It’s always the same feeling. You’re finally getting your Electrolyzer setup to look decent, the base temperature is stable, and then that dreaded red notification pops up: Oxygen Not Included pipe blocked. Everything stops. The gas just sits there. Your Duplicants start gasping for air because the ventilation system has turned into a glorified paperweight. Honestly, it’s one of the most frustrating things in the game because the game doesn't always tell you why it's stuck. It just is.
Managing fluids and gases in Klei’s survival sim is basically a lesson in high-stakes plumbing. If you don't respect the way the game handles "packets," you’re going to have a bad time.
The Absolute Basics of Why Your Pipe Blocked
The most common reason for a pipe being blocked is actually pretty simple: there is nowhere for the stuff to go. If you have a Gas Pipe leading to a Gas Vent, and the room is overpressurized (usually above 2kg of gas per tile), that vent will shut off. When the vent shuts off, the gas packet behind it can't move. Then the packet behind that one stops. Pretty soon, the whole line is backed up to the machine producing it.
You’ve got to check the destination first. Is the liquid reservoir full? Is the coal generator’s input pipe already holding a 1kg packet of Carbon Dioxide? Pipes in this game behave like a single-file line at a grocery store. If the person at the front isn't moving, nobody is moving.
Sometimes the issue is more subtle. You might have accidentally used a Bridge the wrong way. In ONI, Bridges, White ports (Inputs), and Green ports (Outputs) dictate the direction of flow. If you have two Green ports facing each other on the same line without a clear path to a White port, the game's pathfinding logic just gives up. The gas gets "confused" about which way to travel, and the pipe status flips to blocked.
The Bridge Trick and Priority Flow
Understanding the "White to Green" rule is the difference between a failing colony and a thriving one. White means In, Green means Out. Think of it like a vacuum. White ports "suck" packets toward them, while Green ports "push" them out.
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If you want to prevent a pipe blocked message in your main oxygen spine, you should be using Bridges as "overflow" valves. This is a pro move that many players miss early on. By placing a Bridge on a line, the game prioritizes sending the gas through that Bridge first. If the Bridge’s output is blocked, the gas will then continue down the main line. This is how you keep your critical systems running while diverting excess Hydrogen to a generator or an extra storage tank.
Let’s talk about mixing materials. Never, ever put two different types of liquids or gases in a pipe leading to a machine that only accepts one. If you accidentally let a packet of Polluted Water into a Liquid Intake meant for a Water Sieve, the Sieve will stop. It won't just ignore the wrong liquid; it will sit there and hold it, causing an Oxygen Not Included pipe blocked error for every single packet behind it. You'll have to manually click the pipe and "Empty Pipe" to get things moving again, which wastes Duplicant time and spills nasty stuff on your floor.
Troubleshooting the "Ghost" Blockage
Sometimes you look at a pipe and it looks fine. The vent is open. The pressure is low. But the gas isn't moving. Check your junctions. When you have three or four pipes meeting at a single T-junction without a Bridge, the game has to calculate where each packet goes every single tick. This can lead to "stuttering" flow.
If you see packets vibrating back and forth, your plumbing logic is broken. The game doesn't know which way is "away" from the source. To fix this, place a Bridge immediately after the junction. This forces a one-way direction and clears the "confusion" from the pathfinding algorithm.
Advanced Logic: Sensors and Shutoffs
If you’re tired of manually checking for blocks, you need to automate. Using a Pipe Element Sensor paired with a Liquid or Gas Shutoff is the gold standard.
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Imagine your Electrolyzer setup. You're pumping out Oxygen and Hydrogen. If your Hydrogen generators are full and the Hydrogen pipe backs up, it can stop the Oxygen from flowing too if they share a filtered line. By using a sensor, you can detect when the pipe is backed up and redirect the flow to a Gas Range or just vent it into space as a last resort.
Don't forget about the "Bridge Priority" mechanic. If you have a main line and you want to join a side line into it, always use a Bridge to merge the side line into the main one. This ensures that the main line takes priority and doesn't get backed up by the secondary source. It’s a subtle bit of fluid dynamics that the game doesn't explicitly explain but is essential for late-game builds like Petroleum Boilers or Sour Gas Boilers.
Why Temperature Can Block Your Pipes
This is a "gotcha" that kills many colonies. If you are pumping Water through a Cold Biome and the temperature of the water drops below its freezing point while it’s sitting in the pipe, it will turn into Ice.
When a liquid turns into a solid inside a pipe, the pipe breaks. It’s a literal physical block. You'll get a notification for "Pipe Broken," but before it actually bursts, it will show as blocked. The same happens with Steam turning back into Water in a Gas Pipe. This is why Insulated Pipes are your best friend. Use Igneous Rock for insulation early on, and eventually move to Ceramic or Mafic Rock if you’re dealing with extreme space-bound temperatures.
Common Misconceptions About Pipe Capacity
- Gas Pipes hold 1,000g (1kg) per packet.
- Liquid Pipes hold 10,000g (10kg) per packet.
- Insulated Pipes don't stop heat exchange; they just slow it down significantly.
- Radiant Pipes are for cooling things down, but they will block instantly if the liquid inside changes state.
If you try to merge two Gas Pipes that are both full (1,000g), they cannot merge into a single pipe. One has to wait. This creates a "pulse" effect where your machines run at 50% efficiency. Always look at the packet size. If you see small packets of 500g, you can merge those without a block. If they are full 1kg packets, you need a separate line.
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Immediate Steps to Fix Your Plumbing
If you’re staring at a "Pipe Blocked" warning right now, follow this sequence. First, trace the pipe to its very end. Is the final destination—the vent, the machine, or the reservoir—actually capable of accepting more? If it's a vent, check the air pressure of the room. If it's over 1800g-2000g, that's your culprit. Dig out more space or add a Gas Pump to move that air elsewhere.
Second, look for any Bridges that might be facing the wrong way. It happens to the best of us. A single Bridge rotated 180 degrees will stop an entire industrial backbone.
Third, check for "mixed" packets. If there is a tiny 0.1g packet of Chlorine stuck in a pipe meant for Oxygen, and it's trying to enter a Carbon Skimmer, it will never move. Deconstruct that one segment of pipe to let the "wrong" gas out, then rebuild it.
Finally, implement a "Loop" system for your cooling. Instead of having a beginning and an end, create a continuous circle of pipe with a Bridge acting as the "input" for new fluid. This keeps the liquid constantly moving, which prevents temperature spikes and reduces the chances of a "Pipe Blocked" error caused by static fluids.
By mastering the flow of White and Green ports and respecting the 1kg/10kg packet limits, you can ensure your base stays pressurized and your Duplicants stay alive. Plumbing in Oxygen Not Included isn't just about moving stuff from A to B; it's about managing the logic of the grid itself.
Next Steps for Your Colony:
- Audit your Oxygen spine: Replace all T-junctions with Bridges to enforce one-way flow.
- Check your vent pressures: Ensure your main base isn't hovering at 2.5kg of pressure, which causes the "Popped Eardrums" debrief and blocks all incoming Oxygen.
- Upgrade to Insulated Pipes: Focus on the lines running through the Caustic or Frozen biomes to prevent phase-change blockages.