We have a 2001 GMC Sierra pick-up. The headlights will not work. It's been hooked up to a diagnostic computer by a mechanic and they can't find anything wrong with it. The computer says the lights are on and they're not. Fuses have been checked and changed, light bulbs have been replaced, the wiring appeared OK. Has anyone had this problem?

Are you 100% positive that you replaced the bulbs with the right ones? People very easily confuse the 9003, 9004, 9005, 9006 and 9007 bulbs. There is also quite a bit of wiring leading to both headlights; you may have a break somewhere which is going to take a lot of time and effort to locate. Did a mechanic at a dealership do the computer diagnostic or was it an independent shop?

I guess I'm not 100% sure the correct light bulb was used. Will have to double check it. It was checked by an independent mechanic -- one at work. He fixes trucks/heavy equipment, etc. He's helped with other issues in the past.

How did the mechanic determine that the computer says the lights are on? I've never heard of such a diagnostic tool.