How to Diagnose and Fix a Bad Thermostat in Your 2020 Grand Cherokee

How to Fix Thermostat Issue

A failing thermostat is a common and critical issue for the 2020 Jeep Grand Cherokee, often leading to overheating, check engine lights, and cooling system strain. Owners of both the 3.6L Pentastar and 5.7L Hemi engines report this as a frequent failure point that requires prompt attention. As one owner with a 3.6L engine advised, "Thermostats are a common failure on the Pentastar" (source), highlighting its prevalence. This guide synthesizes direct owner experiences and recommendations to help you diagnose, repair, and prevent thermostat-related problems.

Symptoms

The symptoms of a failing thermostat in your Jeep can range from subtle to severe, often escalating if ignored. The most common and alarming symptom is the engine overheating. This isn't just a gauge reading slightly high; it can lead to serious engine damage if the thermostat sticks closed, preventing coolant from circulating through the radiator. You might notice the temperature gauge climbing into the red zone, steam from under the hood, or warning lights illuminating your dashboard.

Another frequent symptom is the illumination of the check engine light, often accompanied by a specific diagnostic trouble code. A common code associated with thermostat failure is P0128, which indicates the engine is taking too long to reach its proper operating temperature. As one owner clarified, "If I remember correctly, thermostat rationality means it took too long to warm up" (source). This happens when a thermostat is stuck in the open position, causing the engine to run too cool, which reduces fuel efficiency and increases engine wear.

You may also observe erratic temperature gauge behavior. The needle might fluctuate wildly, take an unusually long time to move from cold to the normal midpoint, or never quite reach the normal operating temperature. In some cases, poor cabin heater performance is a telltale sign, as a stuck-open thermostat prevents the engine coolant from getting hot enough to provide warm air through the vents. Coolant stains or small puddles under the front center of the vehicle can also point to a leak from the thermostat housing or related seals, which often coincides with a failing unit.

Most Likely Cause

Based on owner reports, the primary cause of thermostat failure in the 2020 Jeep Grand Cherokee is the mechanical failure of the thermostat unit itself, a wear item known to fail on these engines. The thermostat is a wax-pellet type valve designed to open at a specific temperature (typically around 195-200°F for these engines). Over time and through countless heating and cooling cycles, the internal mechanism can fatigue. It can become stuck in the closed position, causing immediate overheating, or stuck open, leading to a persistent low operating temperature and the P0128 code.

While a vacuum leak was identified by owners as a related cause for engine issues, in the context of cooling problems, it's more likely a secondary concern. A significant vacuum leak can cause a rough idle or misfire, which may coincide with cooling issues but isn't the direct failure point of the thermostat. The consensus from experienced owners points directly to the thermostat as a common failure component. This is emphasized for both available engines; as another owner warned, "If it’s the 5.7, watch out for the water pump and thermostat. They go out quick" (source), grouping it with another known weak point in the cooling system.

How to Diagnose

Proper diagnosis is crucial before replacing parts. The most effective method requires a professional-grade OBD-II scan tool, specifically one with advanced capabilities. As owner w00tah recommended, "I would take it to a shop that has a scan tool with Mopar Gateway access, as they will need it to get to the live data" (source). This specialized access allows for precise monitoring of the engine control module's data.

The key diagnostic step is to monitor the live engine coolant temperature (ECT) data stream while the engine warms up. Connect the scan tool, start the engine from cold, and observe the real-time temperature reading. A healthy thermostat will remain closed as the temperature climbs steadily. You should observe a distinct drop in the ECT reading (usually by 10-20°F) shortly after it reaches the specified opening temperature (check your owner's manual, but it's typically near 195°F). This drop indicates the thermostat has opened, allowing cooler coolant from the radiator to mix in. If the temperature climbs past 220-230°F without dropping and continues to rise, the thermostat is likely stuck closed. If the temperature rises very slowly and never exceeds 160-180°F, even after 15-20 minutes of driving, it is likely stuck open.

For a basic visual check, inspect the thermostat housing (located where the upper radiator hose meets the engine) for any signs of coolant leakage or crusty stains. After the engine is fully warmed up, carefully feel the upper and lower radiator hoses (caution: they can be very hot). If the thermostat is functioning, the upper hose should become very hot and pressurized, while the lower hose may be warm but not as hot. If both hoses are hot immediately after a cold start, the thermostat is probably stuck open. Always verify any check engine light codes first, as a P0128 code is a strong direct indicator of a thermostat stuck open.

Step-by-Step Fix

Replacing the thermostat is a moderately challenging DIY job, largely due to access constraints. The following steps are compiled from owner experiences and standard mechanical procedures. Always ensure the engine is completely cool before starting, and relieve any residual pressure in the cooling system by slowly opening the coolant reservoir cap.

1. Disconnect the Battery: For safety, disconnect the negative battery cable to prevent any electrical shorts.
2. Drain the Cooling System: Place a large drain pan underneath the vehicle. Locate the radiator drain petcock on the bottom driver's side of the radiator and open it to drain the coolant. Alternatively, you can disconnect the lower radiator hose at the radiator, but this is messier. Capture all the old coolant for proper disposal.
3. Gain Access: This is the most critical step. The thermostat housing is buried under the intake manifold. On the 3.6L Pentastar, this requires removing the entire air intake assembly and then the intake manifold itself. As one owner who did related work noted, "I advise to do as much as possible while you have it torn down. I just replaced the radiator and thermostat this weekend, ended up removing the nose again" (source). While a full "nose" removal may not be necessary for just the thermostat, his point about accessing multiple components at once is valid. Carefully disconnect all electrical connectors, vacuum lines, and hoses attached to the intake manifold before unbolting and removing it.
4. Remove the Thermostat Housing: Once the intake manifold is removed, you will have clear access to the thermostat housing, which is where the upper radiator hose connects to the engine. Loosen the hose clamp and disconnect the hose. Remove the bolts securing the housing and carefully lift it off. There will be residual coolant, so have rags ready.
5. Clean and Install: Thoroughly clean the mating surface on the engine block of any old gasket material or sealant. Install the new thermostat into the housing (ensure it is oriented correctly, usually with the spring side facing into the engine) with a new gasket or O-ring. Some housings come as a pre-assembled unit.
6. Reassemble: Reinstall the thermostat housing and torque the bolts to specification (typically in the 10-15 ft-lbs range, refer to a service manual). Reconnect the upper radiator hose. Reinstall the intake manifold, carefully reconnecting all lines and connectors. Reinstall the air intake assembly.
7. Refill and Bleed: Close the radiator drain petcock. Refill the cooling system with the correct type and mixture of coolant. For the 2020 model, you must use an OAT (Organic Acid Technology) coolant. An owner stressed, "Later WK2s used OAT (Mopar 10 YR/ 150K Mi Formula)... There are different kinds of OAT so be careful which one you get" (source). Using the wrong coolant can cause damage. They also noted that "Mopar’s OAT isn’t any more expensive than aftermarket brands" (source), making the OEM fluid a safe and cost-effective choice.
8. Bleed Air: Start the engine with the radiator cap or reservoir cap off (or the bleed screw open if equipped). Let it run until the thermostat opens (you'll see coolant flow and the temperature rise on the gauge). As it warms up, air bubbles will escape. Top off the coolant as needed until the system is full and no more bubbles appear. Reinstall the cap.
9. Test Drive and Verify: Take the vehicle for a short test drive, monitoring the temperature gauge closely. Afterward, recheck the coolant level when cold and top off if necessary. Use your scan tool to clear any stored diagnostic trouble codes.

Parts and Tools Needed

• Parts:
  • Thermostat Assembly (includes housing and gasket/seal). Ensure it is the correct temperature rating for your engine. A Mopar OEM part is highly recommended.
  • Engine Coolant: Mopar OAT Antifreeze/Coolant 10-Year/150,000 Mile Formula (MS-12106). You will likely need 1-2 gallons for a drain and refill.
  • Optional but Recommended: New intake manifold gaskets. Since you must remove the manifold, replacing its gaskets is prudent preventive maintenance.
• Tools:
  • Basic socket set (metric) with ratchets and extensions
  • Torque wrench
  • Screwdrivers (flathead and Phillips)
  • Pliers and hose clamp pliers
  • Drain pan (at least 2-gallon capacity)
  • Funnel
  • Shop towels or rags
  • OBD-II Scan Tool (for code reading, clearing, and live data verification)
  • Safety: Safety glasses and gloves.

Real Owner Costs

The cost to fix a thermostat issue varies dramatically between DIY and professional repair, largely due to the significant labor involved in accessing the component.

• DIY Cost: Owners report the part itself is affordable. One stated, "Thermostat is only like $30 too plus coolant if you do it yourself" (source). Adding in the cost of 2 gallons of OEM coolant (approximately $50-$70) and any new gaskets, a DIYer can expect a total parts cost between $80 and $120. Your investment is primarily time and tools.

• Professional Shop Cost: At a repair shop or dealership, the labor is the major expense. The job typically pays 2.5 to 4 hours of labor due to the intake manifold removal. At an average labor rate of $150/hour, labor alone can cost $375 to $600. Adding the OEM thermostat ($50-$80) and coolant ($70), plus shop supplies and tax, brings the total repair bill into the range of $500 to $750 or more at a dealership. This stark difference explains why many capable owners choose the DIY route, despite the complexity.

Prevention

While the thermostat is ultimately a wear item, you can prolong its life and protect the entire cooling system with proper maintenance. The single most important preventive measure is using the correct coolant and changing it at the recommended intervals. As highlighted by owners, using the specified Mopar OAT coolant is non-negotiable. Mixing different types of coolant or using an incompatible formula can lead to gel formation, corrosion, and premature failure of the thermostat, water pump, and radiator.

Adhere strictly to the factory-recommended coolant replacement schedule found in your owner's manual. Avoid the temptation to use universal "mix-with-anything" coolants. Regularly inspect the coolant level in the overflow reservoir when the engine is cold and look for any signs of discoloration or contamination. Addressing small leaks from hoses or the water pump immediately prevents air from entering the system and causing localized hot spots that can stress the thermostat. Keeping the cooling system clean and properly filled with the right fluid is your best defense against repeat failures.

What Owners Say

Real experiences from JEEP owners:

Owner Experiences

"I advise to do as much as possible while you have it torn down. I just replaced the radiator and thermostat this weekend, ended up removing the nose again." — Kabuki_with_an_XJ (source)

"Ended up taking the nose off to pull the headlight enclosure. I got the bulbs out from the port hole, but couldn’t reseat the new one." — Kabuki_with_an_XJ (source)

"I would take it to a shop that has a scan tool with Mopar Gateway access, as they will need it to get to the live data. I would watch the live temp data and see if the thermostat is opening at the specified OEM temperature." — w00tah (source)

Lessons Learned

⚠️ "Later WK2s used OAT (Mopar 10 YR/ 150K Mi Formula): not sure when they changed to OAT. There are different kinds of OAT so be careful which one you get." — grandcherokee2 (source)

⚠️ "There are different kinds of OAT so be careful which one you get. Mopar’s OAT isn’t any more expensive than aftermarket brands, or at least it’s close to it." — grandcherokee2 (source)

⚠️ "If it’s the 5.7, watch out for the water pump and thermostat. They go out quick." — Traditional_Grand_50 (source)

Real Repair Costs

"Thermostat is only like $30 too plus coolant if you do it yourself. But if your going to flush it might as well do the thermostat if your having issues IMO" — ForemostPlanet (source)

"Radiator is not a difficult job either. Thermostat is only like $30 too plus coolant if you do it yourself." — ForemostPlanet (source)

FAQ

Q: How long does it take to replace the thermostat? A: For a first-time DIYer with moderate mechanical skill, plan for a full day (4-8 hours). The majority of this time is spent carefully removing and reinstalling the intake manifold and associated components without damaging connectors. An experienced mechanic in a shop with all tools on hand can typically complete the job in 2.5 to 4 hours.

Q: Can I drive my Jeep with a bad thermostat? A: It is strongly discouraged. Driving with a thermostat stuck closed will lead to rapid overheating, which can cause catastrophic engine damage like a warped cylinder head or blown head gasket in a matter of miles. Driving with one stuck open is less immediately dangerous but causes the engine to run too cool, leading to increased fuel consumption, excessive engine wear, and higher emissions. You should address it promptly.

Q: Is this a common issue on the 2020 Grand Cherokee? A: Yes, based on owner reports across forums, thermostat failure is a known common issue on both the 3.6L Pentastar and 5.7L Hemi engines found in this model year. It is frequently mentioned alongside the water pump as a cooling system component that often requires replacement within the first 100,000 miles.

Q: DIY vs mechanic – what's recommended? A: This repair sits on the border. If you are an experienced DIYer comfortable with tasks like removing an intake manifold, labeling connectors, and following detailed steps, you can save significant money. However, if the phrase "remove the intake manifold" gives you pause, or you lack a comprehensive tool set and a torque wrench, it is best left to a professional. The cost of a mistake (coolant leaks, vacuum leaks, damaged sensors) can quickly exceed the cost of having it done right the first time.

Q: The check engine light came back on after replacement. What now? A: First, ensure the code is cleared after the repair. If the light returns, verify it is the same code (like P0128). As an owner suggested, "Verify the check engine light after the replacement is the same code" (source). If it is, the issue may not have been resolved—perhaps the new thermostat is faulty, there's an air pocket in the cooling system, or the Engine Coolant Temperature (ECT) sensor is also failing and providing bad data to the computer.

Q: Should I replace anything else while I'm in there? A: Absolutely. This is a prime opportunity for preventive maintenance. Owners strongly recommend this approach. Given the labor to access the thermostat, you should seriously consider replacing the water pump, as it is another common failure point and is located in the same general area. Inspect all coolant hoses, especially the heater hoses, and replace if they are soft, brittle, or swollen. Replacing the intake manifold gaskets is mandatory since you are removing it. This "while you're in there" philosophy saves money and labor in the long run.