The P0128 is a less common problem in vehicles. It is a Coolant Temperature below Thermostat Regulating Temperature meaning that the engine is basically cold. In this article, we are going to look at what causes this error code, how to test for it and how to fix it.
Many drivers worry more about the engine getting too hot than it getting too cold. There is no doubt overheating is a much more common problem in vehicles than cooling. However, especially during winter, the engine can overcool.
While this is not a serious problem, it should be fixed immediately for the car to work at optimum conditions. The P0128 error code is found the when the engine is too cold.
It is unlikely that you will experience this error code during summer since when it is hot, the engine is heated up just fine. In winter, the engine might not be able to warm itself and that is when the thermostat problems begin that trigger a P0128 error code.
The P0128 error code means that the engine is not maintaining the right temperature. Within the first 15 minutes of starting the car, the engine temperature should be at 160 degrees Celsius.
However when there is the P0128 error code, the temperature can be even half this when the engine is started. The engine control unit uses this information to regulate the amount of air and fuel mixture. This process will not happen if the engine does not reach the optimum temperature of 160.
Symptoms of the P0128 error code
Most times, there is no noticeable symptom of an overcooled engine other than the error code itself. If any symptoms exist, they are mild and might go unnoticed. They include the following.
Check engine light comes on
The engine light comes on as a P0128 error code affects the engine. When there is the P0128 error code, the temperature can be even half this when the engine is started.
The engine control unit uses this information to regulate the amount of air and fuel mixture. This process will not happen if the engine does not reach the optimum temperature of 160.
The AC and/or the temperature gauge stops working
It can be quite frustrating turning on the ac on a very hot day and being hit by warm air. Unlike the previous issue, this one is a problem with the air conditioning system itself.
The ac system uses a refrigerant and two heat exchangers to cool the air coming into the vehicle. The most common reason of for the ac blowing hot air is if the refrigerant is low due to slow leaks. This happens mostly after a long winter when the ac is in little or no use. The compressor might also be the cause but it is uncommon.
Causes of P0128 error code
This error code is always triggered by a bad thermostat in the vehicle’s engine. The thermostat may be leaking or damaged. However there are other factors that cause this error in cars. The following are the most common causes of a bad thermostat.
Low engine coolant
When the engine coolant is low, the operations of the engine is affected and will cause the engine to cool. This will generate a P0128 error code.
Damaged cooling fan
When he cooling fan is damaged, it may cause the engine to cool when it spins too fast. This will in turn generate a P0128 error code
Poor wiring in the temperature circuit
If there are any loose or damaged wires in the temperature circuit, the engine may fail to warm up to the optimum working temperature. When this happens, the cools more than it is supposed to and a P0128 error code is triggered.
Faulty coolant and air temperature sensor
The ECT (coolant sensor) and IAT (intake air sensor) are responsible for regulating the amount of coolant and air intake in the engine. If either of them is damaged, the engine is affected and cannot warm itself up for optimum working conditions.
How to diagnose P0128 error code
For this process you will need an OBD II scanner and a digital multimeter. The first step is to scan the car for any other error codes. If you find any, fix those first. After doing this, check the coolant system for any rust, dust, debris and leaks.
Clean the dirty parts and repair any leaks you find. Inspect the engine coolant as well. It should be a vibrant green color or red or orange depending on your car. If there is any debris in the coolant, or it appears darkish, flush the coolant and replace it with fresh fluid.
Check the thermostat to ensure it is in good condition. The thermostat is connected to a radiator hose which should barely feel warm before the thermostat opens. The hose will get very hot quickly as coolant flows through it. If it heats up slowly, the thermostat is stuck and needs to be replaced.
Be careful on this step as the hot coolant can burn you. Inspect the radiator fan. Replace it if there is a problem. Finally, use a multimeter to check the ECT. The reading on the multimeter should change as the temperature changes and if it does not, replace the sensor.
How to fix the P0128 error code
After the diagnosis, you should be able to know whether you need to replace the thermostat or coolant temperature sensor (ECT). The following shows steps to replace either of the components. It is important to take the necessary precautions when doing this fix: make sure the car is off and cooled down.
Replacing the thermostat
To replace the thermostat, drain all the coolant out of the radiator tap until the level is lower than the thermostat. Disconnect the hose from the top and unlock the bolts that lock it. Remove the old gasket with scraper tool. Install the new thermostat and reattach the hose. Refill the coolant and you’re now good to go
Replacing the coolant temperature sensor
If the coolant sensor is the triggering the P0128 error code, then you need to fix it immediately. First remove the lead wire from the terminal. Use a wrench to carefully loosen up the sensor.
Clean the threads where the sensor was connected with a dry rug. Install the new sensor by carefully placing and then twisting using your hand. Use the wrench to tighten the sensor into position. Put back the lead wire after cleaning it and inspecting it for damage. Replace the lead wire if damaged and the put it back on.
Since the P0128 error code is not common, it is easy to prevent it from occurring at all. By doing regular maintenance on the car’s coolant system, the error code can be avoided and prevented. Always check the coolant regularly and replace it when it gets dirty and dark. Always flush all the fluid to avoid accumulation of dirt and debris.