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2018 A4 2.0T Misfire on Cylinder 3 - P0303 Code & Rough Idle

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Mike T. · rep 0 · May 25, 3:10 AM · 1 replies · solved · Question

Vehicle: 2018 Audi A4 2.0T Premium Plus

Hey everyone,

My 2018 Audi A4 2.0T Premium Plus recently started acting up. The check engine light came on, and the car has a noticeable rough idle, especially at stop lights. It feels like it's struggling a bit under acceleration too.

I hooked up my OBD-II scanner, and it's throwing a P0303 code, indicating a misfire on cylinder 3. I'm wondering what the most common culprits are for this on these B9 A4s. Could it be a bad spark plug, ignition coil, or something more serious?

Any advice on where to start troubleshooting would be greatly appreciated before I take it to the shop. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:10 AM

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ClearTheCode admin answer:

A P0303 code for a misfire on cylinder 3 is a common issue that can have several causes. Addressing it promptly is important to prevent potential damage to your catalytic converter.

Here are the likely causes and troubleshooting steps:

  1. Ignition System Components:

    • Spark Plug: A worn, fouled, or cracked spark plug is a very common cause. Given the single cylinder misfire, this is a strong candidate.
    • Ignition Coil: The ignition coil for cylinder 3 could be failing. These are known to wear out over time.
    • Diagnosis: You can try swapping the spark plug and then the ignition coil from cylinder 3 with a known good cylinder (e.g., cylinder 1). If the misfire moves to the new cylinder, you've identified the faulty component.

  2. Fuel System Issues:

    • Fuel Injector: A clogged or faulty fuel injector for cylinder 3 could be preventing proper fuel delivery. This is less common than ignition issues but still possible.
    • Diagnosis: This often requires more advanced diagnostic tools to test injector pulse and spray pattern. A fuel system cleaner might temporarily help if it's a minor clog.

  3. Engine Mechanical Problems:

    • Compression Loss: Low compression in cylinder 3 due to issues like worn piston rings, a faulty valve, or a head gasket leak can cause a misfire. This is a more serious and less common cause for a single cylinder.
    • Diagnosis: A compression test or leak-down test would be necessary to confirm this.

When to see a shop:

  • If swapping spark plugs and coils doesn't resolve the issue.
  • If you suspect a fuel injector problem and don't have the tools to test it.
  • If a compression test reveals low compression in cylinder 3.
  • If the misfire is accompanied by unusual noises, smoke, or a strong fuel smell.

Driving with a persistent misfire can lead to unburnt fuel entering and damaging your catalytic converter, which is an expensive repair. It's best to resolve this issue as soon as possible.

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