Thermostat gmc sierra

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How To Replace your GMC Sierra Thermostat

When you have overheating problems in your GMC Sierra, it can lead to check engine codes like P1258 and engine damage. If your Sierra is overheating and the temperature needle is well in the red, knowing how to replace your GMC Sierra thermostat can save your serious money. When your GEN III GM engine is overheating, a good way to see if your thermostat has failed is to carefully touch the lower radiator hose.

Because the thermostat inside your GEN III GM engine is set to open at a specific temperature, when it fails it will cause the engine to overheat. There’s a wax thermal pellet incorporated inside your GM thermostat, and if that’s stuck closed the lower radiator hose will remain cool, even though your temperature meter reads hot.

Because this guide is working on hot components and near your exhaust and cooling fan, if you are not comfortable working on your own vehicle, stop now. Take your vehicle to an experienced mechanic instead, while our how to replace your GMC Sierra thermostat guide is relatively easy and straightforward, it’s still pretty easy to get hurt.


Today I’ll be showing you how to replace your GMC Sierra thermostat in a 2002 Sierra with the 4.8 liter engine in it. Because this guide focuses on replacing your Sierra thermostat, if you are still having overheating problems, check our guide on overheating GEN III engines, and how to troubleshoot overheating.


To replace your 4.8 liter Sierra thermostat, you’ll need coolant as well as a 10mm socket and wrench. The part number for the factory thermostat for your Sierra is GM part number 12562740, 12571261, or 12562740. The temperature rating for these thermostats is 186 degrees, and that is the stock rating for your Sierra.

If you are still having overheating problems, we suggest stepping down to a 160 degree thermostat.

Before beginning our how to replace your GMC Sierra thermostat DIY guide, it’s a good idea to disconnect the negative terminal on your battery, just in case. Because your fan is belt driven, make sure the key is out of the ignition and battery disconnected before you continue. Also it goes without saying that your Sierra being cold or at least not warmed up will go a long way towards you replacing the thermostat.

After you’ve disconnected the battery, drain the coolant from your radiator.


Remove the bolts that secure your Vortec 4.8L air intake box and Mass Air Flow (MAF) housing. Disconnect your MAF sensor by lifting up on the gray lock tab. Once the gray lock tab has been pulled out, you can disconnect the MAF by depressing the clip shown below.


Now with the MAF disconnected, remove your PCV (if applicable) and remove the entire intake tract by undoing the clamp on your throttle body. Now your Sierra should be ready for you to remove the upper fan shroud.

Simply put, this monster of a fan shroud completely blocks your access to your engine as well as your thermostat. To remove the upper portion of the fan shroud, remove the six 10mm bolts that run along either side of the shroud.


Once you have all 10mm bolts removed, lift up and carefully take out your GMC Sierra fan shroud.


Give yourself some extra wiggle room by undoing the bolts that secure the radiator in place.


The thermostat is hidden well behind the fan, you can access the two 10mm bolts that hold the thermostat in place, by rotating your fan. Spin your fan and fit your 10mm socket between the fins in order to remove the GMC thermostat housing.


Once those two 10mm bolts have been removed, pull on the lower radiator hose to completely disconnect the thermostat housing. Remove your old thermostat and discard the unit.


Now use a razor blade to carefully clean the mounting surface of your water pump outlet. This should be free of any debris and clean so that your gasket maker has a good chance to recreate and form a good seal. This is important if you don’t want to repeat this job of how to replace your GMC Sierra thermostat.

Clean and dry this area well, if coolant is left inside the housing, dry this area as well. You don’t want the coolant to interfere with your gasket maker’s sealant.


Prepare the new thermostat by inserting into your old thermostat housing, and apply a thin layer of hi-temp gasket maker.


Carefully push the new thermostat and radiator hose / thermostat housing back into the engine. Be careful aligning the thermostat housing, and when fully installed, insert the 10mm bolts and tighten to specification.

Take a break now and put back your MAF and upper fan shroud. Take some time before pouring in the coolant, give the thermostat housing and the gasket maker a chance to dry and seal. Return after a few hours and fill up your radiator. Test drive your GMC Sierra to make sure that your thermostat is working correctly and routing coolant into your radiator and fans.

You now know how to replace your GMC Sierra thermostat, if you have any questions or comments please leave them for us below.



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Car Thermostat Replacement Service

How much does a Car Thermostat Replacement cost?

On average, the cost for a GMC Sierra 1500 Car Thermostat Replacement is $159 with $64 for parts and $95 for labor. Prices may vary depending on your location.

CarServiceEstimateShop/Dealer Price
2009 GMC Sierra 1500V8-4.8LService typeCar Thermostat ReplacementEstimate$344.15Shop/Dealer Price$407.69 - $571.31
2015 GMC Sierra 1500V8-5.3LService typeCar Thermostat ReplacementEstimate$322.35Shop/Dealer Price$386.46 - $551.75
2012 GMC Sierra 1500V8-4.8LService typeCar Thermostat ReplacementEstimate$318.27Shop/Dealer Price$381.36 - $543.58
2008 GMC Sierra 1500V8-4.8LService typeCar Thermostat ReplacementEstimate$320.15Shop/Dealer Price$383.68 - $547.30
2009 GMC Sierra 1500V8-6.0L HybridService typeCar Thermostat ReplacementEstimate$339.15Shop/Dealer Price$404.73 - $569.89
2013 GMC Sierra 1500V8-6.2LService typeCar Thermostat ReplacementEstimate$318.27Shop/Dealer Price$381.36 - $543.58
2003 GMC Sierra 1500V8-5.3LService typeCar Thermostat ReplacementEstimate$249.91Shop/Dealer Price$297.27 - $417.12
2009 GMC Sierra 1500V6-4.3LService typeCar Thermostat ReplacementEstimate$227.27Shop/Dealer Price$269.34 - $372.50

Show example GMC Sierra 1500 Car Thermostat Replacement prices

What is an engine cooling system thermostat and how does it work?

The car's engine cooling system thermostat is a small valve within a cooling system passageway that opens and closes in response to the temperature of the circulating engine coolant. When your car is first started and the coolant is cold, the valve in the thermostat will be in the closed position, circulating the coolant flow only within the engine instead of flowing to the radiator, which allows the engine to warm up fast, promoting fuel efficiency, low emissions, and reduced engine deposits. Once the engine coolant reaches the design temperature, usually around 195 degrees, the valve will open and close as you drive in order to maintain a coolant temperature typically between 195 to 225 degrees Fahrenheit.


When to consider replacing the engine cooling system thermostat:

  • Engine overheats. A thermostat with the valve stuck closed is a common cause of engine overheating. However, since there are other causes of overheating, a mechanic will thoroughly inspect the cooling system prior to condemning the thermostat. Since it is usually not known how badly the engine overheated, the mechanic will also try to determine if there is evidence that the cylinder head gasket was blown. If so, it would be repaired prior to replacing the thermostat.
  • Temperature gauge reads too low or too high. Thermostats can get sticky, causing the engine temperature to stray from the optimal range. Your dash-mounted temperature gauge will notify you of this malfunction.
  • Cooling system repairs. All thermostats eventually fail. Therefore, if the cooling system is already open for major repairs, such as water pump or radiator replacement, it is recommended to install a new thermostat simply as insurance against future breakdown.

How do mechanics replace the engine cooling system thermostat?

  • Once the engine is completely cold, the radiator cap is loosened and enough coolant is drained from the radiator so the coolant level is below the thermostat housing.
  • The thermostat housing is removed and the thermostat and gasket are extracted. Note that some cars have more than one cooling system thermostat and both might need replacement.
  • All mounting surfaces on the thermostat housing are cleaned of old gasket material. A new gasket is applied and the thermostat is re-installed in the correct orientation and with the temperature sensing cylinder facing downward towards the engine.
  • The thermostat housing bolts are tightened to original equipment manufacturer (OEM) specifications using a calibrated inch-pound torque wrench.
  • The cooling system is refilled with the appropriate coolant type and the system is fully bled of all air.
  • The car engine is run until the normal operating temperature is reached, and while taking precautions with the hot coolant, the coolant level is rechecked while confirming there is flow through the radiator — either visually or with an infrared temperature gun. This will confirm the thermostat is open. Finally, the engine is checked for leaks.

Is it safe to drive with an engine cooling system thermostat problem?

No. If it has been determined that there is a thermostat problem, it should be repaired right away. Failure to do so can cause substantial and costly damage to the engine.

When replacing the engine cooling system thermostat keep in mind:

  • Engine overheating due to causes other than a faulty thermostat, will themselves damage a thermostat. Therefore, always replace the thermostat if the engine overheats, even if the thermostat was not the initial cause of the overheating.
  • The thermostat should be replaced anytime you have major work done on the coolant system. It is convenient and costs little during such repairs and insurance against future breakdown.
  • OEM thermostats are the best ones to use and you can request your mechanic to install only the OEM thermostat. Dealer supplied, OEM thermostats are sometimes more costly but are generally higher quality and specifically engineered for your car.

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How to Change the Thermostat on GMC Sierra

If your GMC Sierra is running hot, whether it is over-heating or just running at a higher temperature, then it might be time to replace your thermostat. A thermostat is small but plays a big role in your car’s temperature. When your engine is running and generating heat, your thermostat is hard at work. It opens and closes according to the temperature of your coolant in your engine, in order to regulate the temperature of your vehicle. When one goes bad, it is usually stuck shut or won’t open enough, which traps the hot coolant in the engine, causing it to overheat.

Changing the Thermostat

Step 1

Remove the thermostat from the engine by following the upper radiator hose to where it meets the thermostat housing. The thermostat housing is where the opposite end of the upper radiator hose mounts to the engine.

Step 2

Drain enough coolant from radiator until it is below thermostat. Failure to do this will result in your antifreeze leaking out onto the ground.

Step 3

Loosen clamp and remove radiator hose from thermostat housing. Some antifreeze will leak out: have some rags available. If your engine is a 4.8L, 5.3L, or 6.0L, remove the air inlet duct first.

Step 4

Remove the bolts holding the thermostat housing on and pull the housing from the engine. The gasket might make it a little difficult, so do not be afraid to pull hard, but do not strike it with any tools as you can crack it.

Step 5

Remove old thermostat and note its alignment.

Step 6

Place a rag in the thermostat’s mounting hole so no gasket material can enter the engine and then scrape the gasket from the housing and the engine. Install the new thermostat in same position as old one.

Step 7

Apply a sealant to the thermostat housing, the part you removed, and then the gasket as well.

Step 8

Reinstall the housing and tighten down bolts snugly. Torque them with your torque wrench. If your engine is one of the following then toque them to 21 ft lbs: 4.3L, 5.0L, or 5.7L. If your engine is a 7.4L then torque the bolts to 27ft lbs. If your engine is a 4.8L, 5.3L, or 6.0L then torque the bolts to 11 ft lbs. Make sure you do not over tighten them.

Step 9

Reinstall the radiator hose to thermostat housing and tighten the clamp.

Step 10

Refill the radiator with coolant and start the engine, allowing it to reach normal operating temperature.

Step 11

Check for any leaks at hose connection and thermostat housing. You can tighten them as necessary to stop any leaks.

Stop the engine and allow it to cool down completely then check the coolant level. Add coolant as required.

Things You'll Need

  • sockets
  • torque wrench
  • new thermostat and gasket
  • gasket scrapper
  • antifreeze
  • screwdriver


  • Always be careful when working around a hot engine.
  • Never open the radiator or any related components while the engine is hot.
  • Never allow anyone or anything to drink antifreeze, it is poisonous.
  • Never pour antifreeze down a drain, it is against EPA regulations.

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