P0128 code is a common problem related to the coolant temperature in vehicles like Honda Civic, Hyundai Accent, Jeep Wrangler, and Chevys. This guide will include all possible causes of the P0128 trouble code and will walk you through all troubleshooting techniques to get rid of this problem.

Bonus Read: P0171 Code

P0128 Code Meaning

The exact meaning of the P0128 trouble code in vehicles is that the ‘Coolant temperature is below the regulating temperature of thermostat’. The engine control module sets the P0128 code when the engine isn’t warming up as quickly as expected due to the low temperature of the coolant. If the P0128 code sets, it doesn’t always mean that the coolant thermostat is bad. It can also mean that the engine has failed to reach a certain temperature within a specific amount of time.

The engine power control module (PCM) throws the P0128 code because it has engine coolant temperature data, intake air temperature data, and engine RPM data. Based on those variables, PCM calculates how long the engine should take to reach operating temperature. If it doesn’t reach that temperature in the expected time, it sets P0128. Modern car engines work at a certain range of temperatures ranging between 192-degree Fahrenheit and 212-degree Fahrenheit.

What Happens If Engine Doesn’t Get Warm Enough Quickly?

When an engine doesn’t get warm enough, it will get bad fuel mileage and the engine oil may start to get sludged up with the deposits in the engine. If the engine takes time to warm up, the engine oil will not properly to the vital parts of the engine. When the engine is cold, motor oil is quite thicker. So, it is important to warm the engine as fast as possible to enhance the flow of the motor oil.

During the engine warmup phase, the engine efficiency is low due to the following reasons

• More fuel is consumed due to high friction at engine components exerted by cold and thick motor oil
• More fuel is consumed due to sess efficient combustion at cold conditions. This is because when the engine is cold, the fuel is less likely to evaporate and create the correct ratio of air and vaporized fuel for combustion. So, the electronic fuel injection system injects more fuel into the air/fuel mixture to compensate for the cold. Due to this reason, the engine runs on the rich (more fuel) mixture. 
• Engine PCM controls the delivery of the fuel. In the start, allows more fuel through the injector so that the car temperature can be warmed up quickly. PCM is also programmed to monitor how long it takes the engine to reach the right temperature. PCM gets this data from the coolant temperature sensor.

Moreover, warming up the engine quickly sets it in the closed-loop operation, meaning that the oxygen sensor and mass airflow sensors of the engine send signals of unburnt oxygen in the air and the flow rate of air through the engine intake respectively.

In a closed-loop operation, the fuel supply to the engine is automatically adjusted by the ECM based on the signals from the sensors. So, if the engine doesn’t warm enough, efficient fuel combustion will not take place. As a result, it will affect the fuel mileage.

More About Error Code P0128

In P0128, P means that the error code is related to the powertrain of the engine. The second digit after P i.e. ‘0’ represents that the code is generic. It means it applies to all vehicles. The third digit i.e. ‘1’ indicates that the issue is with the fuel and air metering of the system. The last two digits i.e. ’28’ indicate where the exact issue lies in the engine.

How Serious Is the P0128 Code?

P0128 trouble code should be taken seriously as it is related to the powertrain of the engine. The engine cooling system is responsible for keeping the engine’s internal components at a constant temperature.

In cars, coolant has the same role as our body sweat. When our body heats up, the sweat absorbs the heat and maintains the body temperature so that our organs function properly. In the same way, the temperature of a coolant should be optimum to warm up or cool down the engine so that its mechanical and electronic components function properly. So, the ultimate results of a P0128 code, if not given attention, will be a reduced fuel economy and increased wear on the engine.

In the past, several studies have been conducted to determine the affect on the engine if its warm up process is delayed. In different studies, Trapy and Damiral (link) and Will and Boretti (link) reported that during the cold-start warm-up phase, only 9% of energy from the fuel is converted to effective work, a reduction of approximately 30%, compared to an optimum warm engine condition.

In another research, it was determined that energy loss due to friction within the engine during the early stages of warm-up (in the range of 20 ⁰C) was estimated to be 2.5 times higher compared to when in optimum temperature conditions. 

Since the engine warm-up process is directly linked with the coolant temperature. So, if the engine doesn’t get warmed up quickly due to the P0128 code and flow of coolant through the radiator, it can cause energy loss and less efficient combustion due to the burning of a rich mixture.

So, instead of ignoring the P0128 code, I would suggest you replace the thermostat if the issue is because the thermostat is stuck open. It is a pretty cheap fix as it only requires around $20 to replace the thermostat.

Symptoms of P0128 Code 

When the OBD throws the engine throws P0128 code and the engine doesn’t reach normal operating temperature within a certain time, the check engine light turns on. One problem or symptom you may observe with the P0128 code is that your heater will never blow hot air just as your engine will never reach normal operating temperature. The other issue with the P0128 code will be that your engine will run rich all the time. So you will have a lower MPG.

Causes of P0128 Code

Here are the causes of the P0128 code:

1. Coolant Thermostat Is Stuck Open or Leaking

The most common cause of a P0128 code is that the engine coolant thermostat is stuck open. The engine thermostat is located between the engine and the radiator. You will find a coolant thermostat close to the water pump on the cylinder head where the top radiator hose joins the engine. Where the hose joins the engine, you will see housing with bolts on it. The thermostat will be enclosed in that housing. It is also called T-stat housing.

A thermostat is a temperature-sensing device that measures the temperature of the engine coolant. The engine thermostat regulates the flow of coolant in the radiator. 

When the temperature rises above a certain level, the thermostat opens the valve to allow more coolant to flow into the radiator, thus lowering the temperature of the engine. Once the temperature of the engine drops below a certain point, the thermostat closes the valve to restrict the flow of coolant.

At cold engine startup, the coolant thermostat closes the valve so that the coolant cannot flow through the radiator, and thus, the engine can be warmed up faster.

Now, when the thermostat is stuck in the open position or the coolant is leaking through it, there is a continuous flow of coolant into the radiator, causing the engine to run cooler than recommended or delaying the warm-up time of the engine.

A very simple way to diagnose the coolant thermostat without removing it is to start the engine and put the hands on the radiator hose close to the thermostat location. If the thermostat is stuck open or leaking, the radiator hose will quickly get hot.

You can follow this video to diagnose a bad coolant thermostat:

Replacing the thermostat is quite cheaper as you only need to locate the housing of the thermostat by tracing the bug radiator hose. It can be done in a few minutes. Make sure to get the OEM thermostat.

2. Low Engine Coolant Level and Poor Coolant Quality

It is mainly because of a poor coolant quality that can also cause a P0128 code. If the coolant is dirty, the thermostat will get clogged up with the dirt, rust and debris in the coolant, causing it to stick open.

Moreover, a low coolant level in the engine can be an indication that your engine is consuming too much coolant, due to which it is not warming up. Also, if the coolant level is quite low, the tip of the coolant temperature sensor will not completely submerge in coolant, due to which it will not be able to measure the coolant temperature sensor correctly. In this way, it will cause a P0128 code.

The color of a clean coolant is green or blue. If you find a coolant discolored. You should flush the engine. If the coolant is discolored, you need to flush the engine coolant system.

To flush the coolant system, keep the radiator valve open and the drain pan in place, and then run water through the radiator. The drain valve of a radiator is usually located at the bottom. Flush the coolant system until you see the transparent water flowing out of the drain valve.

If your coolant color is fine, you need to check whether the coolant level is low or not. To check the coolant level, you need to check the coolant in the overflow tank of the radiator. It is usually white in color. You can easily find it. On the overflow tank, you will the two marks. When the engine is cold, the coolant level should be at the low mark on the tank; when the engine is hot, the coolant level should be at the full mark. In some cars, marks on the coolant reservoir are named the ‘max’ and ‘min’.

The distance between the marks on the coolant reservoir indicates how much the coolant goes from cold to warm. Always check the coolant level in the reservoir when the engine is cold. If the coolant level is below the lower mark, you need to fill the coolant in the overflow tank. If the engine is hot, there can be pressure in the coolant reservoir. If you open the cap in such a condition, the coolant gets onto your face. 

Now, most people do the mistake of filling the coolant in the reservoir up to the full mark. This is not correct as the coolant expands when the engine is heated. So, make sure the coolant level should be just above the lower mark on the coolant reservoir. Also, make that you add a 50/50 mix of the coolant.

If you have a major loss of coolant or you have to flush the coolant system,  you need to add coolant directly to the radiator. Also, note that Also, please note that a radiator cap is dual-stage, meaning you have to tighten it to the second click, not the first. This is to prevent any pressure to release and fluid to boil.

3. Faulty Engine Coolant Temperature Sensor or Wiring

If the engine coolant temperature sensor or its wiring is damaged, it can also cause a P0128 code in your engine.

Many people confuse the thermostat with the coolant temperature sensor. The thermostat is just designed to open or close its valve in response to the coolant temperature. On the other hand, it is the coolant temperature sensor (CTS) that sends real-time coolant temperature signals to the engine PCM. The engine PCM uses this data to control the fuel delivery in order to have maximum engine efficiency.

Engines usually use two coolant temperature sensors. One is primary and the other is secondary. The primary coolant temperature sensor is located near the thermostat in the cylinder head below the air intake pipe.

The primary CTS is used for fuel delivery calculations and turning on/off the radiator fan by sending signals to the ECU. The primary CTS has two wires and is connected to the ECU (ENGINE CONTROL MODULE). It is also called ECT 1.

The secondary coolant temperature sensor, called ECT 2, is usually located on the bottom of the radiator on the driver’s side. It is a single wire sensor that sends signals from ECU to the temperature gauge to tell us how cold/warm or hot our car is running. 

Now, not all cars have two coolant temperature sensors. They are usually found in Honda Civic models. Other vehicles like Jeep Wrangler and Chevys have only one coolant temperature sensor. Now, the exact location of the ECT sensor depends on the make, model, and year of the vehicle. So, to get a detailed guide on how to remove ECT sensors, you must check your vehicle owner’s manual. Below is the location of the ECT sensor in the Jeep Wrangler.

And here is the location of ECT 1 and ECT2 sensors in the Honda Civic.

Usually, to remove the ECT sensor, we have to drain a small amount of coolant from the radiator to drop the coolant level below the ECT sensor. Using a socket and torque wrench, you need to remove the ECT sensor. Make sure to turn off the ignition and disconnect the negative battery cable while removing the ECT sensor.

The best way to diagnose the coolant temperature sensor is to immerse it in the water,  measure the resistance of the sensor at different temperatures and compare the readings to the specifications listed in the service manual.

Follow these steps to test ECT sensor:

• Immerse the tip of the sensor in the water.
• Connect a digital ohmmeter to the two terminals of the sensor.
• Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance
  illustration.
• Repeat the resistance at other temperatures by heating or cooling the water.
• If the sensor does not meet the specification shown in the temperature versus resistance chart, it must be replaced.

You can watch this video to learn more:

Another way to test the ECT sensor without removing it from the vehicle is by measuring the voltage across its terminals.

Follow these steps:

• Turn the ignition on.
• Keep the sensor connected to the wire harness of the PCM.
• Connect the voltmeter across the terminals of the coolant temperature sensor.
• When the engine is cold, the voltmeter will read the voltage at around 3 volts.
• As the engine warms up, the resistance across terminals of the ECT sensor, resistance drops. So, the voltage will also drop.

This video is quite helpful.

Now, remove the sensor and measure voltage across the wiring harness connected to the sensor. The voltage should be around 5V. If it is zero, it means the wiring harness is damaged.

4. Disconnect and Reconnect the Battery

On forums, some people have managed to get rid of the P0128 code just by disconnecting and reconnecting the battery and clearing the P0128 code using a scan tool. Also, make sure that the battery should be fresh. This is the last step you can try after trying the above-explained methods.