Im having a very unique issue with my 2008 f150.
The low beans aren't working but the high beans work. I've had two shops run a diagnostic on the truck and both came up with no solution. I've checked the fuses and replaced the bulbs, nothing.
Any advice or direction will be highly appreciated. Thank you.

 

I'm seeing that model uses 9008 dual element halogen bulbs. Correct? Have you made any recent changes to the electrical system? Any modifications or additional lighting added? Have you checked the wiring going to the bulbs? If the high beams work, they low beam power wire in your bulb connectors may have some corrosion in the connector that is keeping the bulb from getting power. Take off the plug from the bulb and check the wiring with a test light or multi-meter.

 

I've checked the wiring harness and connectors. I've changed the bulbs twice and have not added anything electric to the truck except an after market viper alarm system. Everything seems to be intact.

 

There could be a problem with the dimmer switch (combination switch) itself. Have you tried switching quickly between bright and dim to see if that briefly dims them?

If this has no effect, I'd check for 12 volts at Fuses 25 and 23 with switch in dim position. These 2 fuses should be hot then, but not when switch is in brights position on my 2004 Lariat. You can check and see if they are the same on yours.

Let us know the results if you need further help.

 

Try a third shop. Find one that actually understands electricity unless you're going to try and solve it yourself.

This is correct and applies to the '08 model also. It will, at least, split the circuit and let you know whether or not a portion of the circuit is working or not.

 

when was viper installed and when did lights problem start ?

if previous suggestions don’t work, you could try disconnecting viper and see if symptoms change.