Error code P0128 simply means the coolant temperature is below the ability of the thermostat to regulate. This code may not alert you to the exact nature of the problem, however, it can be an indication of several issues you should check out as soon as possible.
What Does Code P0128 Mean?
Code P0128 stands for Engine Coolant Temperature Below Thermostat Regulating Temperature. It means that the ECM/PCM has “noticed” that the engine coolant temperature is consistently running below the thermostat regulating temperature. In other words, the engine is running cooler than it should.
The P0128 code is set when the engine control module (ECM) determines that the coolant temperature is lower than the original equipment thermostat’s opening temperature.
To put it another way, the engine is running too cool or the temperature does not increase quickly enough. The engine must reach operating temperature promptly so that the vehicle can enter “closed-loop” mode.
In this mode, when everything is warmed up, the ECM begins to use the oxygen sensor(s) for fuel control. It’s all about fuel economy and emissions.
Like so many other components we don’t often think about (like the PCV valve, for example), the ECM/PCM has within its programming the opening temperature of the engine’s cooling system thermostat.
It’s important to note that some OEMs will store a P0125 code if the condition that triggered the P0128 gets bad enough that the engine begins to run even cooler.
Causes of the P0128 Code
The P0128 code is usually associated with the engine’s cooling system. Common causes include:
- Faulty Thermostat: The thermostat regulates the flow of coolant in the engine. If it is stuck open, the coolant will circulate freely, preventing the engine from reaching its optimal temperature.
- Low Coolant Level: Insufficient coolant can result in inadequate heat transfer, causing the engine to run cooler than intended.
- Malfunctioning Coolant Temperature Sensor (CTS): The CTS provides real-time temperature data to the ECM. A faulty sensor may send incorrect signals, leading the ECM to assume the engine is running cooler than it actually is.
- Wiring or Electrical Issues: Damaged wiring or loose connections in the coolant temperature sensor circuit can disrupt communication between the sensor and the ECM.
- Radiator Fan Problems: If the radiator fan is running continuously, it may cool the engine excessively, preventing it from reaching its operating temperature.
- Ambient Weather Conditions: In extremely cold weather, the engine may take longer to warm up, particularly if the thermostat is not functioning efficiently.
Symptoms of the P0128 Code
While the P0128 code itself may not cause severe drivability issues, it can lead to noticeable symptoms. These include:
- Check Engine Light (CEL): The most obvious sign is the illumination of the CEL on the dashboard. This light indicates that the ECM has detected an issue requiring attention.
- Poor Fuel Economy: An engine running cooler than optimal may consume more fuel, as the ECM adjusts the air-fuel mixture to compensate for the lower temperature.
- Reduced Heater Performance: The vehicle’s cabin heating may be less effective, as the engine coolant is not hot enough to transfer sufficient heat to the heater core.
- Prolonged Warm-Up Time: The engine may take longer than usual to reach its operating temperature, especially during cold starts.
- Emission Test Failure: A cooler engine may result in incomplete combustion, leading to higher emissions and a failed emissions test.
Diagnosing the P0128 Code
Diagnosing the P0128 code involves several steps to pinpoint the root cause. Proper tools and techniques are essential for an accurate diagnosis.
- Use an OBD-II Scanner: Connect an OBD-II scanner to retrieve the P0128 code and any related codes. Note the freeze frame data, which provides details about the engine’s operating conditions when the code was triggered.
- Inspect the Coolant Level: Check the coolant level in the reservoir and radiator. Low coolant can cause inaccurate temperature readings.
- Check the Thermostat: Inspect the thermostat for signs of malfunction. A stuck-open thermostat is a common cause of the P0128 code. Testing may involve removing the thermostat and placing it in hot water to observe its operation.
- Test the Coolant Temperature Sensor: Use a multimeter to measure the resistance of the CTS. Compare the readings with the manufacturer’s specifications to determine if the sensor is functioning correctly.
- Inspect Electrical Connections: Examine the wiring and connectors related to the coolant temperature sensor for damage, corrosion, or loose connections.
- Monitor Real-Time Data: Use a scan tool to monitor real-time coolant temperature data. If the temperature remains consistently low despite normal driving, the thermostat or sensor may be at fault.
Fixing the P0128 Code
Once the root cause of the P0128 code is identified, appropriate repairs can be made. Here are some common fixes:
- Replace the Thermostat: If the thermostat is stuck open or not functioning properly, replacing it is the most effective solution. Ensure you use a thermostat that meets the manufacturer’s specifications.
- Refill or Replace Coolant: Low coolant levels can be resolved by refilling the system with the appropriate coolant. If the coolant is contaminated or degraded, a complete flush and replacement may be necessary.
- Replace the Coolant Temperature Sensor: If the CTS is faulty, replacing it will restore accurate temperature readings. Ensure the replacement sensor is compatible with your vehicle.
- Repair Electrical Issues: Address any wiring or connector issues, such as repairing damaged wires or cleaning corroded connectors.
- Inspect the Radiator Fan: If the fan is running continuously, check the fan relay and control module. Replace any faulty components to restore normal operation.
- Address Weather-Related Concerns: In extremely cold climates, using a block heater or ensuring the thermostat is functioning optimally can help mitigate P0128-related issues.
Preventing the P0128 Code
Preventative maintenance can reduce the likelihood of encountering the P0128 code. Consider the following tips:
- Regular Cooling System Maintenance: Flush and replace the coolant at intervals recommended by the vehicle manufacturer. Inspect and replace the thermostat and CTS as part of routine maintenance.
- Monitor Coolant Levels: Regularly check the coolant level and top it off if necessary. Inspect for leaks in the cooling system.
- Inspect Electrical Components: Ensure wiring and connectors in the cooling system are in good condition. Address any issues promptly to avoid sensor-related problems.
- Prepare for Seasonal Changes: In colder climates, take extra precautions, such as using winter-grade coolant and ensuring the cooling system is in optimal condition.
How Serious Is the P0128 Code?
Any time your car’s OBD system triggers an error code it should be taken seriously, particularly one regarding the powertrain such as P0128. As it relates to a P0128 error code, vehicle coolant cools down your car much like how sweat is the body’s natural way of cooling itself.
Therefore, when your car reaches a certain temperature, your vehicle’s thermostat should open up and allow coolant to flow through the engine. Code P0128 monitors how fast the coolant reaches a certain temperature rather than tracking if a car is overheating by default.
A P0217 code would indicate whether or not the vehicle is overheating. Code P0128 is a fault condition that is gauged against similar codes with the main result being an increased emission level and poor fuel economy.
Can I Still Drive with a P0128 Code?
P0128 isn’t the worst code out there, but you should still have a professional mechanic check your car ASAP. Doing so assures you that your vehicle is always operating at its best, while preventing any additional damage.
Ultimately, you should take your car to a nearby mechanic for repairs whenever experiencing a P0 code of any variation.