When your car headlights flicker at idle, you need to check the problem right away because it could be an indication that your car's charging system is failing. If this is true, then you could be left stranded somewhere if it fails completely while you are out on the road. Of course, it could also be a headlight problem. Replacement parts for your cars charging system and headlights vary in price, so before you start replacing parts, troubleshoot the problem.
Open the fuse panel cover that is directly under the steering column by pulling down on the cover with your fingers. The steering column will be in the driver's side foot well and runs from behind the steering wheel to the firewall. You'll probably need to crouch down and climb into the driver's side foot well to access the fuse panel cover on most vehicles.
Visually inspect the wiring in the fuse panel. Any frayed or damaged wiring needs to be replaced by a professional mechanic.
Open the bonnet and check the wiring running to the headlights. Any damaged or frayed wiring needs to be replaced by a professional mechanic.
Check to make sure the electrical connector running to the headlights is fully plugged in and is secure.
Check the tension on the alternator belt. If the alternator is starting to fail, it can cause the headlights to flicker at idle because the car is running at low RPMs. Once the engine speed increases from normal driving, the flicker may disappear, but the alternator problem will only get worse in the future. If the alternator belt is frayed or there is more than a 1/4-inch deflection on the belt (if you can push the middle of the belt between the two pulleys that it sits on down more than 1/4 inch), then the belt needs to be replaced.
Start the engine. If the battery light illuminates, then the alternator is failing and is the most likely cause of your flickering headlights. This is because the alternator pulley is not able to properly charge the electrical system at low RPM and is preventing the proper draw on the electrical system by the car's headlights.