If a modem refuses to connect, the problem is nearly always one of four: No power to the box, a break in the coax or DSL feed, a loss of sync with the ISP, or out-of-date firmware. Those four troubleshooting steps: one, two, three, four. Reboot the modem after you've done the physical checks.
Symptoms
- Power LED is on, but the internet or online light stays off or blinks continuously.
- Devices show "connected, no internet" on both Wi-Fi and Ethernet.
- DSL or cable sync light cycles on and off every few minutes.
- Speed tests time out, or throughput collapses to under 1 Mbps.
- Modem admin page at 192.168.100.1 or 192.168.0.1 is unreachable.
Common Causes
Damaged or loose coax, DSL or Ethernet cable:
A finger-tight coax connector, a bent RJ11 pin or a kinked Ethernet run will reduce the signal levels below the lock point of the modem. Deteriorating splitter and pre-MoCA wall plates exacerbate this.
ISP outage or provisioning problem:
A regional outage, a failed node, or a non-pay account will prevent the modem from reaching DHCP even if the hardware is OK. The sync light will typically be solid, but the internet light dark.
Overheated or aging equipment:
By four to six years, capacitors will have deteriorated with heat. An overheated modem will lose sync temporarily and then come back again when the case cools – just like a flaky ISP link.
Outdated or corrupted firmware:
An ISP will push firmware down the wire, but if an update goes south the modem may end up half configured. Symptoms can range from constant restarts to a modem that won't acquire an IP.
Wrong or missing modem provisioning
A swapped or second-hand modem must have its MAC address registered with the ISP. Without provisioning, the device gets sync but never a public IP.
Step-by-Step Fix
- Check all cabling prior to pressing reset:
Remove the coax from the modem and tighten it finger-tight Hand-tighten the coax back onto the modem Replace the RJ11 filter on the DSL port and test a different phone jack Replace the fibre ONT-to-modem Ethernet patch lead at both ends – in the wall or base unit and at the modem Less than half of "dead" modem tickets actually relate to a faulty modem. Most are caused by cabling issues. - Read the LED pattern before rebooting
The LEDs tell you where the fault sits. Power off means PSU or outlet. Downstream or DSL blinking means no sync with the ISP. Online off with sync solid means a provisioning or account problem. If you reboot first, you lose this diagnostic evidence. - Cold-boot the modem and router in the right order
Unplug the modem, then the router. Wait 60 seconds so the ISP's session timer releases your old lease. Power the modem first and wait for the online light to go solid, which typically takes two to three minutes. Only then power the router. Booting them together is the most common reason a reboot "doesn't work." - Confirm the outage is not on the ISP side
Tether a laptop to your phone and check your ISP's status page and their X or Facebook account. Call the support line and ask them to ping your modem's WAN MAC. If they cannot see it, the fault is between your wall socket and their headend, not inside your house. - Log in at the modem's diagnostic page:
On a wired laptop, load the local admin page for the modem (usually 192.168.100.1 for cable, 192.168.1.1 for DSL). Look at downstream power (aim for about -7 to +7dBmV on DOCSIS) and downstream SNR (aim for greater than 30dB). If they're significantly outside those ranges, you have a line problem your ISP needs to address. - Bypass the router to pinpoint the problem:
Connect your laptop directly to the LAN port of the modem using an Ethernet cable and then restart your modem. If your laptop has internet access, it's the router or Wi-Fi at fault, but if it doesn't, then it's the modem or the link to the ISP. A two-minute test can save hours of 'what if' speculation. - Get new or re-flash firmware:
ISP-provided modems have their firmware remotely controlled, so get the latest image pushed to it by support. If you're using a retail modem, obtain its firmware only from its maker's site and flash it as per the instructions. Halt halfway through a flash, and you'll brick it. - Factory reset in the end, then re-provision:
Press and hold the recessed reset button for 30 seconds, then, after it reboots, call the ISP with the MAC address from the label, and they will reprovision it. (Skipping the provisioning call is the reason the factory reset sometimes appears to make things worse.)
Modem LED patterns and what to do about them
| LED pattern | Likely cause | First action |
|---|---|---|
| Power off | No mains power or dead PSU | Try another outlet, then another power brick |
| Power on, downstream/DSL blinking | No sync with ISP line | Check coax or DSL cable, then call ISP |
| Sync solid, online off | Provisioning or account issue | Call ISP with modem MAC address |
| All lights on, no internet on devices | Router or LAN fault | Bypass router with a wired laptop |
| Lights cycling every few minutes | Overheating or failing hardware | Cool the modem, then plan replacement |
Prevention
- Keep the modem in open air, off carpet, and away from routers or set-top boxes that radiate heat.
- Replace coax splitters older than five years and use screw-on, not push-on, F-connectors.
- Note the current firmware version and signal levels each quarter so you have a baseline.
- Plan to replace a heavily used modem every four to six years, before capacitor drift starts.
FAQ
How long should I unplug my modem for?
Sixty seconds is the sweet spot. Anything less and the ISP's session timer may still hold your old DHCP lease, so the modem reconnects into the same broken state. Longer than two minutes gains nothing. After power-up, give the modem a full three minutes to complete its boot and sync before you judge the result.
Is it the modem or my router that is broken?
Isolate them. Plug a laptop directly into the modem's Ethernet port and reboot the modem. If the laptop gets internet, the router is the problem. If it does not, the modem or the ISP link is. This single test replaces most guesswork and takes about two minutes.
Do you need to replace your modem or just repair it?
Do it when a strong signal is seen, yet the modem fails to maintain sync, restarts by itself, or is so hot you can't touch it. The typical lifespan of a cable and DSL modem is four to six years of continuous operation. Your upgrade to a newer model may also increase your speeds.