P0128 on 2020-2024 Chrysler Pacifica: Coolant Thermostat Fault Causes and Fixes

What's Unique About the 2020-2024 Chrysler PACIFICA

The 3.6L Pentastar V6 engine used in the Pacifica and many other Stellantis vehicles has a well-documented, widespread history of thermostat failures that cause the P0128 code. The issue is so common that mechanics report it's the cause over 90-95% of the time for this code on this engine. Chrysler has issued multiple Technical Service Bulletins (TSBs) acknowledging the problem and providing specific repair instructions, including the use of updated thermostat assemblies. [Bulletin #9100287, Bulletin #S2118000009] The 'Pentastar Upgrade' engine found in 2016+ models, including this Pacifica generation, features changes like a higher compression ratio and two-stage variable valve lift, but the fundamental thermostat design remains a high-failure item.

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
• Engine takes a very long time to warm up
• Temperature gauge on the dashboard reads lower than normal (e.g., stays at the 1/4 mark) or drops at highway speeds.
• Heater blows lukewarm or cool air, especially in cold weather.
• Decreased fuel economy.
• Remote start may be disabled due to the active Check Engine Light.

Most Likely Causes

1. Faulty Thermostat Assembly (Stuck Open) 🔴 High Probability This is a widely recognized failure point on the 3.6L Pentastar engine, as confirmed by multiple manufacturer TSBs and countless owner reports. The original plastic housings can develop hairline cracks and leak, while the internal mechanism fails, causing it to stick in the open position. Mopar has released several revised versions of the part to address the high failure rate.
   How to confirm: Use an OBD-II scanner to monitor the live engine coolant temperature. After a cold start and 15-20 minutes of driving, the temperature should stabilize around 195-205°F. If the temperature rises very slowly, struggles to get above 170-180°F, or drops significantly when driving at highway speeds, the thermostat is almost certainly stuck open.
   Typical fix: Replace the entire thermostat and housing assembly. It is strongly recommended to use an updated OEM Mopar part. Some aftermarket thermostats have been reported to fail in the closed position, leading to dangerous overheating. The assembly often includes a new gasket and ECT sensor.
   Est. part cost: $50-$120
2. Low Engine Coolant Level 🟡 Medium Probability While not specific to P0128, the 3.6L Pentastar can be prone to coolant leaks from the oil filter housing/cooler assembly, which sits in the engine valley. A leak here or from a cracked thermostat housing can cause coolant loss and introduce air into the system, potentially triggering a P0128 code.
   How to confirm: Visually inspect the coolant reservoir when the engine is cold. The level should be between the 'MIN' and 'MAX' lines. If low, inspect the thermostat housing, radiator, hoses, and the area under the intake manifold (engine valley) for signs of pink/purple crusty residue from dried OAT coolant.
   Typical fix: Top off the coolant with the correct type (Mopar 10-year/150,000-mile OAT coolant) and pressure test the system to find and repair the source of the leak.
   Est. part cost: $25-$40 for coolant
3. Faulty Engine Coolant Temperature (ECT) Sensor ⚪ Low Probability This is not a common failure point, but it can happen. The sensor provides the data the PCM uses to set the code, so an incorrect reading can be the cause.
   How to confirm: After the vehicle has been sitting overnight (a 'cold soak'), use a scan tool to compare the ECT sensor reading with the ambient air temperature and intake air temperature readings. All three should be within a few degrees of each other. If the ECT reading is significantly different, the sensor is likely faulty. However, since the thermostat assembly often includes a new sensor, this is often replaced as part of the primary repair.
   Typical fix: Replace the engine coolant temperature sensor. On the 3.6L, it is often included with the new thermostat housing assembly.
   Est. part cost: $25-$50

Rare But Worth Checking

• Cooling Fan Stuck On: If the electric cooling fans run continuously from the moment the car is started, it can over-cool the engine and prevent it from reaching operating temperature, triggering a P0128 code. This is usually caused by a faulty fan relay, control module, or a failed A/C pressure switch.
• PCM Software Issue: In some cases, the parameters for triggering the P0128 code can be too sensitive. Manufacturers sometimes release software updates to the Powertrain Control Module (PCM) to adjust the diagnostic logic. TSB #S2118000009 mentions this possibility, though mechanics widely report that a software update rarely fixes this code and the thermostat is the true culprit.

Diagnosis Steps

1. Check for TSBs: Given the known issues, check for any open recalls or TSBs related to the cooling system for your specific VIN.
2. Verify the Code: Use an OBD-II scanner to confirm P0128 is the only code present. Note any other codes.
3. Inspect Coolant Level and Condition: With the engine cold, check the coolant level in the reservoir. Ensure it is at the proper level and that the coolant is the correct purple Mopar OAT type.
4. Monitor Coolant Temperature (Live Data): Use the live data function on your OBD-II scanner to watch the Engine Coolant Temperature (ECT) as the engine warms up from a cold start. The temperature should rise steadily to around 195-205°F and stabilize.
5. Analyze Temperature Behavior: If the temperature is very slow to rise, never reaches the target range (e.g., stuck at 160-180°F), or drops significantly when you start driving at highway speeds, a stuck-open thermostat is the most likely cause.
6. Perform ECT Sensor Rationality Check: As a final check, after an overnight cold soak, compare the ECT, IAT, and ambient air temp sensor readings. They should all be within a few degrees of each other. This confirms the ECT sensor is reading correctly.
🎬 See this video for the top 3 causes of P0128.
7. Inspect for Leaks: Carefully check the thermostat housing, hoses, and radiator for any signs of coolant leaks (pink/purple crust).
8. Replace the Thermostat Assembly: If all signs point to a stuck thermostat, replace the entire thermostat housing assembly with a quality OEM part.

Parts You'll Likely Need

• Thermostat Assembly (OEM #68217305AH (supercedes previous versions)) — This is the most common failure part for code P0128 on the 3.6L Pentastar engine, as documented in manufacturer TSBs and extensive owner feedback. It typically fails by sticking open. 🎬 Watch: Basic steps for replacing a bad Pentastar thermostat.
  Trusted brands: Mopar (highly recommended)
  OEM price range: $70-$120
  Aftermarket price range: $40-$80
• Engine Coolant/Antifreeze (OEM #68163848AB (Concentrate), 68163849AB (50/50 Pre-mixed)) — You will lose coolant when replacing the thermostat and will need to refill and bleed the system. Use the specified Mopar OAT (Organic Additive Technology) 10-year/150,000-mile coolant (purple in color) to prevent system damage.
  Trusted brands: Mopar
  OEM price range: $30-$45 per gallon
  Aftermarket price range: $25-$40 per gallon

Technical Service Bulletins (TSBs) & Recalls

• 9100287: 'Thermostat Assy For 2022-2024MY 3.6L Pentastar Upgrade with P0128 failure code.'
• S2118000009: 'Customer complains of a MIL-on condition. Upon further inspection, there is a Diagnostic Trouble Code (DTC) P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) set in the PCM.'

Platform-Specific Known Issues

• TSB #9100287 specifically calls out a P0128 failure code on 2022-2024 models with the 3.6L Pentastar Upgrade engine and points to a specific Thermostat Assembly. It mandates an inspection procedure before replacement.
• TSB #S2118000009 describes the P0128 code condition on 2020 models and confirms it is for 'Coolant Temperature Below Thermostat Regulating Temperature'.
• Owner forums are filled with reports of this failure, often occurring between 50,000 and 100,000 miles. Many owners recommend using only Mopar thermostats, as aftermarket versions have a reputation for premature failure, sometimes in a closed position which can cause overheating.

Mechanic-Grade Diagnostic Values

• Engine Coolant Temperature (ECT) Sensor Resistance — expected: 2000-3000 Ω at 20°C (68°F); 200-300 Ω at 90°C (194°F).. Failure: Resistance values that are significantly outside this range for a given temperature, or an open/short circuit.
• Engine Coolant Temperature (ECT) Sensor Signal Voltage (Key On, Engine Off) — expected: Approximately 2.0V - 3.0V on a cold engine. The voltage should decrease smoothly as the engine warms up, reaching approximately 0.5V when fully warm.. Failure: Voltage that is stuck high (e.g., near 5V) or low (near 0V), or does not change as the engine temperature changes. A reading of ~4.6V often indicates an open circuit.

Wiring & Ground Locations

• Engine Coolant Temperature (ECT) Sensor — On the 3.6L V6 engine, the ECT sensor is integrated into the thermostat housing assembly, which is bolted to the front of the timing cover on the driver's side (left side) of the engine.. This is the primary sensor the PCM uses to monitor engine warmup time. A faulty sensor or bad connection here is a direct, though uncommon, cause of P0128. Since it's part of the housing, it's replaced during the most common repair.
• ECT Sensor Connector — A 2-pin connector that plugs directly into the thermostat housing assembly.. A loose, corroded, or damaged connector or wiring can interrupt the signal from the ECT sensor to the PCM, causing incorrect temperature readings and potentially triggering a P0128 or other ECT-related codes like P0117/P0118.

OEM Part Supersession History

• 68217305AD, 68217305AE, 68217305AF, 68217305AG → 68217305AH — Revision by the manufacturer to address the high failure rate of the thermostat sticking open.
  Heads up: The latest version, 68217305AH, is the recommended replacement for all previous versions on the specified vehicles.

Model Year Variations Within This Range

• 2021-2024: The Chrysler Pacifica received a significant refresh for the 2021 model year, including revised exterior styling and a new Uconnect 5 infotainment system. While the core 3.6L Pentastar engine and its cooling system vulnerabilities remain the same, TSB #9100287 specifically targets 2022-2024 models, suggesting there may be a specific batch of thermostat parts or software logic unique to these later years.
• 2022-2024: TSB #9100287, which applies to 2022-2024 models, mandates following an inspection procedure from STAR Online Publication before replacing the thermostat. This differs from the more general TSB for the 2020 model and implies a more specific diagnostic path may be required for these later model years.