P0128 Code: Meaning, Symptoms and Causes

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.

How Did I Fix the Issue Of the P0128 Code?

When my 2001 Honda Civic started taking longer to warm up, the check engine light came on with a P0128 code. I noticed the upper radiator hose never got hot. After checking forums, I learned it was likely the thermostat stuck open.

I located and replaced the thermostat housing near the top hose for about $15. After clearing the code, the engine reached normal temperature quickly. This fixed my P0128 issue and restored good performance.

P0128 Code Meaning

The P0128 trouble code points to an issue with your engine’s temperature regulation system. Specifically, P0128 code means the ‘Coolant temperature is below the regulating temperature of thermostat’.

The P0128 code sets when the engine takes longer than normal to warm up. The power control module (PCM) looks at coolant temperature, air intake temperature, and RPMs to estimate how long heating should take. If it goes over that time, you get a P0128 code.

Modern engines like to operate between 192 and 212 degrees Fahrenheit. The PCM knows the expected warm-up duration to reach this ideal zone. A P0128 code could mean the thermostat is stuck open, slowing down the warmup. But it could also result from other issues preventing normal heat flow.

The main thing is, a P0128 means your engine isn’t getting up to temp as fast as its computer expects. Tracking down why will lead you to the root cause. Once identified, you can take steps to get heat regulation back to normal.

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.

How Serious Is the P0128 Code?

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 reduced fuel economy and increased wear on the engine.

Some interesting case studies:

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. 

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.

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.

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.

How to diagnose?

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 also 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 be unable to measure the coolant temperature sensor correctly. In this way, it will cause a P0128 code.

How to diagnose and fix?

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 make 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 a radiator cap is dual-stage, meaning you have to tighten it to the second click, not the first. This is intended to prevent the release of pressure, ensuring that the fluid does not boil.

3. Faulty Engine Coolant Temperature Sensor or Wiring

The coolant temperature sensor measures the temperature of the coolant. It sends signals to the engine computer to help control the fuel and keep the engine running efficiently.

Most vehicles have two coolant temperature sensors:

1. Primary sensor: Located near the thermostat in the cylinder head. Sends signals to the computer to calculate fuel delivery and turn the radiator fan on/off. Also called ECT 1.
2. Secondary sensor: Located at the bottom of the radiator. Sends signals to the gauge on the dashboard to show the engine temperature. Also called ECT 2.

Some vehicles like Hondas have both sensors. Others like Jeeps and Chevys only have the primary one.

If either sensor or its wires get damaged, it can cause an error code P0128. This means the computer is not getting proper temperature readings.

The exact location of the sensors depends on your vehicle make and model. Check your owner’s manual to find where they are located in your vehicle.

For example, in a Jeep Wrangler the primary coolant temperature sensor is located under the air intake pipe, where the thermostat housing bolts to the cylinder head.

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

How to diagnose?

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 perform after trying the above-explained methods.

Final Thoughts

In summary, the P0128 trouble code indicates the engine is not reaching normal operating temperature quickly enough.

The most likely causes are a stuck open thermostat, low coolant level, contaminated coolant, faulty coolant temperature sensor, or damaged sensor wiring.

Replacing the thermostat or refilling the coolant to proper levels can often resolve the issue. Disconnecting and reconnecting the battery may also reset the error code.

With some basic troubleshooting, the P0128 code can typically be addressed easily and restore proper engine warm up and performance. Regular coolant flushes and sensor checks will help avoid this issue in the future.