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2020 911 Carrera S (992) - Rough Idle and Misfire Codes P0301, P0303, P0305

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Mike T. · rep 0 · May 25, 4:48 AM · 1 replies · solved · Question

Vehicle: 2020 Porsche 911 Carrera S

Hello everyone,

I'm experiencing some concerning issues with my 2020 Porsche 911 Carrera S (992). Recently, I've noticed a rough idle, especially when the engine is cold, and the check engine light came on. I scanned it and got multiple misfire codes: P0301, P0303, and P0305, indicating misfires on cylinders 1, 3, and 5.

The car has about 35,000 miles on it and has been regularly serviced. It feels a bit down on power, and there's a noticeable vibration at idle. It seems to smooth out slightly once I get going, but the rough idle persists.

What could be causing misfires on an entire bank of cylinders? Could it be spark plugs, ignition coils, or something more serious like an injector issue on that side? Any insights would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:48 AM

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ClearTheCode admin answer:

Misfires on an entire bank of cylinders (P0301, P0303, P0305 for cylinders 1, 3, 5) on your 2020 Porsche 911 Carrera S (992) suggest a systemic issue affecting that bank rather than individual component failures. Here's a diagnostic approach:

  1. Fuel System Issues:
    • Fuel Injectors: A clogged or failing fuel injector on one or more of these cylinders could cause misfires. Since it's an entire bank, a fuel pressure issue specific to that bank is less likely but not impossible.
    • Fuel Pressure Regulator/Sensor: A faulty fuel pressure regulator or sensor could cause inconsistent fuel delivery to that bank.
  2. Ignition System:
    • Ignition Coils & Spark Plugs: While less likely to affect an entire bank simultaneously unless they're all at the end of their life, it's worth inspecting. Given the mileage, spark plugs might be due for replacement, but coils typically fail individually.
    • Wiring/Connectors: Check the wiring harness and connectors for the ignition coils and injectors on cylinders 1, 3, 5 for any damage or corrosion.
  3. Engine Management/Sensors:
    • Oxygen Sensor (Bank 1): A faulty O2 sensor on bank 1 could be sending incorrect fuel trim data to the ECU, causing the engine to run too rich or too lean on that side, leading to misfires. Check for related O2 sensor codes.
    • Mass Air Flow (MAF) Sensor: A contaminated or failing MAF sensor can affect overall air-fuel mixture, but usually impacts all cylinders, not just one bank.
    • Vacuum Leaks: A significant vacuum leak on the intake manifold specific to bank 1 could cause unmetered air to enter, leaning out the mixture and causing misfires.

When to See a Shop:

Given the multiple misfire codes on one bank, this issue warrants immediate professional diagnosis. A Porsche specialist will have the necessary diagnostic tools to perform real-time data logging, check fuel pressure, perform injector balance tests, and conduct oscilloscope tests on ignition components. Driving with persistent misfires can damage catalytic converters and other engine components. It's crucial to get this addressed promptly to prevent further, more expensive repairs.

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