Check Engine Light Still On After Filling Coolant: What’s Going On!

You’ve been driving your car for a while now and you’ve noticed that the check engine light is still on after you’ve filled the coolant or have done a coolant flush. You’re not sure what to do and you’re starting to get a little worried.

Before you start to panic, first, you can check if you have filled the coolant to the proper levels in your car. If the level is low, it could be the reason the light is still on.

If the check engine light is still on after filling coolant, the chances are that the coolant thermostat is stuck open or coolant temperature sensor is faulty. Furthermore, there is a possibility that the check engine light of your vehicle was on at the same time when the coolant level was low. You thought that it was because of a low coolant level. So, you filled the coolant and expected that the CEL will go off. But, the cause of the check engine light was something else. You should have an OBD2 scan tool to pull the trouble codes so that you can find the root cause of the check engine light still on after filling the coolant. 

Causes Of Check Engine Light Still On After Filling Coolant

Check engine light still on after the coolant flush is due to the following reasons:

1. Inappropriate Coolant Level

If you have not filled the coolant to the appropriate levels, the engine computer will think that the engine is running low on coolant, due to which it will turn on the check engine light to prevent engine damage from overheating.

If you do a coolant flush and add a new coolant, you need to add coolant in the overflow tank as well as in the radiator.

If you have not properly filled in the coolant, the coolant pressure sensor will detect abnormalities, due to which the engine computer will cause the check engine light to turn on.

Moreover, while filling the coolant, make sure that the engine is cool. The coolant level must be between MIN and MAX marks or FULL/LOW marks on the overflow tank.

coolant level in overflow tank

It is not advisable to fill the coolant up to the MAX level of the overflow tank as the coolant expands when the engine is heated while it is running.

Lastly, after you have filled the coolant to a certain level, make sure the coolant is not leaking from anywhere in the engine. Without the engine running, you should check the water pump.

Some vehicles are known to have water pump problems/leaks. A cold engine will only show signs of a water pump leak as a slight dripping on the pulley attached to the water pump. When the engine is running, the high pressures cause the water pump to reseal. 

Another way to check a coolant leak is to open the oil filler cap and see if you are observing a milky texture or smelling a maple syrup-like smell. A coolant leak also causes white smoke from the exhaust.

Furthermore, you should check the coolant level in the coolant reservoir the next few times you drive your vehicle after topping off the coolant. 

2. Loose Radiator Cap

Loose Radiator Cap

If you see coolant boiling inside the overflow tank, chances are that a radiator cap is loose which will cause the check engine light to turn on after topping off the coolant.

A tight or loose radiator gap causes air to leak into the radiator and form air bubbles in the coolant that can make their way to the engine and damage it.

Moreover, a loose radiator cap will prevent the necessary pressure to build up inside the radiator.

The pressure inside a closed system affects the boiling point of the liquid. The greater the pressure builds up inside the system, the higher will be the boiling point. The lower pressure inside the system will cause liquid to boil at lower temperatures.

So, if the radiator cap is loose, there will be insufficient pressure inside the radiator which will cause the coolant to boil at lower temperatures. This will prevent coolant to flow smoothly to different areas of the engine.

As a result, the engine computer will detect abnormalities in the engine and turn on the check engine light after filling coolant.

So, you should turn the radiator cap clockwise and tighten it fully to the hard stop. Depending on the engine, there will be two clicks when you fully tighten the radiator cap. If the radiator cap has become bad, it is better to replace it.

It is quite a cheaper fix. Make sure to only buy an OEM radiator cap that can hold the designed pressure inside the cooling system of your engine. Otherwise, the high pressure will damage the radiator cap.

3. Loose Radiator Drain Bolt

During the coolant flush, we have to untighten the radiator bolt to remove the old coolant. After the coolant flush, the radiator bolt is not properly air-tightened.

So, it seems a good practice to always apply a thread sealant or Teflon tape to the threads of the radiator drain bolt after you refill the coolant.

Moreover, you should apply the desired torque to tighten the drain bolt as mentioned in the owner’s manual of your engine.

4. Wrong Coolant Type

The wrong coolant type may also turn on the check light after filling the coolant. Every manufacturer specifies the coolant type based on the material of the engine components.

The cooling system contains different metallic components that can corrode if an improper antifreeze or coolant is used.

First, it is recommended to use a 50% concentration of coolant, and use deionized water to mix with the coolant.

Moreover, do not mix two different coolants as their cooling property and flow characteristics can be affected.

You should also keep in mind that the color of coolant or antifreeze is not important. Engine coolant does come in different colors like yellow, blue or green.

If MINI OEM coolant has a blue color, that doesn’t mean you could use any blue-colored coolant. the specs of the engine coolant should match the specs specified by the manufacturer.

Asking from MINI representative, the statement regarding the coolant was:

“Any coolant(no specific brands), with phosphate, silicate, nitrite, and amino-free extended-duty antifreeze with corrosion inhibitor can be used in your MINI.”

5. Air Trapped In Coolant

While doing a coolant flush, air may get trapped in the coolant. Air in the cooling system can cause a host of problems, chief among them being overheating. This is because air is a poor conductor of heat.

When air is trapped in the system, it prevents coolant from circulating properly. This in turn prevents heat from being properly dissipated, leading to an increase in engine temperature. So, overheating of the engine will turn on the check engine light.

Bleeding the engine cooling system with air is important to remove air. The bleeding procedure of the engine cooling system is usually explained in the owner’s manual.

For instance, in Honda engines, there is a bleeder bolt in the thermostat housing. 

Honda advises loosening the air bleed bolt in the thermostat housing, then filling the radiator to the filler neck with the coolant.

When coolant comes out in a steady stream with no bubbles, tighten the bleeding bolt again.

After that, with the radiator cap off, run the engine until it warms up so that trapped air can be removed and coolant properly circulates in the engine.

After that, turn off the engine and check the level of coolant in the radiator. Add a coolant if needed. Install the radiator cap and tighten it fully.

6. Thermostat Is Stuck

The engine thermostat is a temperature-controlled valve 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.

When the engine is cold, the thermostat is closed, blocking the flow of coolant.

As the engine warms up, the thermostat opens, allowing the coolant to flow and keeping the engine at its optimum operating temperature.

If the engine thermostat is stuck in the closed position, the engine will overheat and turn on the check engine light.

To learn more about the T-stat of the engine, you can read my guide on the P0128 code.

While having a routine coolant flush, it is a good practice to also check the thermostat. The chances are that the stuck thermostat is the cause of the check engine light on.

7. Malfunctioning Engine Coolant Temperature Sensor

The engine coolant temperature sensor measures the temperature of the coolant. It’s usually located near the thermostat.

The sensor is there to monitor the engine temperature and relay that information to the car’s computer. If the sensor isn’t working properly, it can cause the check engine light to come on.

Engine coolant temperature can malfunction if a coolant is dirty and may have coated the coolant temperature sensor with dirt.

The exact location of the ECT sensor depends on the engine. In some engines, there are two ECT sensors.

You have to test the primary ECT sensor which is located close to the engine block. You can use the owner’s manual to find the location of the engine coolant temperature sensor.

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.

You can follow these steps to test the engine coolant temperature 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
  • 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.

8. Codes Unrelated To The Engine Cooling System

Sometimes, the cause of a check engine light is unrelated to the engine coolant system.

There are chances that sensors like MAF, Oxygen sensor, Camshaft position sensor or Crankshaft position sensor are malfunctioning and turning on the check engine light. So, you have to use the OBD2 scanner to pull the trouble codes and fix the check engine light issue.

Can Too Much Coolant Cause Check Engine Light?

Too much coolant cannot cause the check engine light. The extra coolant will percolate out the drip tube when it will expand after engine heating. So, if you have a check engine light after overfilling the coolant, it could be due to malfunctioning engine sensors or even the dirty engine air filter.

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