Fault code P0128 is called “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.
Technical description and explained code P0128
The error code means that the transmission control module (PCM) has detected that the engine has not reached the required temperature level. This did not occur within a certain period of time after the engine was started. Often code P0128 indicates a faulty thermostat, most likely it is stuck in the open position.
To determine if the engine has not reached “normal” temperature, the duration of vehicle operation is taken into account. It also takes into account the intake air temperature sensor (IAT) reading, the coolant temperature sensor (ECT) reading, and the vehicle speed.
If the ECM controller detects within a certain amount of time that the engine has not reached the proper operating temperature and cannot go into closed loop mode. It interprets this as a fault and sets error code P0128.
The ECM will store the Diagnostic Trouble Code (DTC) in memory and may command the Malfunction Indicator Light (MIL) to illuminate. The Diagnostic Trouble Code is defined as “Coolant temperature below the thermostat control temperature”.
Symptoms of vehicle malfunctions
The main signal that an error P0128 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.
It can also be warning signs such as:
- Long engine warm-up time.
- Engine temperature drop when driving at high speeds.
Error P0128 is not serious, the car will be able to start and continue driving. But it is better to solve it as soon as possible, because improper heat exchange can lead to more serious consequences.
Factors that can cause this error code
The error code P0128 can mean that one or more of the following problems have occurred:
- The coolant temperature sensor (ECT) is defective.
- Cooling fan does not turn off causing excessive cooling.
- The thermostat is stuck in the open position.
- Engine coolant level is low.
- Faulty intake air temperature sensor (IAT).
How to fix or reset OBD-2 code P0128
Some suggested steps for troubleshooting and fix the error code P0128:
- Read all stored data and error codes with an OBD-II scan tool. To find out when and under what circumstances error P0128 appeared.
- Clear the error codes from the computer memory and test-drive the vehicle to find out if the error appears again.
- Then check the coolant level (the engine should be cold).
- Visually inspect the electrical wires and connections relating to the sensor for looseness and damage. Repair if necessary.
- Test the thermostat; if it has failed, replace it.
- Check the operation of the cooling fan.
- Clear the error code from the computer memory again, test drive the vehicle to see if the problem is resolved.
Diagnose and repair of problems
The first step in troubleshooting error P0128, is to review the Technical Service Bulletins (TSB). For known problems with a particular vehicle.
Perform a thorough visual inspection to check the wiring for obvious defects such as scratches, scuffs, exposed wires or burn spots. Next, check connectors and connections for safety, corrosion, and contact damage.
Be sure to carefully check the integrity of the wire harnesses and secure any loose wires. If you find a burned wire, solder it and make sure it is properly insulated.
Check coolant and thermostat
Make sure the coolant level is correct, both in the radiator (when cold) and in the coolant overflow tank. Make sure that there are no leaks and that the sealed cap is working properly.
If there is not enough coolant in the system, the engine may not warm up properly. Because if the temperature sensor is not in contact with the coolant, it will not read the temperature.
Using a non-contact thermometer, or a real-time streaming scan tool, check to see if the engine is warming up properly. Normally, the upper radiator hose should stay relatively cold until the thermostat opens. After that, it will warm up quickly.
If the thermostat is open, the coolant will leak constantly, preventing the engine from reaching operating temperature. This can also be the cause of fault code P0128.
Use a thermometer or diagnostic tool to check the engine temperature after 10-15 minutes of operation. If it does not reach operating temperature, check the ECT sensor, wiring, coolant level, and thermostat operation.
On which vehicles does this problem occur most frequently
Fault code P0128 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:
- BMW (325ci)
- Chevrolet (Cruze, Impala, Optra, Silverado, Tahoe, TrailBlazer)
- Chrysler
- Dodge (Stratus)
- Ford (Escape, Focus)
- Honda (CR-V, Civic)
- Hummer (H3)
- Hyundai
- Jeep (Renegade, Wrangler)
- Kia (Rio, Spectra)
- Lexus
- Mazda
- Mercedes-Benz (B200, ML164, W164, W221)
- Nissan
- Opel (Astra)
- Pontiac
- Suzuki
- Toyota
- Volkswagen
- Volvo (V70, XC90)
Fault code P0128 can sometimes be found with other errors. The most common are the following: P0087, P0115, P0116, P0117, P0118, P0119, P0125, P0300, P0410, P0420, P0449, P0601, P0671, P0672, P0673, P0676.
