If the radiator fan won’t turn off, there could be a problem with the fan, the engine, the thermostat, or the coolant temperature sensor. You might also have a check engine light on for a related code, like a P0128 code. Find out what to do if you’re having this problem with this article and video.
Causes and Fixes of a Radiator Fan That Won’t Turn Off or a P0128 Code
In the video featured above, code P0128 appeared, which is related to the radiator cooling fans. The coolant is not getting up to the correct temperature, and the computer can’t tell if there’s a problem with the coolant temperature sensor, the engine not working, or the thermostat.
Top Causes of a P0128 Code or a Radiator Fan That Won’t Turn Off
If there is an issue with any of these parts, the computer can turn the radiator cooling fans on high to prevent the engine from overheating.
1. Overheating Engine
The engine could be overheating from a problem with the cooling fans. This can be noticeable at idle, which is when the car is running but parked or braked and sitting still. If you have a car that overheats when at idle but not as much when driving, there may be an issue with the radiator fans. The airflow from driving will help cool the system down, but removing this when at idle can cause the engine to heat up.
More on inspecting cooling fans and an overheating engine at idle
You could also have an overheating engine from a coolant leak. If the coolant level is too low, the radiator cooling fans may stay on.
More on how to check for a coolant leak
2. Coolant Temperature Sensor
The coolant temperature sensor (CTS), also known as an engine coolant temperature sensor (ECT), resistance changes depending on its temperature. The computer takes a reading of its temperature to adjust the engine’s performance. While not common, there can be more than one sensor in some cars and trucks, and one of these may turn the cooling fans on if there’s a problem. Usually the computer turns the cooling fans or a light on the dash on if there’s a problem with how the engine is running.
You can scan your car with a basic OBD-II scan tool. Often you’ll just need to connect it to the port at the bottom left of the driver side dash.
With an advanced scan tool, you’ll be able to gather data about these parts, like the coolant temperature sensor, which can help you find out which part may be causing this problem. You can compare the performance reading of the coolant temperature sensor with the intake air temperature sensor. The readings should be similar. If the coolant temperature sensor reading is drastically different, you’ll know it’s probably defective and needs to be replaced. Spikes or fluctuations could be a sign there’s problem with the sensor.
The CTS will be easier to replace than the thermostat. You may not have to drain any coolant and can just replace the sensor.
3. Thermostat
The thermostat controls the flow of coolant and this helps it regulate the coolant temperature. If the engine isn’t properly heating up to the right temperature, the thermostat could be stuck open. If it is, you’ll need to replace the thermostat.
There could be a problem with the thermostat’s spring. The thermostat is a valve that blocks coolant from just flowing through the engine. Heated coolant on one side will expand the spring, which opens the valve and lets the coolant flow. As the coolant cools, the spring will close to stop the flow. The spring can break down over time and not work as well.
4. Radiator Cooling Fans
If you have a P0128 code or the radiator fans won’t turn off, there could be a wiring issue with the fan. The fan relay could also be stuck on.
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