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2017 IS 350 Rough Idle & Misfire Codes P0301, P0303

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David L. · rep 0 · May 22, 4:38 AM · 1 replies · solved · Question

Vehicle: 2017 Lexus IS 350 F Sport AWD

Hi everyone,

My 2017 Lexus IS 350 F Sport (AWD) has developed a really rough idle and seems to be misfiring. It's most noticeable when I'm stopped at a light or in park. The Check Engine Light came on, and when I scanned it, I got P0301 (Cylinder 1 Misfire Detected) and P0303 (Cylinder 3 Misfire Detected).

The car feels a bit sluggish under acceleration, and the exhaust note sounds a little off. I've got about 80,000 miles on it. I'm wondering if it's time for new spark plugs, or if it could be something more serious like ignition coils or even fuel injectors.

Any thoughts on what might be causing these specific misfires on cylinders 1 and 3? What's the best way to troubleshoot this without just throwing parts at it? Thanks for your help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:38 AM

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ClearTheCode admin answer:

Misfire codes P0301 and P0303 on your 2017 Lexus IS 350 indicate that cylinders 1 and 3 are experiencing combustion issues. This can lead to rough idling, reduced power, and decreased fuel efficiency. Diagnosing misfires systematically is key to avoiding unnecessary repairs.

Here's a diagnostic approach:

  1. Ignition System:
    • Spark Plugs: Given your mileage, spark plugs are a prime suspect. Worn or fouled plugs can cause misfires. Check the condition of the plugs in cylinders 1 and 3.
    • Ignition Coils: A failing ignition coil for cylinder 1 or 3 would directly cause a misfire. You can often swap coils between a misfiring cylinder and a good one to see if the misfire code follows the coil.
  2. Fuel System:
    • Fuel Injectors: Clogged or faulty fuel injectors can prevent adequate fuel delivery to the cylinder. A fuel pressure test and injector resistance check can help diagnose this. Sometimes, a fuel system cleaner can help with minor clogs.
  3. Air/Compression Issues:
    • Vacuum Leaks: Leaks in the intake manifold or vacuum lines can cause unmetered air to enter, leading to a lean condition and misfires.
    • Compression: Low compression in a cylinder (due to worn piston rings, valves, or head gasket issues) will prevent proper combustion. A compression test is crucial if other diagnostics don't yield results.

When to see a shop:

  • Persistent Misfires: If swapping coils doesn't move the misfire, or if new spark plugs don't resolve the issue, it's time for professional diagnosis. A shop can perform fuel pressure tests, injector flow tests, compression tests, and use advanced scan tools to monitor live data for more precise troubleshooting.
  • Flashing Check Engine Light: If the Check Engine Light is flashing, it indicates a severe misfire that can quickly damage the catalytic converter. Drive as little as possible and get it to a shop immediately.
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