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Check Engine Light & Rough Idle on 2019 GLE450 - Misfire codes?

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Brittany E. · rep 0 · May 22, 4:19 AM · 1 replies · solved · Question

Vehicle: 2019 Mercedes-Benz GLE450

Hi everyone,

My 2019 Mercedes-Benz GLE450 (V167 chassis) with the M256 inline-six engine just threw a Check Engine Light, and it's running pretty rough at idle. I can feel a distinct shake, and it feels a bit down on power when accelerating. The car has about 58,000 miles on it.

I managed to pull a code with a basic OBD-II scanner, and it's showing P0303, which I believe is a misfire on cylinder 3. Could this be spark plugs? What's the recommended interval for spark plug replacement on these engines? Is this a DIY job, or should I take it to the dealer?

Any advice on troubleshooting this would be great!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:19 AM

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Hello David,

A Check Engine Light accompanied by a rough idle and a P0303 code (Misfire Detected - Cylinder 3) on your 2019 GLE450 is a clear indication of an engine combustion issue. At 58,000 miles, spark plugs are a very strong candidate for the cause.

Common Causes of Misfires (P030x codes):

  1. Worn Spark Plugs: Over time, spark plugs wear down, increasing the voltage required to fire, leading to intermittent or complete misfires. Mercedes-Benz generally recommends spark plug replacement for the M256 engine around 50,000-60,000 miles or every 5 years.
  2. Faulty Ignition Coil: The coil pack for cylinder 3 could be failing, preventing a strong spark.
  3. Fuel Injector Issue: A clogged or failing fuel injector for cylinder 3 could be delivering too little or too much fuel.
  4. Compression Loss: Less common, but low compression in cylinder 3 due to valve or piston issues can cause misfires.

Troubleshooting Steps (DIY-friendly for initial diagnosis):

  • Swap Ignition Coil: A common diagnostic step is to swap the ignition coil from cylinder 3 to an adjacent cylinder (e.g., cylinder 2). Clear the codes, drive the car, and re-scan. If the misfire moves to cylinder 2 (P0302), you've identified a bad coil.
  • Inspect Spark Plug: If the coil swap doesn't move the misfire, remove the spark plug from cylinder 3 and inspect it for fouling, damage, or excessive wear. Compare it to a new plug.
  • Replace Spark Plugs: If your car is due for spark plugs, replacing all of them is a good preventative measure. This is a moderately difficult DIY job. You'll need a spark plug socket (often thin-walled), a torque wrench, and patience. Ensure you use the correct OEM-specified spark plugs (e.g., NGK SILZKGR8B8S or equivalent).

When to See a Shop:

  • If the misfire persists after replacing the spark plugs and coils.
  • If you suspect a fuel injector issue (requires specialized tools for testing/replacement).
  • If you lack the tools or confidence to perform the spark plug replacement yourself. Incorrect torque can damage the cylinder head.
  • If you get additional codes beyond P030x, such as P0171/P0174 (Lean Condition) or P0420/P0430 (Catalyst Efficiency), which could indicate broader issues.

Addressing a misfire promptly is important to prevent damage to the catalytic converter, which can be very expensive to replace.

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