PoE not working is almost always caused by one of three things: a standards mismatch, a wiring fault, or a cable that’s too long. This guide walks through diagnosing each one methodically — from the quickest checks to the less obvious causes — so you can isolate the fault without swapping parts randomly and hoping something changes.
Step 1 — Check the PoE Standard
The single most common cause of PoE failure, particularly with newly installed equipment, is a mismatch between the PoE standard the injector or switch delivers and the standard the powered device requires.
The two main standards are 802.3af (up to 15.4W sourced, 12.95W at the device) and 802.3at — also called PoE+ — (up to 30W sourced, 25.5W at the device). A powered device that requires 802.3at will not operate correctly — or will not power on at all — when connected to an 802.3af-only injector. The injector won’t fault or alarm; it simply won’t deliver enough power, and the device will either refuse to start or will power cycle continuously.
How to check: find the technical datasheet for your end device — not the product page, the actual datasheet or installation guide — and look for the power consumption specification. It will typically state the PoE standard required and the maximum power draw in watts. Then check the label on your injector. If the injector is 802.3af and your device needs 802.3at, you need a different injector. For a full breakdown of what the standards mean in practice, see our guide on PoE standards explained.
If you’re running an 802.3bt device (PTZ cameras, some ceiling speakers, multi-radio access points), the same logic applies — you’ll need a switch or injector that explicitly supports 802.3bt, not just PoE+.
Step 2 — Check Active vs Passive PoE
This is a related but distinct problem and it’s worth understanding separately. IEEE 802.3af and 802.3at are active PoE standards — before delivering power, the switch or injector performs a handshake with the powered device using a low-voltage detection and classification process. Power is only applied once a compatible device is confirmed on the other end.
Passive PoE works differently. A passive injector simply applies a fixed voltage — often 24V or 48V — to the spare pairs of the Ethernet cable, with no negotiation. This is common on older Ubiquiti hardware (24V passive) and some other vendors. A passive 24V injector connected to an 802.3af device won’t damage it (because 802.3af devices are required to be tolerant of detection voltages), but it also won’t power it — the device is waiting for active negotiation that never happens. Conversely, a passive 24V injector connected to a device that doesn’t expect any voltage on those pairs could potentially damage it.
Check whether your injector is IEEE 802.3af/at compliant or passive. The injector label or datasheet will state this. If you’ve bought a cheap injector from an unbranded source and can’t find specification data, treat it as passive until proven otherwise. Our guide on active vs passive PoE covers this in full, including how to identify which type of passive PoE your device requires.
Step 3 — Check the Cable
Once you’ve ruled out a standards mismatch, the cable is the next thing to examine. PoE has higher requirements than standard Ethernet because it’s carrying current, and a cable that passes a basic continuity test isn’t necessarily adequate for PoE.
First, confirm the cable category. Cat3 — sometimes found in older buildings — does not reliably carry PoE. Cat5e is the minimum for PoE installations. If you’re in a building with legacy cabling, test or verify what’s actually installed before assuming it’s suitable.
Next, check continuity with a cable tester. A link light on a switch port doesn’t tell you everything — you can have a link with a miswired pair, and some wiring faults that don’t break the data link will prevent PoE from working correctly. Use a proper cable tester that verifies all eight conductors, not just a basic continuity checker.
Inspect the RJ45 connectors at both ends. Look for bent or recessed pins in the plug — a common result of a bad crimp or a connector that wasn’t fully seated before crimping. Check that the wires are pinned in the correct order (T568B at both ends for a straight-through cable). A single transposed pair can allow a data link while breaking PoE negotiation.
If you have any doubt about the terminations, re-crimp them. It takes ten minutes and is far faster than spending an hour testing everything else. A poor crimp is a surprisingly frequent culprit, particularly when cable was terminated on-site in difficult conditions — in a ceiling void, in cold weather, or by someone in a hurry.
Step 4 — Check the Cable Length
The 100-metre limit for PoE is the same as for standard Ethernet — it’s a combined limit covering the cable run, patch leads, and any connectors. In practice, PoE becomes marginal as you approach that limit because there is a voltage drop across the cable proportional to cable length and the current being drawn. At shorter cable runs the voltage at the device is close to the injector output; at longer runs the voltage at the device may fall outside the powered device’s acceptable input range.
If your cable run is over 80 metres, start being suspicious. If it’s over 90 metres, test with a known-short cable (5 or 10 metres between the injector and device) to establish whether the cable length is a contributing factor. If the device powers up on the short cable, the run length is your problem — you need a PoE extender, a second switch with an uplink, or a fibre run with a local injector at the far end.
Measure the actual cable length rather than estimating from the floor plan — cable often takes longer routes through trunking, up risers, and across ceiling voids than the straight-line distance suggests.
Step 5 — Check the Power Supply
PoE injectors are powered by a mains adapter, and a failing or undersized adapter is a surprisingly common cause of PoE problems — particularly with budget equipment.
First, check whether the injector’s power LED is illuminated. No LED means no mains power — check the socket, the adapter, and the cable between them. If the power LED is on but the device isn’t powering up, the adapter may be present but delivering insufficient current under load.
Compare the power adapter’s rated output (voltage and amperage, printed on the label) against the injector’s stated power requirements. An injector rated at 30W output needs a power adapter capable of delivering at least that — with losses in the conversion, you typically need an adapter delivering 35W or more. Some budget injectors ship with adapters that are technically the correct voltage but marginally rated for current. Under load they sag, PoE negotiation fails or is unstable, and the device at the far end either doesn’t power on or drops intermittently.
Test by substituting a higher-rated adapter at the correct voltage. If the problem resolves, the original adapter was the fault.
Step 6 — Test with a Different Device
At this point you’ve checked the standard, the cable type, the terminations, the run length, and the power supply. If the fault persists, the question is whether the problem lies in the injector/cable side or in the end device itself.
Swap the end device for something known to work with this injector — another camera, an access point, a PoE tester — and see whether that device powers up. If it does, the fault is in the original device, not the infrastructure. If it doesn’t, the fault is in the injector or cable.
Equally, connect the original device to a different known-good PoE source — a different injector or a PoE switch port you know is working. If the device powers up there but not on the original injector, the injector is the problem.
This kind of substitution testing sounds obvious but it’s genuinely the fastest way to isolate a fault when you’ve worked through the likely causes and nothing has changed. It avoids spending time testing a cable that’s fine because the actual fault is a dead injector.
Step 7 — Check for PoE Budget Issues
If you’re using a PoE switch rather than a standalone injector, and devices were previously working but a newly added device won’t power up, the switch’s PoE budget is a strong candidate.
Every PoE switch has a maximum total PoE power budget. When that budget is exhausted, new devices connecting to remaining ports will get a data link but no power. The switch doesn’t typically send an alert — you just find that the new device won’t come up, while existing ones continue to work.
How to check by platform:
- Ubiquiti UniFi: In the UniFi Controller, navigate to Devices, select the switch, and look at the PoE tab. Total PoE consumption and budget are displayed there. Alternatively, SSH to the switch and run
show poe. - Cisco Catalyst: From the CLI, run
show power inline. This shows per-port allocation and total consumed vs available budget. - TP-Link managed switches: Log in to the web interface, go to PoE → PoE Config or PoE Statistics. Total allocated and remaining budget are shown per switch.
If the budget is exhausted, your options are: reduce the power draw of existing devices (check whether any are over-allocated — some switches allocate the maximum class power rather than actual draw), upgrade to a switch with a higher PoE budget, or add a separate PoE injector for the new device powered from its own adapter.
When the Injector Itself Is Faulty
Injectors can fail, and the failure mode is usually one of the following: completely dead (no power LED, nothing works), intermittently failing under load, or the PoE negotiation circuit has failed while the pass-through data still works.
Signs that the injector itself may be the fault:
- The power LED is off and a known-good adapter doesn’t fix it.
- The injector feels warm — warmer than you’d expect — and the powered device won’t start or keeps dropping. An injector running hot under load is often a sign the internal components are failing.
- The data link works fine (you can ping the device) but PoE won’t negotiate — the device has an external power supply but won’t come up on PoE alone.
- A PoE tester connected to the injector’s output shows voltage below the expected range for the standard.
Testing with a multimeter: if you have a PoE voltage tester or a purpose-built PoE tester device (inexpensive tools are available from suppliers like RS or Toolstation), connect it to the injector’s PoE output port and check whether the correct voltage is being delivered. For 802.3af/at, the output voltage should be 44–57V DC. If you’re seeing significantly lower voltage, or no voltage, the injector’s output stage has failed.
Budget injectors from no-name sources tend to fail more quickly than branded units — particularly if they’ve been running near their rated capacity. If you’re relying on PoE for business-critical devices, invest in a named-brand injector (Cisco, TP-Link, Ubiquiti, Netgear) rather than the cheapest option available. The cost difference is small; the reliability difference over three to five years is not.
Specific: Ubiquiti AP Not Powering Up
If you’re working with a Ubiquiti access point specifically, there are some additional considerations specific to that platform — including the 24V passive PoE requirement for older UniFi APs and the difference between the Ubiquiti supplied injector and generic 802.3af units. See our dedicated guide on powering a Ubiquiti access point with a PoE injector for AP-specific steps.
Working through these steps methodically takes less time than it might appear, and it’s considerably faster than replacing components at random. Start with the standards check — it resolves the majority of faults — move through cable quality and length, verify the power supply, and use substitution testing to isolate the side of the fault. In most cases the answer will be clear well before you reach the end of the list.
