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engine overheating due to failed themostat

Mercedes Thermostat Replacement DIY Guide

In this article, we will show you how easy it is to replace the coolant thermostat on your Mercedes-Benz engine. The thermostat stays closed from the time you first start the motor until the engine operating temperature is reached. At that point, the thermostat opens up and allows the coolant to circulate between the engine block and the radiator where it is cooled down.  The thermostat can fail in open or closed positions. When it fails in the closed position, your Mercedes engine will overheat. When it fails in the open position your Mercedes-Benz engine will take a long time to reach the normal operating temperature.

Symptoms of a failed Mercedes thermostat

The thermostat can fail in open or closed positions.

  • When it fails in the closed position, it can cause your engine to overheat.
  • When it fails in the open position your Mercedes-Benz engine will take a long time to reach the normal operating temperature. In this case, the Check Engine Light may come on. You can verify this by reading the ECU fault codes. In most cases, you will get a P0128 – Coolant temperature below the thermostat regulating temperature fault code.  This article shows you how to use an OBD II scanner to read the check engine / ECU fault code.


Once you confirm that your thermostat has failed, follow the instructions below to learn how to replace it. Replacing the thermostat is easy. Make sure that the engine is cold before you start replacing the thermostat on your Mercedes-Benz.

How to replace the thermostat

Tools you will need:

3/8-Inch Drive Socket Set

Required Parts:

Mercedes Engine Thermostat + Aluminium Housing + Seal Gasket
OES Genuine Coolant/Antifreeze 1 Gallon

Step-by-step instructions

  1. Locate the thermostat. To replace the thermostat, you will need to drain some coolant from the radiator. At the bottom of the radiator of your Mercedes-Benz, there should be a drain plug. Loosen it to release some coolant. If you decide to not remove the coolant note that there will be coolant spilling from the radiator hose and thermostat when you remove the thermostat from the housing.
  2. Loosen the radiator hose clamp. radiator hose to thermostat disconnect
  1. Use theE-Torx socket to remove the two thermostat bolts. Unplug the electrical connector from the sensor if your car has one. The sensor is typically found in 2008,2009,2010 and newer models such as C300 and E350. Remove the thermostat from the engine block. If the thermostat doesn’t come loose, you may need to pry it away using a flat screwdriver.
  2. Install the new thermostat in reverse order. Ensure that you have a new gasket for your thermostat. The old gasket should be removed and discarded.
  3. Tighten the two bolts and reconnect the radiator hose. Do not overtighten the two bolts. If you have a torque wrench, you can tighten these two bolts to approximately 10Nm.install new mercedes coolant themostat e c s clk ml glk gl class
  4. Add coolant to the radiator expansion tank. Make sure there is no air trapped in the cooling system.
  5. Start the engine. Ensure there are no coolant leaks near the thermostat.

Mercedes-Benz Thermostat Problems and FAQ

What is the Best Scanner to use to troubleshoot the Check Engine Light on my Mercedes-Benz?

The YOUCANIC full system diagnostic scanner is a powerful tool that can help you troubleshoot Mercedes-Benz problems quickly and efficiently. This scanner can communicate with all the control units in your Mercedes-Benz, including the engine, transmission, ABS, SRS, and more. By accessing real-time data from these control units, you can pinpoint the root cause of the problem and take appropriate action to resolve it.

Whether you’re experiencing issues with your engine, transmission, or other systems in your Mercedes-Benz, the YOUCANIC diagnostic scanner can provide valuable insight into what’s going on under the hood. This scanner can help you identify error codes, read live data, and perform special functions like reset adaptations or perform component tests. With this information, you can make informed decisions about repairs, maintenance, and upgrades to keep your Mercedes-Benz running smoothly.

Overall, the YOUCANIC full system diagnostic scanner is an invaluable tool for any Mercedes-Benz owner or technician looking to troubleshoot problems quickly and accurately. With its user-friendly interface and powerful diagnostic capabilities, you can save time, money, and frustration by diagnosing and resolving issues with your Mercedes-Benz quickly and efficiently.

What are some symptoms of the thermostat not working properly on my Mercedes-Benz?

Your Mercedes-Benz could overheat or take too long to reach operating temperature. When the engine takes too long to reach operating temperature, the ECU will turn on the Check Engine Light / Warning Light.

Do I need a new thermostat if the check engine light turns on?

In Mercedes-Benz E, C, S, CL, CLK, SLK, GL, G, and ML models, the engine control unit monitors how long it takes for the coolant to warm up. This is important for the engine. Only when the operating temperature is reached, do you get the best fuel efficiency and minimal wear on the engine?

At what temperature does the thermostat open up on a Mercedes-Benz engine?

While this can vary, in the E320  and C240, for example, the thermostat opens up at 87 degrees Celsius.mercede sthermostat

What happens if the thermostat fails in the open position?

If the thermostat fails in the open position, it prevents the coolant from reaching normal operating temperature resulting in an engine that does not warm up properly and an engine warning light illuminating.

Fault codes can be triggered due to engine coolant thermostat failures.

P0128 – Coolant temperature below Thermostat Regulating temperature fault code.

Mercedes thermostat part numbers.

Mercedes M112 M113 Thermostat + Aluminium Housing + Seal 1122030275 will fit the following Mercedes-Benz models:

Mercedes Applications:
2001-2004 SLK320
2001-2005 C320
2005-2005 E320 4Matic
2002-2008 G500
1999-2003 CLK430
2001-2006 CLK55 AMG
2000-2006 S500
2002-2007 ML500
1998-2004 E320
2003-2005 ML350
2001-2002 S55 AMG
2001-2005 C240
1999-2006 SL500
2000-2006 S430
2001-2002 CL55 AMG
2000-2006 CL500
1998-2005 CLK320
2003-2006 CLK500
2003-2004 G55 AMG
2000-2003 ML55 AMG
1998-2000 C43 AMG
2006-2006 S350
2003-2006 E500
1998-2003 ML320
1998-2002 E430
2006-2007 R500
2006-2006 CLS500
1999-2002 E55 AMG
2005-2006 C55 AMG
2005-2005 E320 Base
1998-2000 C280
2002-2004 SLK32 AMG
1999-2001 ML430

272 2722000115  THERMOSTAT will fit the following Mercedes-Benz models:

Mercedes-Benz C230 203.052 2006-2007
Mercedes-Benz C280 203.092 2006-2007
Mercedes-Benz C300 204.081 / 204.054 2008-2011
Mercedes-Benz C350 204.056 2006-2011
Mercedes-Benz CLK350 209.456 / 209.356 2006-2009
Mercedes-Benz E350 211.087 / 211.287 / 211.056  2006-2009  2010-2011 GAS
Mercedes-Benz GLK350 204.987 2010-2011
Mercedes-Benz ML350 163.157 / 164.186 / 164.156 / 164.125 2003-2009  2010-2011 GAS
Mercedes-Benz R350 251.165 2006-2009  2010-2011 GAS
Mercedes-Benz SLK280 171.454 2006-2008
Mercedes-Benz SLK300 171.454  2009-2011
Mercedes-Benz SLK350 171.456 / 171.458 2005-2012

Mercedes Thermostat Replacement Cost

DIY Cost: Under $100 Auto Repair Shop: $200-$350
Applicable Years
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
Applicable Models
E 320, E 500, C 320, C 240, C 32, E 430, E 55, S 500, S 430, S 350, E 350, E 500, ML 320, ML 350 SLK 320 SLK32 AMG CL 430 CLK 320 350 500
Task Difficulty: Easy
Required Time: 30 Min




  1. Hi.i have a C270 CDI(2005)auto(South Africa)
    I have replaced the radiator, water pump and thermostat
    Vehicle swings but won’t Start

  2. thank you for taking the time to put this information out have a good day

  3. I have a Mercedes C200 1996 model when i brought it the man said the head gasket was gone, but there is no steam out exhaust no bubbling in coolant tank, no water in oil, only thing is it leaks coolant out over flow pipe would that be thermostat radiator hose is hot but water in coolant tank is cold. Thanks Anthony.

  4. All very good for the newer gas models. Not even close for my 1992 300D 2.5 Turbo.

  5. Great profesional advise I feel completely honest to say thank you for providing the communities with this great service.my go to place for information.

  6. How tell valve block working S430 220 2000
    Mercedes Benz

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