Classics, Restoration

Electrical Faults On Mercedes-Benz cars

The modern-day challenges of electrical faults on Mercedes-Benz cars.

As we know Mercedes Benz cars have come on leaps and bounds in
technology over the years. Technology is becoming more and more advanced
and with it, a lot more technical than the Mercedes of yesteryear. Each
year when a new car is developed and put into production it is usually
created with new technology, advanced or improved technology from a
model’s predecessor. However, electrical faults will and do still occur.

It is well documented how advanced cars are in this day and age, how
much more convenient they are to use and also with many more warning
signs than before. We have gone from having just one warning symbol (a
generator light on a post-war vehicle) to symbols that can locate a
suitable parking space for your car to a warning sign that can sense if
you are tired when driving and flashes up a coffee sign!

Maybe this just coincides with when your car is thirsty as well! But
nonetheless, your car is more than just a car, it is your ally on the
road to keep you as safe as possible and out of harm’s way. And, should
it need some TLC it will politely advise you on the instrument cluster,
or if the problem is more serious it will let you know in a much sterner
way. But the more equipment a vehicle has and the more technology a
vehicle has, alas, the more that can go wrong.

Mercedes electrical fault diagnosis

Mercedes-Benz = Reliability

Like many situations in life when things are running great, they are running great! but we all know that this isn’t always the case and the same can be said for our Mercedes Benz cars, as loyal and enthusiastic Mercedes Benz owners nothing can frustrate us more than when a warning sign carelessly and nonchalantly pops up on our displays.

Immediate reactions follow:

  • What does that mean?
  • Is it safe to drive?
  • What will it cost?
  • How long will it be off the road?
  • How am I going to get home!?

An immediate call goes out to your local primary dealer, or, in our case a well-respected Independent Specialist for help and advice.

It is quite natural and understanding for people to ask me what I think the solution is and the cost. Of course, this all depends on the type of fault that occurs, some are easy to advise on and some are not.


Electrical Faults – possible examples:

For example, if an ABS/ESP/BAS light should come on the most popular error will be a speed sensor, brake light switch or steering angle sensor. Of course, there are other sensors that can bring these lights on, such as a yaw rate sensor or a lateral acceleration sensor, but in my experience, these are less common.

Another common electrical faults we see are power loss. Especially with diesel engines as there are a plethora of parts on diesel engines that can cause power loss. For example, a turbo, a servo motor on the inlet manifold, an HFM Sensor, an EGR valve, an exhaust back pressure sensor, and DPF pressure sensors.

Electrical Faults Codes:

When you get these type of errors it is important that you take it to a specialist such as ourselves who knows these faults extremely well and know what action/actions to take. Quite often when you are presented with a fault code which demonstrates a faulty part it does not mean that that part is necessarily faulty but something it is making that part fail. This is where it is important that whoever works on your car is more than familiar with the engine and wiring schematics. Quite often we have seen brittle or broken wires between engine control units and engine parts which can cause these issues.

There are also software updates that the engine or gearbox may need in order to cure a running problem. Or a starting problem or even to smooth out gear changes. This is something that only a specialist or dealer will have the ability to do. The same can be said for ABS issues. Even though the ABS sensor is showing a fault it could be wiring to the sensor. Or it could be a defective impulse ring causing an ABS defect. So it is always important to uncover every stone when diagnosing a fault.

Mercedes Care​

One thing to consider when you have an electrical fault with a Mercedes is to ensure that you regularly service your vehicle and take your car to a Mercedes specialist. On many occasions, we have taken on work from other non-Mercedes workshops due to them not having experience with either the car or the fault. They will, of course, attempt to cure the fault but quite often they will hit a dead end and pass the work on to us.

This can sometimes set us back in our diagnosis as we have seen parts either fitted incorrectly, ie, an HFM sensor fitted upside down (which is something you wouldn’t usually look for), fuses missing or fitted to incorrect locations, parts fitted but not coded as others do not know that parts have to be coded, incorrectly fitted gearbox electric plates and cars serviced by other garages but do not know the common issues that certain cars have and will therefore not advise the customer according.

Customers may think taking it to a non-Mercedes garage will be cheaper. However, they may end up paying more than they would have if they had taken it to a Mercedes specialist. The customer could have to pay for our time and the other workshops’ time rather than just paying once.


When a customer has an electrical fault with their car, the first question they understandably ask is how much is it going to cost. Naturally, this is impossible to answer without seeing the car or at least placing it on the Star Diagnosis machine. This will establish the fault code or codes. It is always a stage-by-stage process that I induce.

How to Identify Mercedes Electrical Faults

  • Stage 1, plug the Mercedes into the Star Diagnosis. Read the fault codes.
  • Stage 2 Advise the customer what the fault code/codes are. On many occasions we can then give a price on the cost of the repair as the fault code is an open and shut case. For instance, a turbo fault or servo motor, if there is a loss of power. Or a steering angle sensor should the ESP light be on. Or the need for a new gearbox electric kit if the vehicle will not up-change from second gear for example. When this is the case we can advise the customer of a price to cure the job.
  • However, we may incur a Stage 3 fault scenario where we have the fault code but need to establish why the fault code has occurred and therefore requires further investigation.

Common Mercedes Electrical Faults

One common fault we come across is the EML light on petrol engines. This is where the engine has a misfire or is making a noise. This is a fault which can take some time to establish the reason for the error. So will naturally incur a further diagnosis fee. However, because we know so many variants of Mercedes and their faults we can usually reach a definitive diagnosis within 1-2 hours. Should we be presented with an anomaly (a fault we have not come across before) then, of course, the customer will be informed. This may take longer to reach a diagnosis, either way, the customer is kept informed of our progress and not just given a possibly high bill when it’s all done.

Such is the experience of our technicians, we can honestly say we have never failed to diagnose a fault with a car. Some are more challenging than others, of course. Such as intermittent electrical faults, battery drains etc but we always get there one way or another. This is why and quite often, people come to us from far and wide not only for our expertise but our friendly and professional service.

John Haynes - Mercedes Diagnostic Specialist

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