P0128 on 2022-2024 Ram 1500: Causes and Fixes for Thermostat Rationality

What's Unique About the 2022-2024 Ram 1500

For the 2022-2024 Ram 1500, particularly those with the 3.6L Pentastar V6, this code is frequently linked to a specific part failure at very low mileage. Ram issued Technical Service Bulletin (TSB) #9100287 for the 2022-2024 models, which addresses P0128 failures and points to an inspection and replacement of the thermostat assembly with an 'upgraded' part. This indicates a known defect with the original part, making a faulty thermostat the most probable cause, rather than a sensor or other component. 🎬 Watch: Quick diagnostic tips for inaccurate coolant temperature readings. The issue is so common that it also affects platform mates like the Jeep Wrangler (JL) and Gladiator (JT).

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on with code P0128 stored.
• Engine temperature gauge reads lower than normal (e.g., stuck at the 1/4 mark instead of the usual halfway point).
• Engine takes a very long time to warm up, or the temperature gauge drops at highway speeds.
• Heater blows lukewarm or cool air, especially on cold days, because the coolant isn't getting hot enough.
• Noticeable decrease in fuel economy (MPG).

Most Likely Causes

1. Failing Thermostat (Stuck Open) 🔴 High Probability Thermostats are designed to fail in the open position as a failsafe against overheating. For the 3.6L Pentastar engine in this specific year range, TSB #9100287 explicitly points to thermostat assembly failure for the P0128 code, indicating a known manufacturing issue. Forum users report this failure happening at mileages as low as 15,000-20,000 miles.
   How to confirm: Use an OBD-II scanner to monitor live engine coolant temperature. After a cold start, the temperature should rise steadily to ~200°F and stabilize. If it rises very slowly, struggles to get above 170-180°F, or drops significantly at highway speeds, the thermostat is stuck open. A physical check: after the engine has run for 10-15 minutes, feel the large upper and lower radiator hoses. If they are both equally warm, the thermostat is likely stuck open, allowing coolant to circulate constantly.
   Typical fix: Replace the engine thermostat assembly. On the 3.6L V6 and 3.0L EcoDiesel, the entire thermostat housing assembly is replaced as one unit. On the 5.7L HEMI, the thermostat can often be replaced separately from its housing. It is highly recommended to use a genuine Mopar replacement part, as technicians report a high failure rate for aftermarket thermostats in this application.
   Est. part cost: $50-$200
2. Low Engine Coolant Level 🟡 Medium Probability While not specific to this platform, leaks can develop. On the 3.0L EcoDiesel, the EGR cooler is a known potential leak point that can lead to coolant loss and other codes.
   How to confirm: Visually inspect the coolant reservoir to ensure the level is between the 'MIN' and 'MAX' lines when the engine is cold. If low, inspect for leaks from hoses, the radiator, water pump, or thermostat housing itself.
   Typical fix: Find and repair the source of any leaks, then top off the cooling system with the correct Mopar OAT coolant and bleed any air from the system.
   Est. part cost: $20-$40 for coolant
3. Faulty Engine Coolant Temperature (ECT) Sensor ⚪ Low Probability On the 3.6L Pentastar engine for these model years, the ECT sensor is often integrated into the thermostat housing assembly. This means it gets replaced along with the thermostat, making it an even less likely standalone cause.
   How to confirm: With the engine cold, use a scan tool to compare the ECT sensor reading to the Intake Air Temperature (IAT) sensor reading. They should be within a few degrees of each other. If the ECT reading is significantly different (e.g., reading 150°F on a 50°F morning), the sensor is likely faulty.
   Typical fix: Replace the engine coolant temperature sensor. On 3.6L models, this often requires replacing the entire thermostat housing. Clear the trouble code afterward.
   Est. part cost: $25-$60

Rare But Worth Checking

• Cooling Fan Stuck On: If the electric cooling fan runs constantly at high speed, even when the engine is cold, it can over-cool the engine and prevent it from reaching operating temperature, triggering a P0128 code. This is uncommon but worth checking.
• PCM Software Issue: Older TSBs for previous model years (like #18-002-23 and #18-017-16) have mentioned software enhancements for 'Thermostat Rationality'. While less likely for these newer models which already have a known hardware fault, it's a possibility that a dealer software update could be required if a new thermostat doesn't fix the issue.

Diagnosis Steps

1. Verify the code with an OBD-II scanner. Confirm P0128 is present.
2. Check the engine coolant level in the reservoir. Ensure it is at the proper level when the engine is cold. A low level can introduce air into the system and cause this code.
3. Start the cold engine and monitor the temperature gauge on the dashboard. Note if it takes an unusually long time to warm up or never reaches the middle mark (typically straight up).
4. For a more precise diagnosis, use a scan tool to view live data for the Engine Coolant Temperature (ECT) sensor.
5. Observe the ECT reading as the engine warms up. It should climb steadily to around 195-203°F and then hold steady. If it plateaus at a lower temperature (e.g., 160-180°F) or drops when driving at highway speed, the thermostat is the primary suspect.
6. If the ECT reading is erratic or seems incorrect from the start (e.g., reads -40°F on a warm day), the ECT sensor or its wiring is likely the problem.
7. If the temperature rises normally but the code persists, check for any applicable TSBs or software updates at a dealership, referencing TSB #9100287 for 3.6L engines.

Parts You'll Likely Need

• Thermostat Assembly (3.6L V6) (OEM #04893865AC (or its superseding part number)) — This is the most common failure part for P0128 on the 3.6L engine, as noted in TSB #9100287. The thermostat is integrated into the housing and must be replaced as a unit. This part number has been superseded multiple times (previously 68105533AB, 4893865AA, 4893865AB), indicating revisions to fix the issue.
  Trusted brands: Mopar (Recommended), Dorman, Gates
  OEM price range: $70-$120
  Aftermarket price range: $40-$80
• Thermostat (5.7L HEMI V8) (OEM #52028898AJ) — On the HEMI engine, the thermostat is a common failure item and can be replaced separately from the housing, making for a less expensive repair.
  Trusted brands: Mopar, Gates, Motorad
  OEM price range: $60-$90
  Aftermarket price range: $15-$40
• Thermostat Assembly (3.0L EcoDiesel V6) (OEM #68253514AA) — The thermostat is also a known failure item on the EcoDiesel, leading to P0128. It is sold as a complete housing assembly.
  Trusted brands: Mopar
  OEM price range: $100-$150
  Aftermarket price range: $70-$110
• Engine Coolant / Antifreeze (OEM #Mopar 68163848AB (or equivalent)) — You will lose coolant when replacing the thermostat and will need to refill and bleed the system. Use Mopar OAT (Organic Additive Technology) 10-year/150,000-mile coolant specified for your vehicle to prevent corrosion and system damage.
  Trusted brands: Mopar
  OEM price range: $25-$40 per gallon
  Aftermarket price range: $20-$30 per gallon

Technical Service Bulletins (TSBs) & Recalls

• 9100287: Mentions 'Thermostat Assy For 2022-2024MY 3.6L Pentastar Upgrade with P0128 failure code.'
• 18-002-23: Notes a software enhancement for 'P0128-00 - Thermostat Rationality' on earlier models.
• S2118000009: Defines the code as 'P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)' on earlier models.
• 18-017-16: Lists P0128-00 - Thermostat Rationality as a potential code on 3.0L Diesel engines that can be addressed with a PCM software update for other driveability issues.

Platform-Specific Known Issues

• TSB for 3.6L Pentastar V6: Ram has issued Technical Service Bulletin #9100287 specifically for the 2022-2024 Ram 1500 with the 3.6L engine. It addresses the P0128 code and instructs technicians to inspect and potentially replace the thermostat assembly with an 'upgraded' part, confirming this is a known defect from the manufacturer.
• 3.0L EcoDiesel Considerations: On the EcoDiesel engine, the thermostat is designed to maintain a higher operating temperature of around 208°F for emissions purposes. A P0128 code is particularly problematic as it can prevent the engine from getting hot enough to perform a DPF regeneration cycle, potentially leading to more serious exhaust filter issues. Also, check for coolant leaks from the EGR cooler, a common issue on these engines.
• Aftermarket Part Warning: Multiple technicians and experienced owners on forums for Ram and Jeep vehicles strongly advise against using aftermarket thermostats for this repair. They report a very high premature failure rate, with the P0128 code often returning within weeks or months. Using a genuine Mopar part is highly recommended to ensure a lasting repair.

Mechanic-Grade Diagnostic Values

• Engine Coolant Temperature (ECT) Sensor Resistance — expected: Between 2000Ω and 3000Ω at 20°C (68°F); between 200Ω and 300Ω at 90°C (194°F).. Failure: Resistance is infinite (open circuit), near zero (short circuit), or does not correspond to the temperature/resistance chart for the sensor.
• Engine Coolant Temperature (ECT) Sensor Signal Voltage (Key-On, Engine-Off) — expected: Approximately 2.0V - 3.0V with a cold engine (e.g., 20°C / 68°F).. Failure: Voltage is stuck near 5V (indicating an open circuit or disconnected sensor) or near 0V (indicating a short to ground).
• Engine Coolant Temperature (ECT) vs. Intake Air Temperature (IAT) at Cold Soak — expected: With the engine cold after sitting overnight, the ECT and IAT sensor readings on a scan tool should be within a few degrees of each other and the ambient air temperature.. Failure: If the IAT reads 60°F and the ECT reads 120°F on a cold engine, the ECT sensor is skewed or faulty.

Hidden / Shadow Codes Worth Checking

• P0128-00: This is the manufacturer-specific version of the code, often displayed on dealer-level scan tools like wiTECH. The '-00' suffix typically indicates a generic or primary fault type with no additional subtype information. Its presence confirms the 'Thermostat Rationality' fault as defined by Stellantis/Ram in TSBs #18-002-23 and #18-008-21. (see via Advanced scan tools like wiTECH, AlfaOBD, or high-end Snap-on/Autel scanners.)

Wiring & Ground Locations

• PCM Connector C2, Pin 12 (3.6L V6) — This is one of the main connectors on the Powertrain Control Module (PCM).. For the 3.6L engine, Pin 12 on the C2 connector is the ECT Signal circuit (K2). A technician can test for voltage here to verify the signal the PCM is receiving from the sensor. The wire color is typically Violet/Orange.
• G301 / G302 — Located on the left front body structure of the truck.. These are primary body ground points. While not directly tied to the ECT sensor's dedicated sensor ground within the harness, a poor main ground can cause a host of electrical issues and erratic sensor behavior. Verifying major grounds are clean and tight is a good practice if sensor readings are nonsensical.

Real Owner Repair Stories

• RamForum.com user 'buckeyexx' (2016 Ram Power Wagon, 6.4L Hemi (different engine, but same code and principle)) — Truck was not getting up to temperature, but did not have a check engine light initially. Later, another user with a P0128 code chimed in.
  ✅ What actually fixed it Replaced the entire thermostat and housing assembly. The user noted it was a simple, two-bolt job that took about 1-1.5 hours.

OEM Part Supersession History

• 68105533AB, 4893865AA, 4893865AB → 04893865AC — This part is the thermostat housing assembly for the 3.6L V6 engine. The multiple supersessions indicate that Mopar has released several revisions of this part, likely to address the high failure rate that causes the P0128 code, as referenced in TSB #9100287.

Model Year Variations Within This Range

• 2022-2024: There are no significant known changes to the base 3.6L, 5.7L, or 3.0L EcoDiesel cooling systems within this specific 2022-2024 model year range that would alter the diagnosis or common causes of a P0128 code. The primary issue remains the thermostat failure, especially on the 3.6L as identified by TSB #9100287 which covers the entire range.