BMW M5 F90: Flickering Check Engine Light & Rough Idle – What’s Going On?

Why is the light flickering and the idle rough?

When your M5’s check engine light flickers instead of staying steady, that’s your engine control unit telling you it’s seeing misfires severe enough to damage the catalytic converters. The rough idle is almost always the same story – cylinders are not firing cleanly at low RPM. On the S63 engine in the F90, this is commonly caused by worn ignition coils, fouled spark plugs, or a combination of both. But before you start throwing parts at it, there are a few other usual suspects worth checking.

What to check first

• Read the fault codes – Even if the light flickers and goes out, stored misfire codes will point you to the exact cylinder(s).
• Ignition coils and spark plugs – These are the top cause on the F90, especially if you’re past 30,000–40,000 miles. Swap coils between cylinders to see if the misfire follows.
• Fuel injectors – High-pressure injectors on the S63 can leak or clog, causing rough idle and intermittent misfire. A fuel pressure test or injector balance test helps confirm.
• Vacuum leaks – Cracked crankcase ventilation lines or intake boots let unmetered air in, leaning out the mixture and making the idle hunt.
• Crankshaft/camshaft position sensors – A failing sensor can cause erratic timing and misfire-like symptoms, though less common than coils.

Is it safe to keep driving?

Honestly, I’d avoid long trips and definitely avoid hard acceleration.

A flickering check engine light means ongoing misfires, and each misfire dumps raw fuel into the exhaust, which can overheat and destroy your catalytic converters. That’s a very expensive repair. If the car is still driving smoothly enough to get you to the shop, take it easy and go there directly. If it’s shaking badly or loses power, get it towed. The good news is that the F90 M5 is a tough engine, and catching this early usually means a simple coil or plug replacement gets you back on the road.

This material is AI-generated and for informational purposes only — it does not replace an in-person diagnosis by a qualified mechanic.