??/
any decent leak can cause a pressure loss. pressure loss can result in coolant not circulating properly...
remember coolant/water flows rapidly under pressure...