The S-Class is the Mercedes-Benz flagship model and without a doubt a masterpiece. The S-Class, W221 chassis was manufactured from 2006 until 2013. Even though it is such a complex luxury sedan, it is one of the most reliable S-class Mercedes-Benz has made in the last couple of decades. 

That doesn't mean that the S-Class W221 has no issues. In this article, we go over the most common problems that affect models such as S350, S550, and S400 Hybrid. 

Auxilary Battery

w221 s class auxiliary battery problem

The W221 S-Class has two batteries. The main battery starts the engine but the second, auxiliary battery, runs various electronics. Because the W221 has an electronic transmission shifter, it is also used to put the vehicle in Park if the main battery fails. 

Just like with any battery it needs to be replaced. The auxiliary battery mostly fails if the car is over 7 years old or if the vehicle has over 100,000 miles. Replacing the auxiliary battery on a W221 is relatively easy and can be completed at home. 

Air Suspension Compressor

mercedes s class s550 air suspension compressor problem

The S-Class is equipped with air suspension. The air compressor is the heart of the air suspension. It is utilized to raise and lower the vehicle.

It often fails, or it might wear internally to the point where it's not generating enough pressure to raise the car. Once the air compressor fails, it causes the vehicle to drop to the ground and the warning message "Stop Vehicle Too Low" displays on the dashboard. 

The compressor is located behind the front bumper, passenger side. It can be easily replaced in about an hour.  It used to be a costly repair, but the price of a W221 air compressor has come down significantly, You can find a replacement W211 air suspension compressor for under $150 online.  It is an easy job that any car owner with a few basic tools can complete in their driveway. 

Transmission Electronics 

transmission valve body s550

Another common problem that affects primarily the pre-facelift model (from 2007 to 2009) has to do with the transmission electronics. 

The valve body or the conductor plate can fail, which sends the car in limp mode, and the transmission no longer shifts.

In the early days, the only one who could replace the valve body was at the dealer. The valve body has the Transmission Control Unit (TCU) mounted on it which requires coding. Mercedes-Benz was refusing to sell these parts even to independence auto repair mechanics.

Today not only will the dealer sell the valve body to Independence repair shops, but now you can find third party transmission experts that are capable of restoring your old valve body and conductor plate. Also, the right transmission expert can take a used conductor plate and program it to match your Mercedes-Benz VIN. 

Amplifier 

amplifier

The amplifier is what does all the heavy work of running the COMAND. It is located on the left side of the trunk. S-Class COMAND uses a fiber optic loop to connect different components such as radio, navigation, and CD changer. 

When the amplifier fails, you will have a radio that turns on, but you don't hear any sound. Other problems caused by a lousy amplifier include the command does not turn on, or your battery drains when the vehicle is parked 

OM642 Diesel Engine

s class diesel problems

If you have an S-class with the diesel engine such as the S350 BlueTech with over 200,000 miles, you are very likely to start expiring problems with the oil cooler, chain, swirl flaps, turbo actuator, EGR, and injectors especially if the vehicle wasn't maintained properly.

This engine is very popular in the European market. Very few S350 BlueTech models with diesel engines were sold in the US market.

Window Problems

s550 door module problems

It's not uncommon to see Mercedes Benz S-Class with a window that will not go up or down. The problem, in most cases, is the window regulator, but it can also be the window module itself.

The window regulator is the mechanism that moves the window up and down. The window motor itself can also fail, but it much less likely to happen.  To fix the window problem, you need to need to remove the door panel and replace the window regulator.

The door modules are the brains that control the window operation. They often fail due to corrosion or internal failure and need to be replaced. 

Air Suspension Airbag

s550 common problem air suspention airbag

The S-Class suspension is overall more reliable than the suspension of the W220. It is not uncommon for the air suspension airbags to fail. It is a repair that you should expect once the vehicle passes the 100,000-mile mark.

It is most likely to happen sooner if you do a lot of City driving or drive on roads that are poorly maintained. 

With that said, we often see S550 with over 150,000 miles with original air suspension airbags.

Rear Level Height  Sensor 

rear level senosr

The rear level suspension sensor is located above the rear axle and determines the height of the vehicle on the back. The sensor can fail, which causes the car to drop to the ground, again only at the back.

It will affect both rear struts, unlike when you have a bad rear airbag that causes the car to drop at one corner. 

It is an inexpensive repair, and many S-Class owners with DIY skills can fix this problem themselves. The challenging part is accessing the rear level sensor mounted above the rear axle.

Gear Selector Module 

gear selector module

The S-Class no longer has a mechanical connection from the gear shifter to the transmission. It uses what is known as a shift by wire.

When you change the gear between PRND, the shifter module sends a signal to the transmission valve body, which then controls the valve body. 

The module responsible for this is known as the Intelligent Servo Module (ISM) and can fail. 

Inverter Converter Assembly - Hybrid Only

mercedes problems

The High Voltage Inverter Converter is only found on Mercedes-Benz S400 Hybrid. Unfortunately, it is a widespread problem that seems to affect virtually all S400 vehicles as they age. 

Once the Inverter Converter fails, the S-Class no longer starts. The part is challenging to find and is not available from any aftermarket parts supplier other than China suppliers.

At the dealership, the Inverter Convert Assembly (Part number:221 540 44 50) costs between $2500-$3000. The repair itself can range between $5000 and $6500 if you take into account time and parts.

Depending on how many miles are on the S400 Hybrid, fixing this problem can sometimes cost more than what the car is worth. 

A/C Blower Motor

ac blower motor

The blower fan may fail which means your A/C and heat stop working. The blower motor usually fails due to moisture. At first, the blower motor will work intermittently, turning on and off at randomly.

Eventually, the blower motor fails completely and will not turn on at all leaving you with no air conditioner and heat. 

When the blower fan starts working intermittently it is time to get the vehicle checked. Don't wait until it completely fails because it will happen when you least expect it. Replace the blower motor before it completely fails. 

Replacing the blower motor at the dealer costs between $650-$900. It is not a difficult repair and if you decide to replace it yourself plan on up to two hours. 

Other possible problems that can affect the W221 include:

  • Camshaft Adjuster Solenoid Can Fail
  • Crankshaft Position Sensor Can Fail
  • Driveshaft Flex Disc May Crack
  • Auxilary Battery 

Conclusion

The most common problems with Mercedes-Benz S-Class W221 (S350, S400, S550) are the transmission valve body, air suspension compressor, suspension airbag, and amplifier. These repairs are generally expensive with each individual repair ranging between $700 and $1500. If you have DIY skills fixing these problems yourself, it will cost you around $150 - $300 for the repair. 

A well maintained Mercedes-Benz W221 S-Class, especially the S550 4Matic, can be a pleasure to drive and, overall is a very reliable vehicle.

The W221 doesn't have any common major problems that require a complete engine or transmission overhaul. Keep in mind that any problem with the S-Class will be expensive to fix if you depend on your mechanic or Mercedes-Benz dealer.

If you don't plan on repairing your S-Class yourself, it is worth considering an aftermarket vehicle warranty. 

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