Why did low beams stop working?

Solution: If both headlights aren't working, it's unlikely that a bulb is at fault. The likely cause is a fuse, headlight relay, headlight switch, dimmer switch or a wiring fault. ... Consult your owner's manual to locate the main fuse for the headlight circuit and replace that fuse with one having the same amp rating.
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Most total headlight failures are caused by a bad component like a fuse, relay, or module. Wiring problems can also cause both headlights to stop working. High beam headlights don't work or low beams don't work. The cause: A burned out bulb, or a problem with the high beam switch or relay.
https://www.lifewire.com/fix-non-working-headlights-4137021

High current draw and bad connection at the relay, fuse box or loose ground.

If the bulbs are good then could be a fuse or relay, high beams and low beams are always on seperate fuses and relays.

more than likely a fuse or a relay is bad. locate the fuse for the headlights (owners manual) and check if it is bad, if the fuse is good, find the relay and give it a flick or two and try the headlights then, if they work, its time for a new relay

if the fuses are good then you need to check for power and ground at the headlight plug that attaches to the headlight bulb if you have a good power and ground there and the connections are good then the bulbs are both no good.or if you can tell the bulbs are burnt out by looking at them then you don`t have to check powers and grounds.you also can directly feed power and ground to the bulb and check both the terminals for high beam and low beam as you cannot tell which is which by looking at the terminals

On the van you have a high beam fuse and relay. On the low beam you also have a fuse and relay. Check the fuse/relay panel for a blown fuse controlling the low beam lights. If you do not find a blown fuse have some one to turn the key on and turn on the high beam lights> During this time locate the low beam headlight relay and place your finger on top of relay, have the other person to switch from high beam to low beam lights. You should feel a click in the relay when they make the switch from high to low beam. If not replace the relay and the problem should go away. Hope this will help you.

test and check the headlight switch -wiring , connections and sockets for power

Power flows from the battery to the main fuse. Then it flows to the control coil and contact side of the headlight relay. When you turn on the headlights, the headlight switch provides ground to the control coil in the headlight relay and that operates the contacts. The contacts allow power to flow to the right and left hand headlight fuses. Power flows to the headlights. The headlight switch is what provides ground to the high and low beams on the headlights.

To diagnose, remove the headlight relay. Using a voltmeter, check for battery voltage on terminals 3 & 4 in the socket with the key in the RUN position. Then turn on the low beams. Test for good ground on terminal 1 in the socket. If you're not getting ground, then you've got a problem with either a bad headlight combo switch or a bad ground. If you get good ground on terminal 1, reinstall the relay, turn on the low beams and test for power in the headlight sockets. If you get battery voltage, that confirms you've got good fuses and wiring up to that point. Next, check for good low beam ground on the red/green wire in the bulb socket. If you're not getting good ground, it's either a bad combo switch or bad ground from the combo switch. Then turn on the high beams and check for good ground on the red/yellow wire in the bulb sockets.

It wouldn't hurt to clean the ID ground on the left side of the instrument panel, but based on everything you've said so far, this sounds like a bad combo switch

At first 1994 C280 high beams began to not connect regularly when stick pushed forward, then not at all. Drove high beams by pulling and holding stick back towards steering wheel until that stopped working, too. Shop said the turn signal switch was at fault-- had seen it many times before with W202; about $160 to replace. Haven't done it yet, but should be an easy fix?