Piston Slap: That ballast diagnosis flex?

B writes:

I have a 2013 Flex with HID headlights. One side goes out after 20 minutes of driving, and looking online I see that it has a ballast for each of the headlights. The headlight also comes back on when I turn the lights off and back on again.

Question is: Do I replace the bulb or the ballast? The bulb can be reached under the hood but the ballast requires the bumper to be removed.

Sajeev answers:

Bulbs are usually not picky on when they want to illuminate: Either they light up (to some degree) or they burn out. Again, usually. Even worse, HID ballasts often possess the same symptoms. So what’s the best way to get to the bottom of this?

You could probably (maybe) test the ballast via Ford service manual* and a multimeter. But the easier way is to swap the bulbs from left to right and see whether the problem persists. This is even easier considering you (apparently) can swap bulbs without the all-too-common bumper removal.

Now let’s do the armchair quarterbacking part. I reckon both of your bulbs are fine and the ballast went bad due to exposure to moisture. If so, there are plenty of replacements available online—not just at parts stores—and they are likely to be many, many times cheaper than their in-store or factory-spec equivalent. When replacing ballasts with non-OEM parts, try to stick with a big-name aftermarket brand (Dorman, Osram, etc.) if your budget allows for it.

*This asterisk exists only because I had to check the resistance of an ABS wheel sensor in my 1995 Lincoln Mark VIII and the factory Ford service manual did not list the resistances in their diagnostic charts. I found the info elsewhere, thankfully.