Dodge, Jeep, Ram P0128 Engine Code Guide

Austin Parsons

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The P0128 fault code is the diagnostic trouble code (DTC) used by Dodge, Jeep, Ram, and numerous other manufacturers to signify that the coolant temperature in the engine is too cold for the thermostat to regulate. The P0128 engine code can result from numerous issues including a malfunctioning thermostat, a faulty coolant temperature sensor, a coolant fan stuck in the ‘on’ position, or a problem with the wiring for the coolant temp sensor.

A P0128 fault code isn’t a severe problem that will prevent you from driving your truck. However, it is a good idea to diagnose and solve the issue as soon as possible. Continuing to drive with a malfunctioning cooling system can cause issues later down the line. 

The cost of repairing the source of a Dodge, Jeep, Ram, or other vehicle’s P0128 diagnostic code can vary significantly depending on the problem. If the P0128 code is caused by a broken thermostat, repairs could range between $50-$400. However, if the cause of the code is less time intensive to repair, like a stuck coolant fan or faulty sensor, the repair could be significantly cheaper. Keep reading to learn more about the Dodge, Jeep, Ram P0128 engine code. 


Engine Code P0128 Meaning

P0128 – Engine coolant temperature is below the thermostat regulating temperature

While that might sound like a mouthful, the explanation of what a P0128 fault code means is relatively simple. However, you need to know a little bit about how an engine’s thermostat works to be able to understand what is actually going on. 

An engine’s thermostat is responsible for cutting off coolant flow from the radiator until the engine is warm. This allows the engine to reach operating temperature quickly. This is important because engines are designed to operate in a specific temperature window. If an engine can’t reach that window, its internal components will wear more quickly and fuel efficiency will be hurt dramatically.

There are a number of sensors controlled by the engine’s control module (ECM) that are responsible for reporting the temperature of your engine’s coolant and also the time it takes for the engine to reach operating temperature. The coolant temperature sensor and O2 sensors continually monitor the system and report to the ECM. They also report if it took too long for the engine to reach operating temperature, or if the coolant temperature is too low when driving. If the engine took too long to warm up or if the coolant temperature continued to be low when driving, the ECM will throw a P0128 engine code on the next startup.

Dodge, Jeep, Ram P0128 Symptoms

  • Illuminated check engine light
  • Decreased gas mileage
  • Abnormally low temp gauge
  • Heater not working

Most of the time, the first symptom of a P0128 engine code is an illuminated check engine light. Most of the potential causes of a P0128 engine code are pretty under the radar and are rarely catastrophic or immediately recognizable. 

In addition to the check engine light, you might also notice that your Dodge, Jeep, or Ram’s temperature gauge is sitting abnormally low, towards the cold side of the gauge while driving. This is a very good indicator that the engine did not reach its operating temperature and that there is a potential issue with the cooling system. This will also cause the heater to stop working as effectively, which is especially noticeable in the colder months.

Decreased fuel efficiency is another indicator that there might be something wrong with your truck’s cooling system. However, most people won’t catch this symptom early. Drivers typically only recognize it once they have diagnosed the cause of the P0128 DTC. 

Is Check Engine Code P0128 Serious?

Severity Rating: Low

Ultimately, a P0128 fault code on a Dodge, Jeep, or Ram vehicle is not a serious and immediate problem. That is as long as that is the only fault code that your vehicle is displaying. With that being said, it is not a good idea to go an extended period of time without diagnosing and repairing the cause of a P0128 DTC. 

One of the most common causes of a P0128 trouble code is a thermostat that is stuck open. If the engine is never allowed to reach operating temperature, internal engine damage from excessive wear can occur over time. You’ll also likely be spending a fortune on gas that you otherwise wouldn’t have to. Our recommendation is to limit how much drive the vehicle until you can take it to a repair facility.

Causes of P0128 Engine Code

  • Faulty or failing engine thermostat
  • Failing coolant temperature sensor
  • Damaged or incorrect coolant temp sensor wiring

The primary cause of a P0128 check engine code on Dodge, Jeep, or Ram vehicle is a thermostat that is stuck open. A thermostat works like a valve. It opens once the coolant within the engine is warm enough to supply it to the radiator for additional cooling. If the thermostat breaks in the open position, it allows coolant to freely flow from the engine into the radiator. That shouldn’t happen before the engine is warm. 

Additionally, a faulty coolant temperature sensor, which is normally responsible for alerting your engine’s engine control unit, can also cause a P0128 diagnostic code. In this case, the thermostat and cooling system itself are working properly and the coolant is reaching the correct temperature. However, the sensor is sending incorrect information to the ECM. This will still throw the code, despite the cooling system working correctly.

Another possibility is that there is something wrong with the cooling system’s wiring. This is a less common cause of a P0128 fault code, but it is still a possibility. If a sensor’s wiring has been damaged or is not properly secured, it can also send incorrect, or no, information about the cooling system to the ECM, also triggering a P0128 code.

Diagnosing a Dodge, Jeep, Ram P0128 Engine Code

Since there are a number of causes for a P0128 engine code, there are numerous ways to diagnose it. Luckily, there are only a few main components that are responsible for triggering a P0128.
If you lack mechanical training or feel uncomfortable diagnosing the cause of the code, it would be wise to take your vehicle to a trained mechanic. 

Diagnosing a Faulty Thermostat

Since a stuck-open thermostat is the most common cause for a P0128 engine code, that is a good place to start the diagnostic process. To do this most easily, open the vehicle’s radiator cap when the engine is cold, start the engine, and look into the radiator filler neck to observe any coolant movement. Since the vehicle’s thermostat should be in the closed position, there shouldn’t be any coolant movement in the radiator. Keep in mind, this only works if the vehicle has been sitting for an extended period. Do not remove the radiator cap if the engine is warm; the pressurized system can cause serious injuries if the coolant is hot.

In addition to doing a visual inspection of the coolant through the radiator cap, another way of checking is by measuring the temperature of the radiator hose. This is typically done with a laser thermometer. If the thermostat is stuck open, the hose will warm very slowly. If the thermostat isn’t the issue, the hose will begin cold and increase in temperature quickly as the thermostat opens and allows warm coolant from the engine into the radiator. 

Diagnosing a Bad Coolant Temp Sensor

One of the other most common causes of a Dodge, Jeep, or Ram P0128 engine code is a malfunctioning coolant temperature sensor. If the sensor is being supplied with either too much or too little electrical current, it can report faulty information to the engine’s ECM. 

Using a voltmeter to test the voltage being supplied to the sensor is the easiest way to diagnose this issue. The first step is to attach the voltmeter’s positive lead to the signal terminal of the sensor. Then attach the negative lead of the voltmeter to a grounding point. Once you have attached the leads, start the engine. The voltmeter should be reading around 3V-4V upon initial startup. 

As the engine warms, the voltage should continue to drop gradually to around 1.2V-0.5V. If the voltmeter reads 5V or above, it indicates that there is an open circuit. In that case, you should check the sensor’s sensor terminal for a good connection and also the sensor’s ground connection. If the voltmeter reads 0V, it is a sign that there is a short circuit in which case you should check the wiring from the ECM to the sensor. 

Dodge, Jeep, Ram Fault Code P0128 Repair Cost

  • Thermostat: $25-$400
  • Coolant Temperature Sensor: $25-$250
  • Loose connection/wiring issue: $0-$500

In most instances, a P0128 engine code is a relatively inexpensive repair. Almost all of the parts that can cause a P0128 code are cheap to purchase and easy to replace.

Most of the Dodge, Jeep, and Ram vehicles that experience this engine code have easy-to-access engine thermostats. The thermostat itself typically only costs around $15-$45 depending on the make of your vehicle. If you have some mechanical knowledge and come access to tools, the job should only take around 2 hours. That includes the time that it takes to drain the vehicle’s coolant. If the thermostat is the cause of the code and you are planning to take it to a certified repair center to have it replaced, the cost could range anywhere from $200 to $400.

The same can generally be said for replacing a faulty coolant temperature sensor. Like the thermostat, a Dodge, Jeep, or Ram coolant temp sensor usually ranges in price from $15-$40. It is also a pretty easy component to replace, but it does depend on the make and model of your vehicle. A certified technician will likely charge in the ballpark of $75-$250 to replace it. 

Wiring issues are a bit harder to pin down an exact repair cost for. However, they are also the rarest cause of a P0128 engine code. Obviously, if the problem arose due to a loosely connected wire or a poor connection, you won’t have to replace anything. However, if there is damage to the engine’s wiring harness, it would be a much more costly repair.

P0128 FAQ

What does a P0128 engine code mean?

Engine code P0128 means that the vehicle’s engine coolant temperature is below the thermostat regulating temperature. In essence that means that there is a fault somewhere in the vehicle’s cooling system. Whether it be electrical or mechanical, a failed component is either not allowing the vehicle’s engine to reach its proper operating temperature in the proper amount of time or the vehicle’s engine control module is getting an incorrect reading of how hot the vehicle’s coolant actually is.

Is it safe to drive with a P0128 engine code?

For the most part, a P0128 is not a serious engine code that will present any major issues initially. You can still drive your car, but it is a good idea to have the issue diagnosed as soon as you can to prevent a sharp decrease in fuel efficiency, a non-functional heater, and potential engine damage down the line if the problem is left untreated for an extended period of time.

How much does it cost to fix P0128?

The cost of repairing a P0128 engine code can vary depending on the cause of the code. Generally speaking, assume to spend around $250-$350 on solving the problem if the code was the result of a broken thermostat or faulty coolant temperature sensor.

How do I fix engine code P0128?

The first step in fixing engine code P0128 is diagnosing the cause of the code itself. There are a number of common causes of the code on Dodge, Jeep, and Ram vehicles including a stuck-open thermostat, bad coolant temperature sensor, loose sensor connections/wiring damage, or an overactive radiator fan. After pinpointing the cause, you’ll need to replace the part, clear the engine code, and make sure that it doesn’t come back on at a later time.

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