#OBD Codes

P0128 – Coolant temperature below thermostat regulating temperature

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P0128 – Coolant temperature below thermostat regulating temperature
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Technical definition 

 
Coolant thermostat (coolant temperature below thermostat regulating temperature) 
 

What does P0128 mean? 

 
Trouble code P0128 means that your engine's coolant temperature is much lower than expected. The ECU estimates the expected engine temperature by considering the time since startup and intake air temperature, then compares it with the actual engine temperature.  
 
Typically, this problem occurs when the thermostat is stuck in an open position, which prevents the engine from warming up properly. A faulty engine coolant temperature (ECT) sensor can also cause this issue. 
 
Engine coolant temperature plays an important role in the engine's longevity. Maintaining the proper operating temperature keeps the oil at the right viscosity, provides adequate lubrication to all engine parts, and promotes long-term engine health. 
 

How serious is P0128? 

 
Diagnostic trouble code P0128 is moderately severe. It doesn't usually cause immediate damage but ignoring it can lead to bigger problems in the long run – as running the engine at a lower temperature increases wear on the engine's mechanical components. 
 
Normally, the engine temperature stays between 70-105°C. Lower temperatures make the engine oil thicker, which doesn't provide proper flow and adequate lubrication to all engine components. 
 
In addition, when the engine runs cold, it can produce more emissions and decrease fuel economy. 
 

Can I still drive with P0128? 

 
Yes, you can still drive with a P0128 code. 
 
In the short term, driving with this code probably won't cause serious damage, and you won't notice a big change in engine performance right away.  
 
However, prolonged driving with a P0128 code can put stress on engine components. The main risk is reduced engine life due to inefficient lubrication. While these problems may seem minor initially, they can result in costly repairs down the road. 
 

Symptoms of P0128 

 
The P0128 code often triggers several noticeable symptoms. One of the most obvious signs is the check engine light coming on, along with the temperature gauge not reaching the normal operating range. 
 
In colder weather you may also notice that your vehicle's heater is not working efficiently. This happens because a cooler engine can't generate enough heat. 
 
Another symptom is decreased fuel economy. When the engine operates at lower than normal temperatures, it uses a richer air-fuel mixture and experiences higher energy losses due to thicker oil and bigger gaps between the cylinder walls and pistons. 
 

Common causes of this DTC

 
Several common issues can cause the P0128 code. A stuck open thermostat is a primary culprit which prevents the engine from warming up properly. Replacing the thermostat usually solves this problem.  
 
Another possible cause is a faulty engine coolant temperature sensor. This sensor provides critical data to the ECU about the engine temperature. 
 
Low engine coolant levels can also contribute to the P0128 code. Insufficient coolant prevents the engine from maintaining optimal temperatures. 
 
Problems with the intake air temperature sensor may trigger this code, too. The ECU needs this sensor reading to compare with the coolant temperature. 
 

How to troubleshoot this DTC 

 
Use these tips to accurately diagnose and fix a P0128 code:  
 
Check your vehicle's coolant level
 
While low coolant isn't the most common cause of a P0128 diagnostic code, it is the easiest to fix. Simply pop the hood, check the coolant level, and top off if necessary. It's also worth checking for coolant leaks. 
 
Check for other codes
 
Use a diagnostic scanner to check for other related trouble codes. These may provide additional clues to the underlying problem. Other common diagnostic trouble codes to look for include:
 
  • P0115 - P0119: These codes are related to the engine coolant temperature sensor. 
  • P0115: ECT sensor circuit malfunction 
  • P0116: ECT sensor circuit range/performance problem 
  • P0117: ECT sensor circuit low input 
  • P0118: ECT sensor circuit high input 
  • P0119: ECT sensor circuit intermittent 
  • P0125: Insufficient coolant temperature for Closed Loop Fuel Control. This code also indicates that the engine is not reaching the expected temperature, similar to P0128. 
  • P0126: Insufficient coolant temperature for stable operation. This can indicate problems with the thermostat or the cooling system as well. 
 
Get diagnostic scanner
 
Inspect the thermostat
 
A thermostat stuck in the open position is the most common issue that triggers trouble code P0128. You can check its function by placing your hand on the top hose of the radiator immediately after starting the engine. 
 
Normally, the thermostat opens when the engine coolant reaches operating temperature. Initially, you should feel the hose is cold. Once the engine reaches the proper temperature, the thermostat opens, allowing coolant flow, and the hose becomes hot. 
 
If the thermostat is stuck in the open position, the upper radiator hose will gradually warm up along with the engine coolant temperature instead of staying cold until the thermostat opens. 
 
Another way to check the thermostat is to remove it from its housing and visually inspect its condition. You can also drop it into boiling water to check its functionality. Once removed from the hot water, make sure it closes when it cools. If it stays open when it's cold, it's most likely defective and needs to be replaced. 
 
Analyze live data readings 
 
Live data refers to real-time information provided by the vehicle's sensors. This data includes various parameters such as engine coolant temperature and intake air temperature (IAT). Understanding how to read OBD2 live data can help you find the root cause of the problem.
 
In the OBDeleven VAG mobile app, you can find these values under the engine control unit details.
 
 The image shows the OBDeleven interface with the "Live data" option highlighted on the engine control unit screen, indicating no current engine faults.The image shows the OBDeleven "Live data" screen, listing various selectable engine parameters for real-time monitoring.
OBDeleven live data feature
 
Analyze these values carefully. IAT depends on engine load and ambient air temperature. The higher the engine load, the higher the temperature. The IAT should never be below the ambient air temperature and should not exceed 200°F (95°C), even on hot summer days. If you notice readings outside this range, it may indicate a faulty temperature sensor. 
 
Test the engine coolant temperature sensor
 
You can also use a multimeter instead of an OBD2 scan tool to check the functionality of the engine coolant sensor. 
 
The coolant temp sensor has three pins. Connect the multimeter's terminals to the left and right pins and see how the resistance changes when the sensor is dropped into water bowls of different temperatures. 
 
If the sensor is working properly, you should get about 5,000 or 6,000 ohms in ice cold water and about 150-250 ohms in boiling hot water. 
 
Compare your results to the chart below:
 
This graph shows the relationship between temperature (°C) and resistance (Ω) for a coolant temperature sensor.
Resistance-temperature characteristics of a coolant temperature sensor
 

FAQ

 
How does a thermostat cause a P0128 code?
 
A thermostat stuck in the open position keeps coolant flowing continuously through the radiator, which prevents the engine from warming up to its optimal temperature. Replacing the faulty thermostat usually solves the problem. 
 
How does a coolant temperature sensor cause a P0128 code? 
 
A faulty coolant temperature sensor provides incorrect data to the engine control unit. This causes the ECU to misinterpret the engine temperature. 
 
Is it expensive to fix a P0128 code?
 
The cost to fix a P0128 code varies. Replacing a thermostat or coolant temperature sensor is usually affordable, but more extensive repairs can be costly. 
 
How does a P0128 code affect fuel economy? 
 
A P0128 code can reduce fuel economy. A cold engine runs on a richer mixture, so it is less efficient and uses more fuel. 
 
Can cold weather cause a P0128 code? 
 
Cold weather can make it more difficult for the engine to warm up. However, this should not be the case because the ECU compares the readings from the engine coolant temperature sensor with the readings from the intake air temperature sensor before it triggers the P0128 code. 
 
 
 
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.

Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.