Mercedes OM656 Engine Problems (6-Cylinder – 350d, 400d)

The OM656 diesel engine from Mercedes-Benz ended the long-lasting reign of the legendary but somewhat complicated OM642 V6 engine. With the introduction of the OM656 engine in the new W222 S-Class, the expectations were sky-high.

This article dives deep into all the common OM656 engine problems and concerns. We base the contents of this article on NHTSA reports, owners forums and personal experience.

Basic OM656 Engine Specs

The OM656 engine brought the term “clean diesel” to life by featuring exhaust gas recirculation and AdBlue injection to further reduce the harmful emissions. The engine lays in a straight 6 configuration with an aluminium alloy block and head.

With 4 valves per cylinder and variable valve timing (VVT) and a single BorgWarner turbo configuration, the OM656 brings more than enough power to the table.

There are 2 versions of this engine:

  • OM656 D29T R SCR (350d)
    This version pushes out 210 kW or 282 horsepower and 600 Nm or 443 lb ft of torque.
  • OM656 D29T SCR (400d)
    The 400d, the stronger version of the OM656 engine, pushes out 250 kW or 335 horsepower and 700 Nm or 536 lb ft of torque.

Common Mercedes OM656 Engine Problems

If you’re just here for a brief second, here’s everything you need to know served in a form of a short list.

Common Mercedes OM656 engine problems include:

  • Vacuum/Tandem oil pump failure (recalled in 2022)
  • Rocker arms bearing failure
  • Carbon build up
  • AdBlue and NOx sensor failures

There are not a lot of things going wrong with the OM656 engine at the moment, and we can’t exactly say that these engines haven’t had the time to show their issues. They are now 6 years old and plenty of them have covered a significant amount of miles.

If you want to learn more about common Mercedes engine problems, consult our ultimate guide!

OM656 Vacuum pump failure and recall

When this issue first started occurring, people were quick to call it a second iteration of the infamous “diesel gate”. Mercedes was reluctant to admit the issue at the beginning, but the number of failure was hard to ignore.

In January 2022, Mercedes recalled almost 850,000 vehicles with the OM654 and the OM656 engine due to a potential loss of braking fire and a fire hazard. This was due to a leak that could occure on the vacuum controlled coolant pump.

The leak may appear between the coolant and vacuum circuits. As you can imagine, if the coolant came into contact with the electrical components, this could spark an electrochemical reaction which could result in a fire.

Rocker arms bearing failure

While it is not as common as on the OM654 engine, the OM656 engine is not completely void of the rocker arms bearing failures. Ignoring this problem can also lead to camshaft lobe damage and significantly higher repair costs.

We’ve covered this issue extensively in our review of the OM654 engine problems. Please read that guide to learn more.

Carbon build up

Just like any other modern diesel engine and directly injected petrol engine, the OM656 hates short trips and stop-and-go traffic.

Cars with this engine are meant to be mainly driven on long distances. If you need a city commuter, an OM656 powered Mercedes diesel is not the car for you. Not only will there be carbon build-up in the engine, you’ll also experience problems with oil dilution and emission regeneration systems.

Read our full guide on carbon build up in the engine and how you can prevent it.

AdBlue and NOx sensor failures

Modern diesel engine, like the OM656, all feature some sort of Selective Catalytic Reduction or SCR exhaust system. The purpose of this system is to reduce harmful exhaust emissions and to comply with all the demanding environmental policies.

While it isn’t exactly an engine problem, there are a number of owners that experiences problems with:

  • failing NOx sensors (not exactly cheap to replace)
  • clogged AdBlue lines and injector
  • failed heating elements of the AdBlue system
  • cracked AdBlue tanks

Some repairs of this system can quickly amount to $1000 and more.

To learn more, check our dedicated Mercedes AdBlue SCR system guide!

Which Cars Have the OM656 Engine?

As we mentioned earlier, there are two variants of the OM656 engine, here are all the cars that carry either the 350d or the 400d variants.

OM656 D29T R SCR – 350d

  • W222 Mercedes-Benz S-Class S 350d
  • W223 Mercedes-Benz S-Class S350d
  • C257 Mercedes-Benz CLS 350d
  • X167 Mercedes-Benz GLS 350d
  • V167 Mercedes-Benz GLE 350d
  • W213 Mercedes-Benz E-Class E 350d
  • W463 Mercedes-Benz G-Class G350d

OM656 D29T SCR – 400d

  • W222 Mercedes-Benz S-Class S 400d
  • W223 Mercedes-Benz S-Class S 400d
  • C257 Mercedes-Benz CLS 400d
  • X167 Mercedes-Benz GLS 400d
  • V167 Mercedes-Benz GLE 400d
  • W213 Mercedes-Benz E-Class E 400d
  • W463 Mercedes-Benz G-Class G 400d


Roughly 6 years after the release of the OM656 engine, this engine is still known to be a reliable 6-cylinder workhorse. Compared to the OM642 engine which I have in my W211 E-Class, there’s certainly less common problems.

Frequently Asked Questions

Is the OM656 engine reliable?

Yes, the OM656 6-cylinder diesel engine from Mercedes is considered highly reliable. Our research shows that there is very little common problems and the problems that do exist are generally known and easy to resolve.

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