What does error code P0128 mean on a Dodge Grand Caravan?

The quick answer: P0128 means the engine coolant temperature is not reaching the thermostat’s regulating temperature, usually signaling a thermostat stuck open or another cooling-system fault that prevents the engine from warming up properly.

In the Dodge Grand Caravan, this diagnostic trouble code points to how the engine management system expects the cooling system to behave during startup and warm-up. When the coolant stays too cool after a reasonable period of engine operation, the PCM sets P0128 and may trigger the check engine light. The result can be uncomfortable cabin heat, longer warm-up times, and potential impacts on fuel economy, emissions readiness, and overall engine efficiency. While not always dangerous, it is a sign that the cooling system is not operating as designed and should be addressed to avoid further wear or drivability issues.

What P0128 means for your Grand Caravan

P0128 is a generic OBD-II code indicating the coolant temperature is below the thermostat’s regulating temperature. On most Dodge Grand Caravan models, the thermostat is designed to regulate engine warmth to a specific range. If the engine remains cooler than that range after a normal startup period, the system flags P0128. This can happen even if the engine seems to run normally, but it often leads to reduced heater performance and longer cold-start times.

Causes

The main categories behind P0128 include mechanical, sensor-related, and cooling-system issues. Understanding these helps narrow down the most likely fault.

Mechanical issues

Before the list, note that many P0128 cases center on the thermostat or the cooling system pathway. The following mechanical problems are common culprits.

• Thermostat stuck open or slow to open


• Thermostat valve failure or incorrect thermostat installation


• Cooling system air pockets or improper bleed after service


• Low coolant level due to leaks or improper coolant fill


• Blocked or restricted coolant passages

Addressing these mechanical issues typically involves inspecting and replacing the thermostat, checking for leaks, and ensuring proper coolant flow through the engine and heater core.

Electrical and sensor issues

In some cases, the problem isn’t the thermostat itself but related sensors or wiring that misreport temperature to the PCM.

• Faulty engine coolant temperature (ECT) sensor or wiring


• PCM or wiring harness issues causing incorrect temperature readings


• Grounding problems or poor electrical connections in the cooling system circuits

Electrical problems may require testing the sensor resistance, checking for corroded connectors, and repairing or replacing faulty wiring or modules.

Symptoms you might notice

Several signs can accompany P0128, especially during cold starts or in winter driving. These symptoms help you decide whether to pursue a DIY check or professional service.

• Engine takes unusually long to reach operating temperature


• Weak or inconsistent cabin heating when the engine is cold


• Cold air from the heater despite running the heater for several minutes


• Check Engine light or MIL illuminated


• Temperature gauge lingering below the normal range after startup

If you notice these symptoms, it’s a good idea to read the codes with an OBD-II scanner and verify whether P0128 is present, possibly alongside related codes such as P0125 (insufficient coolant temperature) or P0118 (coolant temperature circuit high).

Diagnosis and fixes: steps to take

Below is a practical sequence for diagnosing and addressing P0128 on a Dodge Grand Caravan. Each step builds on the previous to identify whether the issue is mechanical, sensor-based, or a simple maintenance item.

1. Check coolant level and look for leaks. Top up if necessary and repair any leaks to ensure the system isn’t losing coolant.


2. Inspect for air pockets and properly bleed the cooling system. Air in the system can prevent proper coolant circulation and warm-up.


3. Test the engine coolant temperature sensor (ECT) and its wiring. Look for damaged connectors, corrosion, or broken wires, and verify the sensor resistance against the service manual spec.


4. Examine the thermostat and replace if it’s stuck open or not regulating correctly. Ensure you use the correct part for your model and follow proper installation torque and coolant refill procedures.


5. Check for restrictions in the cooling circuit, including radiator flow, water pump operation, and heater core flow. Clear obstructions and ensure coolant can circulate freely.


6. Reset the PCM, drive cycle the vehicle, and recheck for codes. If P0128 returns, rerun diagnostic checks or consult a professional to verify no parasitic electrical issues are present.

Costs vary by vehicle age and labor rates. A thermostat replacement tends to be a mid-range repair, but additional cooling-system work or sensor replacement can raise the total. If you’re not comfortable performing cooling-system work, seek a qualified technician.

Summary

P0128 on a Dodge Grand Caravan signals that the engine isn’t reaching the expected coolant temperature, usually due to a thermostat issue or a cooling-system/dependent sensor fault. Start with the basics—check coolant levels, bleed the system, and inspect the thermostat—then expand to sensor tests or electrical repairs if needed. Prompt diagnosis can restore proper warm-up performance, heater effectiveness, and fuel efficiency, while helping prevent longer-term engine wear.

How do I fix the engine code P0128?

To fix a P0128 code, first check and correct the coolant level, as low coolant is a common cause. If that doesn't resolve the issue, the most common fix is to replace the thermostat, as it is likely stuck open. If a new thermostat doesn't work, further steps include checking the coolant temperature sensor and its wiring for damage. 
Steps to fix the P0128 code
This video demonstrates how to replace a thermostat and fix a P0128 code: 55snondaYouTube · Oct 27, 2020

1. 1. Check and top off the coolant: Ensure the engine is cool, then check the coolant level in the reservoir. Add the correct type of coolant if it is low. 
2. 2. Inspect for leaks: Look for any visible signs of leaks in the cooling system hoses and around the radiator cap. 
3. 3. Replace the thermostat: This is the most common fix for a P0128 code. A thermostat that is stuck open will prevent the engine from reaching its proper operating temperature. 
4. 4. Inspect the coolant temperature sensor (ECT): If replacing the thermostat doesn't solve the problem, the ECT sensor itself or its wiring could be faulty. 
5. 5. Check the wiring and connections: Inspect the wiring connected to the ECT sensor for any damage, corrosion, or loose connections. Repair or replace any damaged sections. 
6. 6. Clear the code: Use an OBD-II scanner to clear the P0128 code from the vehicle's computer. 
7. 7. Test drive the vehicle: Drive the car for a period to see if the engine reaches its proper operating temperature and if the code returns. 

What is the code P0128 on a Dodge Caravan?

The P0128 code on a Dodge Caravan indicates that the engine coolant is not reaching the proper operating temperature, most commonly because the thermostat is stuck open. Other causes include low coolant levels or a faulty coolant temperature sensor. Symptoms can include a check engine light, poor fuel economy, and the engine temperature gauge not reaching normal levels. 
Common causes

• Faulty thermostat: The most common reason, as the thermostat may be stuck open, allowing too much coolant to circulate through the radiator before the engine warms up. 
• Low coolant level: A low coolant level or an air pocket in the system can prevent the engine from reaching the correct temperature. 
• Faulty engine coolant temperature sensor: The sensor may be providing an incorrect reading to the engine control module. 

Symptoms to look for
Check engine light is illuminated, Engine takes a long time to warm up, Poor fuel economy, Heater may not work efficiently, and Remote start function may be disabled. 
This video shows how to replace the thermostat on a Dodge Grand Caravan: 57sGearHeads with FinallyHeSleepsYouTube · Apr 16, 2025
How to diagnose and fix

1. 1. Check the coolant level: Ensure the coolant is at the proper level. If not, top it off and re-test. 
2. 2. Inspect the thermostat: A stuck-open thermostat is the most likely culprit. The solution is to replace it, along with the integrated thermostat housing if necessary. 
3. 3. Check the coolant temperature sensor: A faulty sensor can trigger the code. If the thermostat is okay, consider replacing the sensor. 
4. 4. Clear the code: After the repair, use an OBD-II scanner to clear the code and then take the vehicle for a road test to confirm the engine temperature is now normal. 

How serious is the P0128 engine code?

Driving with a P0128 error code for a short distance is generally safe, but it's not a good idea to ignore the issue for an extended period. While your vehicle might still operate, it's likely to experience reduced fuel efficiency and potentially more serious mechanical issues over time.

What are the common mistakes when fixing P0128?

So as the name suggests here the coolant temperature. Sensor. Simply reads the temperature of the engine coolant. And so of course if you have a faulty sensor.