Symptoms of a bad or stuck thermostat - 2002 Jeep Grand Cherokee

T stat stuck open will not let the engine get up to operating temp. Stuck closed, the engine overheats. You would see the on the temp gauge. If the temp control is bad, then it won't open or close the blend door to regulate the temp. Could be the blend door motor also.

Yes, that could easily be a thermostat going bad. Listen for you water pump as well. Symptom of that going bad is a high pitch squeal upon start up.

no, the fans are temperature activated. all electric fans are.
no year car stated. so no real help.
cracks leak AF
and the level drops
air in water jacks are overheat , for sure.
check the rad level each dead cold, morning, at the filler neck on rad.
if it's low each day, there is a leak. inside or outside.

bad thermostat,
so it can be stuck open, stuck closed, be way off temp or stuck ,or just too slow, at anywhere, its 100% mechanical and do all that. at any time , and will.
on cars 1996 and newer in use we can plot the thermostat temp
with any scan tool. 180F or more is good.
here is my double bad thermostat. (I love the truly sick for photos)
http://www.fixkick.com/my96-16v/TP-MAF-VSS-ECT-FUEL.JPG

see that, it took like 20 miles to reach 160, (marked 180 onside)
so is too low and way too slow, causing engine ware, and wasted fuel.
id think 5 years is life span.

The idle is generally higher along w the fuel consumption when cold. Poss coolant sensor,, all sensors work together to adjust idle & adjust air/fuel mixture. A bad thermos would give u other symptoms too, {bad heating/ AC) Good luck scan it 1st & have all the SENSORS READING READ & ck for codes.

thermostat might be stuck shut, most likely reason for those symptoms

hi Planeman.
Why is the heater in my car not working? Ken Mattson, owner of Len's Automotive in Bellevue, Wash., says a car heater can stop working for a number of reasons, including: A low antifreeze/water level in the radiator due to a leak in the cooling system. A bad thermostat that isn't allowing the engine to properly warm up.11 Dec 2013

Why Isn't My Car Heater Working? ' Angie's List

https://www.angieslist.com/articles/why-isnt-my-car-heater-working.htm Search for: Why is the heater in my car not working?
Why is my car heater not blowing hot air?Hot coolant from the engine passes through a heater core, which looks and functions a lot like a small radiator, and a blower motor forces air through it. ... It's also the reason that a plugged heater core, stuck thermostat, or air in the cooling system can all cause a car's heater to blow cold.9 Jul 2017When Your Car Heater Blows Cold Air - Lifewirehttps://www.lifewire.com/when-your-heater-blows-cold-air-534667
How do you fix the heat in your car?Begin with the vehicle on level ground, engine off (cold) in park with the emergency brake set.Heater Controls. Step 1 - A heater cannot operate unless the engine is full of coolant/antifreeze. ...Refilling Cooling System. ...Engine Thermostat. ...Blend Door Actuator. ...Broken Vacuum Line. ...Heater Control Valve. ...Vacuum Gauge. ...Heater Core.How to Fix a Car Heater in Under 30 Minutes - 2CarPros
What are the symptoms of a bad thermostat?A motor with a bad thermostat will run fine for the first few minutes, then overheatrapidly. Other symptoms include the engine and radiator making a rumbling, gurgling, boiling sound: like a steam pipe knock, if you've ever heard one of those. A thermostat sticking open can make your heater not work in the winter.Auto, Car, Truck THERMOSTAT Repair bad thermostat symptoms
or go to this link if you need more help.
2008 ford ranger has heater that won get hot Google Search

If the temperature is cold outdoors, this can be a symptom of a stuck open thermostat. With the thermostat stuck open the engine never gets warm, so it won't give good heat. It's not expensive to get a new one, and they are easy to replace, so take it in and have it checked. Another symptom of that is very low gas mileage in the cold weather.

If it's stuck closed, the truck will overheat in under 10 minutes, if it's stuck open, it won't reach operating temp, gauge would show it running a bit below normal or low normal range. To check thermostat operation, remove it, drop it in a pan of boiling water, it should open right away.

A stuck thermostat is fairly common (especially if it's original on your truck). If the thermostat is stuck, its common for antifreeze to back up into the reservoir tank. I

A bad water pump will normally overheat at lower speeds. If you notice the engine temp comes down when you turn on the heater, it's most likely the water pump.
Best of luck.
Greg

There are two primary symptoms of a faulty thermostat (assuming you mean the coolant thermostat in the engine not the A/C Heater Thermostat).  The thermostat can fail in basically two ways:
1)  Stuck open - Engine runs colder, heater takes forever to heat up. 2)  Stuck closed - Engine runs very hot, then will suddenly drop in temperature, then run very hot again.  Or engine overheats constantly.  When the engine is hot, feel the upper radiator hose.  If it is not hot, you can bet the thermostat is stuck closed.  If it is hot, the thermostat may not be stuck close, but not opening at the proper temperature.
Stuck open is annoying, but not dangerous.  Stuck close is dangerous and can cause overheating-related damage to your engine.
If you are handy, you can replace it yourself.  Otherwise it is an easy job for a mechanic.