Low beam not working on 2006 GMC Sierra

Check both bulbs, power and ground to the bulbs and connections.

I wonder if you actually checked the power in and out of the relay or just heard it clicking and assumed it is working ok?

It isn't unusual for high beams to be relayed but low beams to be direct from the dipswitch. I suggest you take a look at a wiring diagram. The fault could be the dipswitch, especially if the driver smokes.

Check relays might have a bad switch

use dc voltage meter and see if you have power in the sockets and if the fuse is good and relay , also check to make sure you haven't lost the ground.

{: ) I wonder how you checked the fuses and the bulbs. Did you check also the HDM fuse 15amp and the HDM relay both at the underhood fuseblock? Turn on the low beam and check for 12v at the LO beam fuses. If there is voltage at the fuses but the low beam ok bulbs won't light, then there's a wiring issue from your fuseblock going to the bulbs.
But if no voltages on the LO fuses when the low beam is switched on, then the problem could be on the HDM fuse or relay, the headlamp switch, the BCM, or even the fuseblock.

You need a new headlight switch my friend.

If high beams work and low beams do not, then it is probably the high/low relay. It is part number 15016745 and has four prongs. To test if your relay has gone bad locate the relay in your fuse box. There will be two of these relays located next to each other. One controls the low beam headlights and the other the cooling fan. Swap the relays and check your low beam headlights. If the low beams work you have a bad relay.

There may be a relay that operates the low beam lights, check the owners manual to see if there is a relay, you should be able to hear it go "click" if there is one.

i had same p w 1996 gmc chgd bulb chgd switch checked wires in steering coloumfinmally went under dash where your left boot would be and wiggled a 3quarter by half inch relay and done it lite up all day of jackin 2sec fix

hi from uk provided you have checked bulbs are ok? then this fault may be the relay for the low /dipped beam has gone faulty if you check h/book you may find location of low beam relay there is relay for high and one for low beams if you find both relays ? try switching? this will confirm relay fault if you then have low beam but no high ? b4 you buy new relay other than tis the other possible fault may be the actual di/high beam stick control ?

Did you check your fuses by any chance? otherwise check all your connection from the headlights back. Check your relays also.