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How to know when to change thermostat or sensor, for engine runing too cool.

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In all probability the thermostat stopped working, and it is not blocking the flow of fresh water from radiator. The only way i can think a sensor might lower engine temp, is by tripping the fans continuously, and even then a thermostat in good working order would block water flow, and let engine reach normal operating temperature. With the engine idling, in park, with the a/c off, if the temp is low, look in the engine bay, and make sure that the fans are NOT working.

Posted on Sep 17, 2010


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When bleeding cooling system on Linc ls v8 can you fill engine at engine fill keeping cap off and adding coolant at that point as needed

You must wait for the thermostat to open up, crank your heater on full, you will get only cold air once the thermostat opens it will start to heat up, heather runing wait for thermostat to open youll see your coolant bottle bubble up and go down , then you loosen the bleeding valve if you got one, cooling system service should be done all together , change thermostat,flush the coolant system, replace hoses if needed, if thermostat is shut and wont open up your more likely will blow a hose from the pressure .ty

Jan 01, 2017 | Cars & Trucks

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Why doesn't the cooling fan turn on, causing the engine to overheat?

Check your thermostat it my not opening all the way

Apr 07, 2015 | 2005 Ford Freestar

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2004 Trailblazer Left it runing for 2-3hrs, woke up engine off with ac giving hot air. Way home service engine came on. Codes for bad thermostat or bad heat sensor, but frirnd believed iy migjt be fan...

Suggest changing thermostat and temperature sensor, they are right next to each other and are relatively inexpensive. Whoever does the repair should disconnect the battery during the repair to reset the computer.
The fan clutch will usually not engage fully until the engine is really hot, somewhere around 250+ deg.

Jul 09, 2012 | 2004 Chevrolet TrailBlazer

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Hard starting

it could be an electrical problem.. temperature sensors may tell the engine its getting too hot and wont let it start until its completely cool.. you may need the sensors changed because they are giving false readings.. but the best thing you could do is to take your car to your nearest garage soon in case it turns out to be a serious problem.

Apr 24, 2012 | 1999 Chrysler 300M

1 Answer

Are there problems with the thermostat gauge not show proper temperture readings. 1999 vw jetta 2.0 gauge shows no temp reading. Is it a bad thermostat and needs to be changed. And can you harm the...

If you are not getting a signal to your gauge, your car can still run. You just won't know if your car is overheating until it is too late. A bad thermostat won't keep your gauge from working. The temperature sensor that sends the signal to the gauge will keep it from working.

The thermostat helps your car in extreme weather. It helps your engine warm up in extreme cold so your heater will work sooner. And it helps you car cool itself off in extreme hot weather. As your engine water temp rises, the thermostat will open allowing the water in the radiator (that has been cooling) to flow into the engine block. The water that was in the block will go into the radiator to begin cooling.

In fair weather, you don't need a thermostat.

Oct 09, 2010 | Volkswagen Jetta Cars & Trucks

2 Answers

P0128 coolant temp below thermostat/regulating temp

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
* Low engine coolant level
* Leaking or stuck open thermostat
* Faulty cooling fan (running too much)
* Faulty coolant temperature (ECT) sensor
* Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember to rate and comment this).

Sep 27, 2010 | 2006 Dodge Caravan

3 Answers


I had this problem with a 2007 Chevy Cobalt. I changed the coolant sensor and even though the check engine light went out I still had a reading of "---" on the coolant gauge. The problem is most likely one of three things: a low coolant level, a bad coolant sensor, or the thermostat needs to be replaced. The only other possible thing it could be is a bad BCM (Body Control Module) which in that case you would most likely be having other weird problems.

The Thermostat and the sensor are both very easy to get to and replace. Take off the engine cover by removing the oil cap and they are both to the rear of the engine on the driver's side. They are right next to each other.

Hope this helps!

Apr 14, 2010 | 2007 Chevrolet Cobalt Coupe

1 Answer

How to replace thermostat engine is over heating dont know the cause yet.

Before you change the me a favor and start your Jeep. Squeeze the hose leading into your engine.....your should feel the pressure pushing into the pressure, bad pump. Pressure? Change the thermostat.

Let me know what you find.....If you would like me to post the they are:

Removal & Installation
  1. Disconnect the negative battery cable.
  2. If necessary, disconnect the coolant temperature sensor electrical connector.
  3. Disconnect the upper radiator hose from the thermostat housing.
  4. Remove the attaching bolts and lift the housing from the engine.
  5. Remove the thermostat from the engine

    1. Remove the thermostat and gasket.
    To install:
    1. Clean all gasket surfaces thoroughly.
    2. Place the thermostat in the housing with the spring inside the engine.
    3. Install a new gasket with a small amount of sealing compound applied to both sides.
    4. Install the water outlet and tighten the mounting bolts to 200 inch lbs. (23 Nm) on V-8 engines and 15 ft. lbs. (20 Nm) on non-8 cylinder engines.
    5. Install the upper radiator hose to the housing and tighten the hose clamp.
    6. Connect the coolant temperature sensor connector to the housing.
    7. Refill the cooling system.

Oct 10, 2009 | 1995 Jeep Grand Cherokee

2 Answers

2003 pontiac running hot but get a reading of runing cool

When the car is completely cool,check the electric fan(s) for smooth rotation.

Clean/check/change the thermo sensor,contact and wire.


Excavate air pocket in coolant system / check for head gasket leak

This test will kill two birds with one stone.




You will spill some coolant during this air pocket purge test.......BE KIND TO THE ENVIRONMENT and ANIMAL please clean up after the test!


Put the front end on a pair of ramp or park your car on a VERY STEEP HILL (radiator facing top of the hill) .

Top of the coolant reserve tank

Let it ran for 10-15 minutes.

Monitor for air pockets escaping from coolant reserve tank.

Small amount of bubbles is OK at 1-5 minute mark

After the thermostat open up (after 195 F warm up) at
5-12 minute mark or after high idle you should see less bubbles.

If you do not see any in rush of bubbles then your thermostat may be partially stuck or rusted badly inside the thermostat hosing.

Give the thermostat host few gentle taps.

If you see larger bubbles surfacing after 15 minutes then should do a hydrocarbon (HC) dye test to test for potential head gasket leak.

Let engine cold down and top off coolant reserve tank.

Start monitor for coolant lost


A coolant flush is require every 2 years or 24,000 miles.

I recommend the thermostat that has a relief pop-let to reduce the change of burst radiator and coolant hoses.

Make sure you get a new thermostat gasket,black RTV and fresh coolant for the job.

Please post more information by clicking the comment link on the top right corner.

DON'T FORGET to RATE ME if my tip is helpful to you!

Sep 21, 2009 | 2003 Pontiac Grand Prix

2 Answers

Overheating 1993 Buick Skylark

it could be a fuse or fan relay, or a stuck thermostat, i will recomend to aply stright power to radiator fan to see if works, if don't replace fan and fuse, but if does, then it might be the thermostat, one very easy way to test the cooling fan system is by unpluging the coolant sensor with the car runing, if fan come on whit car runing and sensor disconected, the the system is ok, and might be a bad sensor, or stuck closed or open thermostat. please leave feedback if this take care of your problem

Jul 15, 2008 | 1993 Buick Skylark

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