Fallout 4 Explained: The Flux Sensor Quest Walkthrough

When the Brotherhood Sends You Diving for Pre-War Tech

There’s something about the Brotherhood of Steel that makes every errand feel like a mix between a holy mission and a suicide run. You might think you’re just fetching another random piece of pre-war junk, but to them? It’s sacred relics of lost power. And that’s exactly what happens when Proctor Quinlan drops “The Flux Sensor” quest on your Pip-Boy.

At first glance, it’s a typical Brotherhood radiant mission , but beneath the surface (literally, in this case) lies a fascinating chunk of Fallout 4 world-building, complete with murky waters, radiation, and the remnants of pre-war innovation at Poseidon Reservoir.

Let’s dive in , pun absolutely intended.

Brotherhood Business as Usual

Once you’ve climbed aboard the Prydwen and proven yourself to the Brotherhood, Proctor Quinlan, the guy in charge of all things data and tech, will start tossing you “retrieval missions.” One of these is the Flux Sensor assignment.

He explains it in his trademark, monotone Brotherhood way:

“We’ve located a piece of pre-war sensor technology , a flux sensor, used in high-precision energy transfer systems. Recover it for analysis.”

Sounds fancy, right? What he doesn’t mention is where it’s located. Usually, your map pings somewhere deep, dark, and crawling with ghouls or turrets , and in many cases, that place is Poseidon Reservoir.

What’s a Flux Sensor, Anyway?

To understand why the Brotherhood wants it, you need to know what the flux sensor actually is. In pre-war America, flux sensors were specialized instruments used in energy research, particularly in nuclear and water power regulation. They measured electromagnetic flux , basically, the stability of energy flow in reactors or turbines.

In Fallout 4’s world, that means Poseidon Energy, the massive pre-war corporation behind half the Commonwealth’s power grid, used them in facilities like Poseidon Reservoir to keep their water turbines running smoothly.

So when the Brotherhood catches wind that one of these things is still intact, they don’t hesitate , because to them, every working sensor, every old capacitor, and every power relay is one more brick in their dream of a rebuilt, technology-driven civilization (with them in charge, of course).

The Poseidon Reservoir: History, Vibes, and Dangers

The Poseidon Reservoir is tucked in the southwestern Commonwealth, a rusting shell of what was once a critical part of Boston’s energy infrastructure. Back before the bombs fell, it provided hydroelectric power through water turbines , think of it as the Commonwealth’s own miniature Hoover Dam.

Now? It’s a haunting site of cracked concrete, corroded pipes, and irradiated water shimmering under a sickly green sky. Raiders, mirelurks, and feral ghouls have made it their personal spa.

The moment you step inside, you can almost hear the echo of engineers shouting over turbine noise , now replaced by the low growl of mutated creatures lurking below the walkways.

Step-By-Step Walkthrough

Step 1: Accept the Mission
Talk to Proctor Quinlan aboard the Prydwen. He’ll brief you on the flux sensor’s coordinates. The quest will appear under “Miscellaneous” as Retrieve the Flux Sensor.

Step 2: Head to Poseidon Reservoir
Travel southwest of the map. The reservoir’s structure is easy to spot , big, industrial, and half-submerged. Be ready for radiation and close-quarters combat. A Power Armor suit with a few Rad-X and RadAway doses is your best friend here.

Step 3: Enter the Main Building
The entrance is guarded by raiders or feral ghouls depending on your level. Inside, expect low lighting, flooded hallways, and tripwires. The interior feels like a mix between a power station and a tomb.

Step 4: Find the Control Room
Follow your Pip-Boy marker through the flooded lower floors. You’ll likely hear the water dripping and turbines humming faintly. The Flux Sensor is usually located in a metal case near the central control terminal or inside a Poseidon Energy toolbox on a desk.

Step 5: Clear the Enemies (Optional)
You might run into a Legendary Raider or Mirelurk King lurking near the lower platforms. If you’ve got a suppressed weapon, it’s worth picking them off quietly.

Step 6: Retrieve the Flux Sensor
Once you find it, grab the device , a small metallic cylinder with glowing indicators , and check the area for extra loot like Fusion Cores, Mini Nukes, or Technical Documents (Quinlan loves those too).

Step 7: Return to the Prydwen
Report back to Proctor Quinlan. He’ll thank you with the enthusiasm of a man who’s been awake for three days scanning old floppy disks, then reward you with caps and XP.

The Lore Beneath the Mission

What’s cool about the Flux Sensor quest is how it quietly ties into Fallout’s bigger story about humanity’s relationship with technology.

The Brotherhood of Steel sees tech as sacred : a force that must be controlled and protected.
The Flux Sensor is a perfect metaphor: it measures energy flow, something invisible but powerful : just like the Brotherhood’s influence over post-war science.
The Poseidon Reservoir acts as a symbol of the old world’s arrogance : building massive machines to control nature, only to drown in the radiation that followed.

Every time you bring Quinlan another relic like this, you’re not just completing a fetch quest . you’re helping a faction rebuild the very world that destroyed itself. And that’s the moral contradiction that makes Fallout 4 such a masterpiece.

Rewards & Replay Value

Like all of Quinlan’s radiant jobs, this one is repeatable and varies slightly each time. You’ll earn:

💵 100–300 Caps
🧠 75–150 XP
⚙️ A small Brotherhood reputation boost

Combine it with other Brotherhood radiant missions (like “Cleansing the Commonwealth”) for double efficiency while grinding XP.

Pro Tips from the Wasteland

Bring Power Armor: You’ll likely swim or walk through irradiated water.
Pack a Gauss Rifle or Suppressed .308: Raiders and ghouls drop fast with precision shots.
Loot Technical Documents: Quinlan will accept these for extra caps and XP.
Explore the Sub-Rooms: Some side chambers in Poseidon hide holotapes and terminals that hint at Poseidon Energy’s pre-war experiments.

Final Thoughts

The Flux Sensor quest might look like another Brotherhood fetch mission, but it’s one of those underrated gems that quietly expand Fallout 4’s world. It’s not about shooting your way through a base , it’s about recovering echoes of the old world.

Every time you dig up another relic from Poseidon’s flooded ruins, you’re reminded of why the Brotherhood exists , to worship the past while unknowingly repeating it.

And that’s the magic of Fallout: even in a flooded power plant full of ghouls, there’s always a story humming beneath the surface.