'99 Sienna has no low beams or running lights. Both left and right went out suddenly and at the same time. High Beams work okay. Is this a bad dimmer relay?

if this unit has a multifunction(turn signal switch with head lights) switch its about 99% the problem. for sure it is the dimmer switch wherever it is located.

Do you have power in and out of the dimmer switch on the low beam circuit ?

Do you have power in and out of the dimmer switch in the low beam position ?

It could be a ground problem, or the dimmer switch. I would check for power and ground at the lights with them unplugged and then take a look at the dimmer switch.

maybe the dimmer switch is bad! the dimmer switch is what turns the headlights on low and high beam.
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spare car keys

Your dimmer switch is bad (lever used to flip between hi/low beams) they're only plastic and they go bad. Before you change check fuses, buy I highly suspect dimmer switch.

IF LOW BEAMS BULB OKAY,MORE LIKELY DIMMER SWITCH FAULTY.

Either both bulbs are bad(I've seen it before) or your
dimmer/headlight switch is bad. If you can without pulling the bulb, pull plug from the light. If it's a 4 lamp system pull the low beam. Do this on the left side as the right feeds from the left. With a 12v test light, check for power. (with lo beam on) if you have
power, change the lamp, if you don't it's most likly
the dimmer switch at the steering collum.
Hope this helps Good luck

FIRST THE HEADLIGHT SWITCH??? AND CHECK THETURN SIGNAL MECH

Don't know that much about 99 Surburbans but you may won't to see if you could just unplug it from the cluster or maybe see if thiers a fuse for your lights that you can take out