How to test bufferbloat after an ISP modem replacement

Your internet may feel slower for calls or games after an ISP modem replacement, even if a speed test shows a good download rate. The replacement may have changed how your router handles busy traffic, or the problem may be Wi-Fi rather than bufferbloat.

Use the Bufferbloat Test to see whether latency rises while the connection is downloading or uploading. Compare a wired test with a Wi-Fi test, then decide what to check next.

Test bufferbloat after the modem change

1. Open the Bufferbloat Test on the computer you want to check. If you saved a result from before the replacement, keep it nearby for comparison.
2. Stop downloads, video streams, cloud backups, and game updates on the network. Ask other people to pause heavy internet use while you test.
3. Run the test once with the computer connected to the router by Ethernet, if possible. Let the test finish so it can check how delay changes under download and upload traffic.
4. Run it again over Wi-Fi from the same room as the router. Keep other conditions the same, including the computer, browser, and time of day as much as practical.
5. Compare the grades and latency changes. If you have an older result, compare that too, but check whether the earlier test used the same device and connection type.

The test puts traffic through your connection to measure how much delay grows under load. It exchanges test data with our server, so pause other activity to avoid mixing its results with someone else's downloads.

What the result means

The wired result is good, but Wi-Fi is worse

The modem and router can handle the test load over Ethernet, but the wireless connection is adding a problem. Distance, walls, interference, or a weak Wi-Fi link can increase delays. Test closer to the router before changing modem or router settings.

The wired result has a low grade or a large delay increase

The issue is less likely to be Wi-Fi alone. The router may be building a long queue when traffic is busy, a setting may have changed during the replacement, or the modem and router may not be working together as expected. The test identifies a latency problem under load, but it cannot prove which device caused it.

The new result is similar to your old result

The modem replacement has not clearly changed bufferbloat in this test. A good grade does not rule out brief dropouts, weak Wi-Fi coverage, or a problem with a specific service. If the original symptom remains, test at the time it happens and consider whether other devices are using the connection.

The result changes each time

Other network traffic, Wi-Fi conditions, and internet congestion can change the outcome. Repeat the test with other devices idle. If results vary mainly on Wi-Fi, compare Ethernet and Wi-Fi again from the same location rather than treating one result as proof of a modem fault.

How to fix a worse result

1. Confirm how the computer is connected. On Windows 11, check Settings › Network & internet and select Ethernet or Wi-Fi. On a Mac, open System Settings › Network and select the active connection. Run the wired test using an Ethernet cable connected to the router, not to a different network device by mistake.
2. Restart the modem and router. Unplug both devices, wait about 30 seconds, then reconnect the modem first. Wait for it to come online before reconnecting the router. Use the ISP's instructions if the modem has a specific restart process. Do not factory reset an ISP-managed modem unless support tells you to.
3. Check the router's traffic settings. Look in the router's app or administration page for Quality of Service, QoS, or Smart Queue Management. If a setting was reset when the modem changed, restore the previous configuration or ask the router manufacturer what setting suits your connection. Avoid changing several options at once, so you can tell which change helped.
4. Check the modem and router arrangement. If you use a separate router, confirm whether the new modem is also acting as a router. Two devices both routing traffic can cause connection problems. The correct bridge or passthrough mode depends on your ISP and equipment, so ask the ISP before changing it.
5. Update router software if needed. Check the router manufacturer's instructions for a firmware update, especially if the replacement changed compatibility or settings. Save a copy of the current configuration first if the router offers that option.
6. Ask the ISP to check provisioning. If the wired result remains poor after a restart and the router settings look correct, contact the ISP. Ask whether the modem is provisioned correctly and whether the replacement changed any router or gateway settings. Share the test grades and whether the results came from Ethernet or Wi-Fi.

When the modem may be the problem

A poor bufferbloat grade by itself does not mean the modem is defective. Repeat the test over Ethernet, with other network use paused, and check the router settings first. If the connection also drops, the modem loses its online status, or the wired result stays poor after the ISP confirms its setup, ask the ISP to test or replace the modem. If the ISP owns the equipment, let them handle repairs or replacement.

Why the router matters too

The modem connects your home to the ISP, while the router manages traffic between your devices and that connection. Some ISP boxes do both jobs. Bufferbloat can come from the way traffic queues in the router, so swapping only the modem may not change the result. If you also replaced or reset the router, compare its settings with the ones you used before. For a router-focused comparison, see how to tell if a new router fixed bufferbloat.

The Bufferbloat Test measures response delay while adding network traffic, so results can differ from a speed test that only reports download and upload rates.