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. This article discusses the most common problems that affect models such as S350, S550, and S400 Hybrid.
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 has over 100,000 miles. Replacing the auxiliary battery on a W221 is relatively easy and can be completed at home.
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 essential tools can complete in their driveway.
3. Transmission Electronics
Another common problem that affects the pre-facelift model (from 2007 to 2009) primarily has to do with transmission electronics found inside the 722.9 7-speed automatic transmissions.
The transmission control unit (TCU), 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 top. This means they are both located inside the transmission. In addition, the TCU requires coding.
Mercedes-Benz was refusing to sell these parts even to independent auto repair mechanics. There are a number of companies that specialize in reprogramming used TCUs, a service that is not offered by any Mercedes-Benz dealership.
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 TCU, valve body, and conductor plate. The right transmission expert can also take a used conductor plate and program it to match your Mercedes-Benz VIN.
The pre-facelift W221 S600 was equipped with the 5 Speed 722.6 transmission, as the 722.9 could not handle the torque of the V12 at the time.
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 not turning on or your battery drains when the vehicle is parked.
5. OM642 Diesel Engine
If you have an S-class with a 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.
6. Window Problems
It's not uncommon to see a Mercedes Benz S-Class with a window that will not go up or down. In most cases, the problem 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 is much less likely to happen. To fix the window problem, you 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 or updated.
To update the door module on a W221, you need an authorized Star Diagnostic scanner with an active Daimler login. This problem was common for models up to VIN# A161249.
7. Air Suspension Airbag
The S-Class suspension is overall more reliable than the suspension of the W220. It is not uncommon for 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 many City driving or drive on poorly maintained roads.
With that said, we often see S550 with over 150,000 miles with original air suspension airbags.
8. Rear Level Height Sensor
The rear level suspension sensor is located above the rear axle and determines the vehicle's height 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.
9. 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 controls the valve body.
The module responsible for this is known as the Intelligent Servo Module (ISM) and can fail.
10. Inverter Converter Assembly - Hybrid Only
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 consider 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.
11. A/C 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 randomly.
Eventually, the blower motor fails 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. Please don't wait until it ultimately 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 complex repair, and if you decide to replace it yourself, plan on up to two hours.
Other possible problems that can affect the W221 include:
The front, optional dynamic multi-contour seats look simple, yet they are complex.
They contain numerous air bladders, valves, hoses for the air bladders, electronic control modules, Motors for seat adjusting fail for the seat ventilation, and the seat heating pads. On top of that, there is also a separate external air pump for the bladders located in the trunk. And what can go wrong? Well, mostly, the air bladders can leak somewhere. Then occasionally, the electronic modules can be faulty, or they will only need a software update, and sometimes even the seat wiring can be damaged.
Sunroof drains can clog. There are four drains on the regular sunroof, but if you don't want to remove the headliner, you will be able to access only the front drains, so occasionally carefully clean them with some fish tape or edge trimmer line.
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 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 most reliable W221 is the 2008-2011 S550, equipped with the naturally aspired M273 engine and 7-speed automatic transmission. If you are looking for an upgraded look, the 2010-2011 S550 is a great choice. The 2012 S550 has a new twin-turbo engine. Even though this engine has a few issues, it is not expected to last as long as the M273 engine.
The W221 doesn't have any common significant 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 a mechanic or Mercedes-Benz dealer.
If you don't plan on maintaining the S-Class yourself, it is worth considering an aftermarket vehicle warranty.