The Five-Minute First Pass
Before pulling out test equipment, do the visual checks. Most no-signal calls are resolved in the first five minutes by people who know what to look at.
- Power LED on the ONT -- if dark, the ONT power adapter is the problem, not the fiber.
- LOS LED state -- solid red means no downstream signal; blinking means low/intermittent signal.
- PON or AUTH LED -- solid green means OLT registration is good; blinking means the ONT is trying to register and failing.
- Visible fiber damage -- check the patch cord between the wall plate and ONT for sharp bends, kinks, or pet damage.
- Connector seating -- unplug the SC/APC at the ONT, blow out dust, reseat firmly. Repeat at the wall outlet.
If everything looks clean and the LOS light is still on, move to instrumented checks.
ONT LED Decoding
| LED | State | Meaning |
|---|---|---|
| POWER | Off | No AC, bad adapter, dead ONT |
| POWER | Solid green | ONT powered normally |
| LOS | Off | Downstream signal present and in range |
| LOS | Solid red | No downstream optical signal detected |
| LOS | Blinking red | Signal present but below sensitivity |
| PON / AUTH | Off | No registration attempt (no signal) |
| PON / AUTH | Blinking green | Registering with OLT |
| PON / AUTH | Solid green | Registered and authenticated |
| LAN / WAN | Off | No upstream service |
| LAN / WAN | Solid green | Service active |
The combination of LEDs tells you where to focus. LOS solid red = optical path. LOS off but PON not registering = OLT/provisioning. Both healthy but no LAN = service or downstream router.
Step 1: Measure Downstream Optical Power
Disconnect the SC/APC fiber from the back of the ONT and connect it to a PON power meter. The XGS/GPON Power Meter reads downstream and upstream wavelengths simultaneously without disturbing the live PON.
What the reading tells you:
- -18 to -25 dBm: Healthy. Look elsewhere for the problem.
- -25 to -28 dBm: Marginal. Inspect connectors, check for tight bends, verify splitter is correct.
- Below -28 dBm: Out of spec. The fiber path has too much loss.
- No reading at all: No light reaching the customer. Walk the fiber back toward the OLT.
- Above -8 dBm: Receiver overload. Add a fixed attenuator (5 dB or 10 dB).
Step 2: Inspect the Connector End-Faces
If downstream power is low or absent and the fiber feels intact, the most likely cause is a contaminated connector. A single particle of dust or a smear of skin oil can kill a connection.
Use the WiFi/USB Fiber Inspection Microscope to inspect every mated pair from the ONT back toward the splitter:
- ONT pigtail (the one you just unplugged)
- Wall outlet bulkhead
- Splitter output port
- Splitter input port (if you can access it)
Clean any contaminated end-face with a CLEP 2.5mm cleaner for SC connectors or CLEP 1.25mm for LC. Re-inspect after cleaning. See our cleaning best practices guide for the full inspect-clean-inspect protocol.
Step 3: Verify Fiber Continuity with a VFL
If the connectors are clean and downstream power is still missing, the next step is a continuity check with a visual fault locator. Connect the VFL Pen 5km at the ONT side and walk the fiber back. Red light should be visible at the splitter output.
What you might see:
- Red light glowing through the cable jacket at one point: Macrobend or kink. Repair or replace.
- No light at the splitter input: Fiber is broken between the splitter and customer. Use an OTDR to find the distance to the break.
- Light reaches the splitter but not the OLT: Problem is upstream of the splitter, often a damaged feeder fiber.
For longer drops, use the 30km VFL for higher launch power.
Step 4: OTDR Trace for Fault Distance
If the VFL shows a fiber break or unexpected loss, the OTDR pinpoints exactly where. Connect the Fiber Ranger Mini OTDR at the customer side with a 100m launch reel.
Read the trace for:
- Distance to first major event: If less than the known distance to the splitter, that is your fault location.
- Excessive splice loss: Field splices that have degraded over time, water-damaged splice closures.
- High reflection events: Loose, broken, or open SC/APC connectors.
OTDR tracing is the bridge between "the fiber is broken somewhere" and "the fiber is broken 1.7 km from the customer, between pole 47 and pole 48." For more on OTDR interpretation see our OTDR basics guide.
Step 5: Identify Live Fiber Before Disconnecting
If you have to disconnect a fiber at a splitter cabinet to test a feeder, use the Optical Fiber Identifier first. Clamping it onto the cable identifies live traffic and direction without breaking the circuit. Disconnecting the wrong fiber takes down service for an entire splitter group of customers, and you will get an angry call from operations.
Step 6: OLT-Side Verification
If everything on the customer side reads in spec but the ONT will not register, the issue is upstream or provisioning. Things to check on the OLT (or escalate to NOC):
- OLT port admin state: Is the port enabled?
- Provisioning: Is the ONT serial number assigned to that PON port?
- Upstream signal at OLT: Does the OLT see the ONT's upstream burst? PON meters with upstream readout help confirm.
- Splitter port assignment: Is the customer's drop on the right splitter port for their service profile?
- Firmware compatibility: Is the ONT running a firmware version supported by the OLT?
Common Root Causes (In Order of Frequency)
| Cause | Frequency | Fix |
|---|---|---|
| Dirty connector | ~40% | Inspect and clean every mated pair |
| Damaged drop fiber | ~20% | Replace drop, check for staples/pinch |
| Dead ONT or power adapter | ~15% | Swap ONT or adapter |
| Tight bend or kink | ~10% | Re-route cable, replace patch jumper |
| OLT provisioning | ~5% | Re-provision ONT serial |
| Failed splitter | ~5% | Replace splitter cassette |
| Feeder fault upstream | ~5% | OTDR from splitter to OLT |
The Test Kit You Need
A no-signal call is hard to resolve without these tools in the truck:
XGS/GPON Power Meter
$484.99 -- Wavelength-selective downstream and upstream readings on live PON.
WiFi Fiber Microscope
$1,249.99 -- Catches the dirty connectors that cause 40% of no-signal calls.
VFL Pen 5km
$104.99 -- Visible-light continuity check, locates breaks and bends.
Fiber Ranger Mini OTDR
$579.99 -- Distance-to-fault for any break the VFL cannot resolve.
Fiber Identifier
$539.99 -- Confirms which fiber is live before you disconnect anything.
Frequently Asked Questions
The customer says it worked yesterday. What changed?
Ask: was there construction nearby, weather event, pet damage, or did they move furniture? FTTH drop fiber is fragile to cuts, sharp bends, and water intrusion. A landscaper hitting a buried drop is a daily occurrence in residential builds.
How long should troubleshooting take?
A standard no-signal call should resolve in under 60 minutes once a tech is on site. Most are 20-30 minutes. If you are still troubleshooting after an hour without a clear cause, escalate -- the issue is probably outside the customer side and needs OLT-side investigation.
Can I troubleshoot without a PON meter?
Not effectively. A generic power meter cannot distinguish wavelengths and will mislead you on a live PON. The cheapest way to start is the PON Power Meter Pro for GPON-only networks.
What if the OLT shows the ONT registered but the customer still has no service?
Service problems above Layer 2 (DHCP, PPPoE, VLAN, IP) require NOC engagement. Your job as an installer ends at confirming optical power and OLT registration. If both check out, hand off to the service desk.
The Bottom Line
FTTH no-signal troubleshooting is an ordered workflow: visual checks, optical power, connector inspection, continuity, distance-to-fault, OLT verification. Run the steps in order with the right tools and you will find the cause within an hour on every call.
For deeper coverage of customer-side issues see our customer-side troubleshooting workflow. For drop cable testing specifics see how to test an FTTH drop cable.