Here are the potential issues:
* Faulty DRL Module: The module controlling the DRLs might be malfunctioning. This is a common culprit.
* Wiring Issues: A break or short in the wiring harness leading to the headlights or the DRL module could prevent the lights from activating.
* Faulty Headlights or Bulbs: Even if the DRL system is working, burnt-out headlights or DRL bulbs will prevent you from seeing them. Check your headlights' low beams — these are often used as the DRLs. They'll be at reduced intensity.
* Fuse Problem: A blown fuse in the DRL circuit could be the problem. Check your owner's manual for the fuse diagram and locate the fuse(s) related to headlights or daytime running lights.
* Incorrect Headlight Setting: It's unlikely on a 1999 Sable, but some vehicles allow you to select between different headlight modes, potentially disabling DRLs. While this car probably doesn't have that option, it's worth a quick check of your headlight switch positions.
* Computer Glitch: The car's computer system may have a minor software or electrical issue that's interfering with the DRL function.
What You Should Do:
1. Check the Bulbs/Headlights: This is the easiest and cheapest thing to do. Make sure your low-beam headlight bulbs are working.
2. Check the Fuses: Carefully inspect the relevant fuses in your fuse box (under the hood or inside the car—check your owner's manual).
3. Return to the Dealer (or a Different Mechanic): Explain that you *still* aren't seeing the DRLs function. The dealer's initial diagnosis may have been incorrect. A second opinion from a different mechanic is highly recommended. Describe exactly what happens when you start the car.
Without being able to physically inspect the vehicle, it's impossible to pinpoint the exact problem. A qualified mechanic will need to diagnose the issue using diagnostic tools and possibly replace parts.
