A flickering check engine light with a rough idle usually points to active misfires on your Audi A4 B9. Here are the likely causes, what to check first, and when to stop driving.
What a flickering check engine light really means
On your Audi A4 B9, a check engine light that flickers or flashes is not the same as a steady light. It almost always means the engine is misfiring right now—unburned fuel is being pushed into the exhaust, which can quickly damage the catalytic converter. A rough idle alongside it supports that: one or more cylinders are not firing cleanly, especially at low rpm. If the light is flashing constantly, stop safely and have the car towed. If it only flickers intermittently and the idle shakes but the car still drives, you can usually limp gently to a shop—but avoid hard acceleration and high rpm.
