P0128 – Coolant temperature below thermostat regulating temperature

 

Coolant thermostat (coolant temperature below thermostat regulating temperature) 

 

 

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. 

 

 

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. 

 

 

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. 

 

 

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

How to troubleshoot this DTC 

• P0115 - P0119: These codes are related to the engine coolant temperature sensor. 

 

 

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. 

 

 

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.

 

 

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. 

 

 

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:

 

 

 

 

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. 

 

 

A faulty coolant temperature sensor provides incorrect data to the engine control unit. This causes the ECU to misinterpret the engine temperature. 

 

 

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. 

 

 

A P0128 code can reduce fuel economy. A cold engine runs on a richer mixture, so it is less efficient and uses more fuel. 

 

 

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.