2011 Prius low beams not working. Help please!

Did you check if you were getting voltage at the bulb?
There is a recall for the low beams from Toyota.

 

There are fuses for the high beams but not for the low beams. Those are fed by the Integration Relay which, while called a "relay", is really a spiffy semiconductor multi-switch with its own current limiting, so instead of blowing like a fuse, it's supposed to just be able to limit fault currents until a problem is fixed. (I think. Anyway, there are no user-visible fuses in it.)

I don't have any idea how often an Integration Relay actually fails. As a no-moving-parts semiconductor thing, I'd guess not very often. Wiring damage might be more likely.

You can see the integration relay in the underhood fusebox on the left. It comes out and has three plug-in connectors on the bottom, though I do not know if there's enough slack in those to really just pull it out and unplug them from above. Annoying if you have to free up the fusebox and fuss with things through the bottom cover.

Never mind the green line here, I'm cribbing this image from a different thread about horns. The low beam headlights are connector 1G, pin 1 (right) and 2 (left). Both wires are purple. The one for the left is thinner wire, to go a shorter distance but still drop equal voltage. That's important; early Gen 3s have been known to eat their low beam bulbs really fast by overvolting them.



And yes, as ASR suggested ... if you haven't yet just checked for voltage at the bulb sockets ... that's the easiest place to start.

 

I will assume you have halogen lights and not led. I would verify you have the correct low beam bulbs and verify the integration relay's 40 amp fusible link. I believe the bulbs are H11. Also make sure both bulbs are not burned out. The bulbs connect direct to the integration relay and ground. A simple voltage check at the bulb's connector verifies everything.

 

The integration relay is sneaky because it gets power from more than one place. Those 40 amps from the P/I 2 fuse feed into it on connector 1H pin 7 ("BATT"), and there's also an ignition-only feed into it on connector 1G pin 14 ("IG1"), but it also has a giant connection "DCDC" directly to the car's DC/DC converter inside the fusebox. (The converter isn't inside the fusebox, I mean the connection is.) That doesn't have a connector or pin number because it's just a big metal blade at the top of the relay that mates into the fusebox. It also doesn't have a fuse. (There's a 125 amp DC/DC fuse buried in the fusebox, but this connection is on the converter side of it.)

So this "relay" is an intricate beast that gets power from multiple sources and distributes it to a whole bunch of things. The wiring diagrams never "look inside" it to show which outputs are derived from which inputs. (I've never found anything that even describes it enough to say what the two test points on top of it are for.) But for most of the skinny-wire circuits coming out of it, clearly it has to be doing overcurrent protection itself; they would not be protected only by the 40 A P/I 2 fuse, much less the monster DC/DC feeder.

 

The cheap bulbs burn out quickly, but I just buy a bunch on ebay. Mine usually last over a year and I run them day and night for safety. Also wear gloves when you handle the new bulbs. Skin oils will create a hotspot and burn out halogens.

 

The wiring issue is that some had too-heavy wiring that did not drop the voltage enough on the way to the headlights, so their lifespan was reduced. The official fix was I-kid-you-not to splice in additional lengths of wire that were calculated to get the right voltage drop.

It makes a big difference whether you are using the standard bulbs or "premium" extra-bright ones. The extra-bright ones are already built to have shorter lifetime in trade for brighter output, and when you combine that with too-high voltage in a Prius without the wiring fix, they get vaporized in mere months. The standard ones last a lot longer.

The last owner of my 2010 complained about how often it ate low beams. So I bought the official harness extensions, but never got around to splicing them in. The last pair of bulbs he put in before selling me the car were standard ones, and those never burned out in three years.

Usually, at the auto parts stores, they put the "premium" extra bright bulbs on the pegs right up at eye level where they hope you will see them, and they hang the standard ones down by your ankles where they hope that you won't.