Most GPON problems come down to a handful of causes: a red LOS light, no internet even though lights look normal, or slow speeds. This guide walks through the most common GPON issues in plain language, what each ONT light means, and the exact steps to fix them before calling your ISP.
Your ONT (the box on your wall) uses small indicator lights to tell you what is happening. Power means the unit is on. PON (sometimes labeled PON or Auth) should be solid green when the connection to your provider is established - blinking means it is still trying to connect. LOS stands for Loss of Signal and should normally be off; a red or blinking red LOS light means the fiber signal is not reaching your ONT. LAN or Internet lights show your local devices are connected.
A red LOS light is the most common serious GPON fault and almost always points to a physical fiber problem. Work through these steps in order: 1) Check that the fiber cable is fully seated in the ONT - unplug the green or blue fiber connector, inspect the tip for dust, and click it back in firmly. 2) Look for sharp bends, kinks, or crushed spots along the fiber run; fiber breaks easily if bent too tightly. 3) Power the ONT off for 30 seconds and back on. 4) If the LOS light stays red after all of this, the problem is on the line side (outside your home) and you should contact your ISP - the fiber, splitter, or OLT port may need service.
If your Power and PON lights are solid green and LOS is off, but you still cannot get online, the fiber link is fine and the issue is usually with the router, IP session, or Wi-Fi. Try these: reboot both the ONT and your router; if you use a separate router, confirm the cable runs from the ONT LAN port to the router WAN port; check whether one device or all devices are affected (a single device points to Wi-Fi or that device, not GPON); and confirm your account is active and not suspended for billing. If the PON light is blinking rather than solid, the ONT has not authenticated with your provider - this often needs the ISP to re-provision your ONT serial number.
If you are connected but speeds are lower than your plan, the cause is often local rather than the GPON line itself. Run a wired speed test directly from the ONT or router using an Ethernet cable to rule out Wi-Fi; Wi-Fi loss, old devices, and distance from the router account for most slow-speed complaints. Confirm your device and cables support the speed you pay for (a device with a 100 Mbps port cannot deliver a 500 Mbps plan). Check for background downloads or many devices sharing the connection. Remember GPON is a shared architecture - up to 32 or 64 homes share a splitter - so brief congestion at peak times can occur, but sustained slow speeds well below your plan are worth reporting to your ISP.
Contact your provider if the LOS light stays red after checking the fiber, if the PON light will not go solid, or if wired speeds stay far below your plan after ruling out local issues. These point to problems on the provider side - a cut or degraded fiber, a bad splitter, an OLT port fault, or an ONT that needs to be re-provisioned - which only your ISP can fix. When you call, tell them your ONT light status (which lights are on, off, or red); it helps them diagnose the issue faster.
Related: What the ONT Lights Mean is covered above. For related reading, see How GPON Works and GPON Security.