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2020 GLC300 Rough Idling and Misfire Codes P0301, P0302

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Mike T. · rep 0 · May 25, 3:04 AM · 1 replies · solved · Question

Vehicle: 2020 Mercedes-Benz GLC300 X253

Hello everyone,

I'm having some trouble with my 2020 Mercedes-Benz GLC300 (X253). Recently, it's started to idle very roughly, and I've noticed a slight hesitation during acceleration. The Check Engine Light came on, and when I scanned it, I got P0301 (Cylinder 1 Misfire Detected) and P0302 (Cylinder 2 Misfire Detected).

This is a relatively new car, so I'm a bit surprised. I'm wondering if it could be something simple like spark plugs or ignition coils, or if it points to a more serious fuel injector issue. The car has about 45,000 miles on it.

Has anyone else experienced multiple misfire codes on a newer GLC300? What was the resolution? Any advice on what to check first would be great.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:04 AM

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ClearTheCode admin answer:

Misfire codes P0301 and P0302 on your 2020 Mercedes-Benz GLC300 indicate that cylinders 1 and 2 are not firing correctly. This can definitely lead to rough idling, hesitation, and reduced fuel economy. While your car is relatively new, components can still fail.

Here's a structured approach to diagnosing these misfires:

  1. Common Causes for Misfires (P030X codes):

    • Ignition System: Faulty spark plugs, ignition coils, or spark plug wires (less common on modern engines).
    • Fuel System: Clogged or faulty fuel injectors, low fuel pressure, or contaminated fuel.
    • Air/Vacuum Leaks: Leaks in the intake manifold or vacuum lines can cause an unmetered air intake, leaning out the mixture in certain cylinders.
    • Compression Issues: Low compression in a cylinder due to worn piston rings, valve issues, or a blown head gasket (less likely on a newer engine unless there was overheating).
    • Engine Control Unit (ECU): Faulty ECU, though very rare.

  2. Diagnostic Steps You Can Take:

    • Swap Ignition Coils: A common DIY diagnostic is to swap the ignition coils from cylinder 1 and 2 with coils from known good cylinders (e.g., 3 and 4). Clear the codes, drive the car, and re-scan. If the misfire codes move to cylinders 3 and 4 (e.g., P0303, P0304), then the coils you moved are faulty.
    • Inspect Spark Plugs: Remove the spark plugs from cylinders 1 and 2. Look for signs of fouling, wear, or damage. Replace if necessary, and ensure they are gapped correctly. It's often recommended to replace all plugs if they are near their service interval.
    • Check for Vacuum Leaks: Listen for a hissing sound around the intake manifold and vacuum lines. You can also use a can of unlit propane or carburetor cleaner (carefully!) around these areas; if the engine RPM changes, you've found a leak.

  3. When to See a Shop:

    • If swapping coils or inspecting spark plugs doesn't resolve the issue, or if the misfires persist, it's time for professional diagnosis.
    • A shop can perform more advanced tests, such as fuel injector testing (flow and spray pattern), fuel pressure tests, and compression tests. They can also use specialized diagnostic equipment to pinpoint intermittent issues.
    • Mercedes-Benz engines are complex, and some issues (like fuel injector replacement or deep vacuum leaks) require specialized tools and expertise. Addressing misfires promptly is important to prevent potential damage to the catalytic converter.
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