9 Common Causes Why a Mercedes Won’t Start (+ Solutions!)

9 Common Causes Why a Mercedes Won’t Start (+ Solutions!)

There’s nothing quite like the sinking feeling of turning the key (or pushing that start button), and… nothing happens.

And if you’re reading this, I’m guessing you’re in a similar boat right now.

Now, cars as finely tuned as Mercedes models can have various reasons for not starting.

The good news is that most of these reasons are more common than you’d think, and with a bit of detective work, you can get to the root of the issue.

So, before you go pulling your hair out or calling for a tow, let’s walk through some of the usual causes.

Before we dive into the details, here is a summary of why your Mercedes won’t start:

  1. You have a low-voltage battery.
  2. It’s caused by a faulty starter motor or starter relay.
  3. There is a malfunction with your key fob or ignition switch.
  4. You have a blown fuse.
  5. The fuel pump is defective.
  6. The fuel filter is clogged.
  7. You have bad spark plugs or ignition coils.
  8. The gear selector or brake light switch is defective.
  9. The crankshaft position sensor is defective.

1. Low-voltage battery

Low-voltage battery illustration

Okay, so here’s the deal: before we dive into the complexities, we should start with the most accessible and common issue.

And more often than not, that’s the battery.

Your Mercedes’ battery stores electrical energy for later use, like when you want to start your car or use electrical systems when the engine isn’t running.

How do you know if you have a battery issue?

  • Visual inspection: First, look for obvious signs of wear or damage. Corroded terminals, a bulging case, or leaks are telltale signs of a faulty battery. If you see corrosion on your battery’s terminal, cleaning it using a mixture of baking soda and water can help.
  • Multimeter test: Use a multimeter to measure the voltage. According to Mercedes-Benz, a healthy, fully charged battery should read around 12.6 volts. If it’s between 12.2 and 12.4 volts, it’s okay, but it might need charging. Anything below 12.2 volts indicates a problem.
Measuring a car battery
  • Load test: This is best done by professionals. It measures the battery’s ability to hold voltage under a load similar to what it faces during starting.
  • Jump-start: If the car starts immediately after a jump, it’s a clear hint that the battery may be the issue. However, if the car dies shortly after being jump-started, it could be related to the alternator.

If any of these tests reveal that you have a battery issue, replacing your battery with a new one will likely solve your problem.

What are the causes of battery problems?

  • Age: A car battery generally lasts 3 to 5 years. Beyond this, its efficiency can decrease, leading to potential starting issues.
  • Temperature extremes: Batteries don’t particularly like extreme temperatures. Cold can slow the chemical reactions inside them, while heat can speed those reactions up too much or cause evaporation of the battery fluid.
  • Electrical drains: Modern cars have many systems that can drain batteries when they malfunction or are left on; think infotainment systems, lights, or even alarms.

2. Faulty starter motor or starter relay

A starter motor on a mechanic's bench

Let’s move to the next critical component in the starting process: the starter motor and the starter relay.

If your Mercedes won’t start, but you do hear a clicking sound, then it points towards an issue with the starter motor or the starter relay.

If you don’t know what the starter motor or starter relay does, here’s a simple explanation:

When you turn the key or press the start button, the starter motor is responsible for physically turning the engine over, allowing it to “start” running on its own.

Think of the starter relay as a middleman. Instead of sending a high current through the ignition switch, the relay ensures the starter motor gets precisely enough energy to operate.

How do you know if you have a starter motor or starter relay issue?

  • The sound test: When attempting to start, listen carefully. A single loud click may indicate a malfunctioning starter motor, while rapid clicking often points to the relay.
  • Visual inspection: Check for any obvious damages, corrosion, or loose connections on the starter and the relay.
  • Bypass test: With utmost caution and preferably with professional guidance, you can bypass the relay and supply power directly to the starter. If the engine cranks, the relay could be the culprit. Here’s a video explaining it:

If you find that your starter motor or starter relay is defective, the only (worthwhile) solution is to replace any of them with a new part.

According to RepairSmith, replacing the starter motor on a Mercedes-Benz typically costs between $440 and $551.

What are the causes of starter motor or starter relay problems?

  • Worn-out components: Over time, the starter motor’s internal components, like the brushes, can wear out, leading to inconsistent or failed starts.
  • Electrical issues: Loose or corroded connections can interfere with the flow of electricity, rendering the starter motor ineffective.
  • Relay malfunction: A faulty relay might not send the required power to the starter motor, even if the motor itself is in perfect condition.

3. Key fob or ignition switch malfunction

The key fob or ignition switch is the next potential cause.

It might sound basic, but if either of these has issues, it could be why your Mercedes isn’t starting.

Do you have a key fob or ignition switch malfunction?

  • Battery check: If your Mercedes’ won’t start after pressing the push-to-start button, this is the easiest place to start. Replace the internal battery of your key fob, which is typically a small coin cell, and test again.
  • Physical inspection: Next, look for visible damages. Does the physical key seem bent or damaged? Does the ignition switch feel loose or overly tight?
  • Reprogramming: Sometimes, key fobs lose their programming and need a reset. Often, it requires a visit to the Mercedes-Benz dealership.

4. Blown fuse

A box of fuses

Now, there’s a little detail that might be stopping your Mercedes from starting: the fuses.

Fuses act as safety devices that protect your vehicle’s electrical circuits.

They’re designed to “blow” or “trip” when there’s too much current, effectively breaking the circuit and preventing potential damage or fires.

Each fuse is responsible for one or more electrical functions in your car, from starting the engine to turning on the light.

How do you check for blown fuses?

  • Fuse box location: First things first, locate the fuse box. In most Mercedes models, you’ll find one under the hood and another inside the cabin, often by the driver’s side.
  • Inspect visually: Many fuses are transparent, allowing you to see a broken or burnt wire inside, which indicates it’s blown.
  • Use a test light: Turn on your car’s ignition (without starting it). Use a test light and touch its probe to each side of the fuse. A good fuse will light the tester on both sides. The fuse is blown if it lights on one side but not the other.
  • Check the manual: Your Mercedes-Benz’s manual will have a diagram showing which fuse is responsible for each function, helping you pinpoint potential causes related to starting issues.

Always replace a blown fuse with one of the exact same rating.

Using a higher-rated fuse might seem like a quick fix, but it can lead to damaged components or even a fire.

If you notice that a specific fuse keeps blowing frequently, it’s a sign of a deeper issue that requires professional attention.

5. Defective fuel pump

The fuel pump pushes gasoline from the tank to the engine.

Without it working efficiently, the engine can’t receive the fuel it needs to run.

What are the signs of a defective fuel pump?

  • Humming noise: Before starting, turn the ignition to the ‘ON’ position and listen. A working fuel pump will make a brief humming noise as it primes the engine with fuel.
  • Fuel pressure test: Using a fuel pressure gauge, you can measure if the pump delivers fuel under the correct pressure.
  • Check the fuse and relay: Before concluding the pump is defective, check its fuse and relay. A blown fuse or a bad relay can cut power to the pump.

A failing fuel pump often also gives several signs before it stops functioning completely. You may be able to recall the following:

  • Engine sputtering: One of the most common symptoms. If you’re driving and the engine starts to sputter at high speeds, it could indicate the fuel pump is deteriorating.
  • Rising temperature: The temperature gauge rising paired with the engine stalling can be another indication.
  • Fuel pressure loss: If the car doesn’t start after multiple tries but eventually does, the fuel pump might not maintain consistent pressure.
  • Whining noise: A high-pitched sound from the fuel tank, especially when starting the car or during acceleration, can suggest a pump issue.

If the fuel pump is indeed the culprit, replacing it is usually the way to go. The cost usually varies between $611 and $894 (including parts and labor), depending on your Mercedes model.

6. Clogged fuel filter

Clogged fuel filter - illustration

The fuel filter is an often-underestimated component that ensures that any impurities in the gasoline don’t make their way into your engine.

A blockage here, and your Mercedes might not start.

What are the signs of a clogged fuel filter?

  • Struggling acceleration: If your car hesitates or jerks during acceleration, it might be due to an inadequate fuel supply from a clogged filter.
  • Frequent stalling: A restricted fuel flow can cause the engine to stall, especially when driving uphill or at high speeds.
  • Visual inspection: Ultimately, it’s best to perform a visual inspection. Unfortunately, depending on your Mercedes model, the fuel filter can be a bit hard to reach as a DIYer.

If you’ve nailed down the fuel filter as the cause, replacing it is often the recommended route.

Thankfully, this is generally one of the more affordable fixes. According to KBB, the replacement cost of a fuel filter on a Mercedes-Benz is between $90 and $207 (including parts and labor).

7. Bad spark plugs or ignition coils

A row of firing spark plugs - illustration

If one of your spark plugs or ignition coils is defective, your Mercedes may not start.

Spark plugs provide the spark to ignite the air-fuel mixture in the engine’s combustion chambers, while ignition coils transform the car’s low voltage to the thousands of volts needed to create that spark.

How do you confirm bad spark plugs or ignition coils?

  • Visual examination: For spark plugs, signs of wear, soot buildup, or an eroded electrode can indicate a defect. A damaged ignition coil might show cracks or signs of burning.
  • Onboard Diagnostics (OBD) scan: An OBD tool can be invaluable when diagnosing a Mercedes-Benz. A faulty ignition system can trigger the check engine light. By reading the error codes, you can determine whether the issue lies with the spark plugs or the coils.
  • Swap and observe: If you suspect one coil might be bad, try swapping it with another. If the problem moves with the coil, you’ve found your culprit.

Should you determine that the spark plugs or ignition coils are to blame, replacement is often the most straightforward fix.

Keeping up with routine maintenance, like timely spark plug changes, can prevent such issues in the future.

8. Defective gear selector or brake light switch

Mercedes A-Class automatic gearbox selector

Modern vehicles, including your Mercedes, are equipped with safety mechanisms that prevent the car from starting if it’s not in the correct gear or if the brake isn’t engaged.

And you guessed it already: a glitch in this system can prevent your Mercedes from starting.

Unfortunately, this issue is a bit harder to identify and fix without professional diagnostics tools, but there are some signs that point to it.

First of all, if the brake lights don’t illuminate when pressing the brake pedal, there’s likely an issue with the brake light switch.

Also, check if you see any warning lights or messages on your dashboard related to the transmission or brakes.

If you’re technical, you can check the continuity of the brake light switch when the brake is pressed and released with a multimeter. The brake light switch is usually located near the top of the brake pedal.

Ultimately, if everything else seems fine and the car doesn’t respond when you try to start it – no clicks, no sounds – it might be one of these components acting up.

Related article: The 10 Most Reliable Mercedes-Benz Models And Years

9. Defective crankshaft position sensor

This sensor monitors the position and rotational speed of the crankshaft, sending this vital data to the Engine Control Unit (ECU).

Based on this, the ECU adjusts the engine’s timing for optimal performance and efficiency.

A bad crankshaft position sensor can lead to unpredictable engine behavior like stalling, especially during acceleration. In worse cases, your Mercedes may crank but not start, or in some cases, not crank at all.

The car’s computer will often recognize a malfunctioning sensor, triggering the check engine light.

Therefore, an OBD scan is your best friend in this scenario, as you’ll usually find a P0335 error code in the diagnostics, which is related to a defective crankshaft position sensor.

According to RepairPal, replacing the crankshaft position sensor on a Mercedes usually costs between $240 and $288.


So, there are quite a few reasons why your Mercedes isn’t starting, aren’t there?

If you’re feeling a little overwhelmed, that’s okay. Cars are complex products, and even the best of us can get a little puzzled now and then.

If you’re still looking for a cause, don’t hesitate to write a comment below. There’s no harm in seeking a bit of help.

And if you’ve solved the puzzle? Give yourself a pat on the back! Great job!

Until next time!

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  1. My Merecedes 380 SEL 1985 – Got hot while it was being driven – the vehicle stalled – now it won’t start. Could the head gaskets be blown? What’s the best way to get it started?? Appreciate any assistance.



    1. Hi Rick,

      It sounds like your Mercedes 380 SEL might indeed have a blown head gasket, especially considering it overheated and then stalled. This is a common issue when a car gets too hot. Before trying to restart it, I’d recommend checking for any obvious signs of a blown head gasket, such as milky oil or coolant leaks. Try to remove and inspect the spark plugs. If they’re coated in oil or have a milky residue, it might be a sign of a blown head gasket. If these symptoms are present, it’s best to avoid starting the car to prevent further damage.

      Hope this helps!
      – Patryk

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