---
title: "W5500 Ethernet Link Failure on Some Units – Temperature Dependent, Fixed by Lowering Series Resistor"
url: "https://maker.wiznet.io/forum/16093"
markdown_url: "https://maker.wiznet.io/forum/16093/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "Huseyin_Ozdil"
created: "2026-06-11T17:04:21+09:00"
last_activity: "2026-06-11T17:12:52+09:00"
language: "en"
tags: ["W5500", "PoE"]
views: 4
replies: 0
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# W5500 Ethernet Link Failure on Some Units – Temperature Dependent, Fixed by Lowering Series Resistor

## Question

Asked by Huseyin_Ozdil on 2026-06-11 in Ethernet Chips.

Some of our devices in the field have been reported to experience Ethernet connectivity issues. After recalling the affected units for investigation, we obtained the following findings:

We would appreciate your support in identifying the root cause(s) of this issue.

- Devices that fail to establish an Ethernet connection start operating normally after being warmed up. While some units work at room temperature and others do not, our laboratory tests showed that certain units fail at 0°C but operate correctly at 10°C. However, in the field, some affected units fail to operate even at ambient temperatures as high as 24°C.

- An additional observation is related to the startup behavior. Devices that successfully establish an Ethernet link while at normal temperature continue to operate correctly even when they are subsequently cooled down to lower temperatures. However, if power is removed and reapplied while the device is cold, the Ethernet link can no longer be established.

- Conversely, devices that fail to establish an Ethernet link at low temperature will automatically recover and establish the link after being warmed up for some time, without any hardware changes or resets being applied during the heating process.

- When the Ethernet cable of a non-functional device is disconnected and a PoE Extender is inserted between the device and the network, the device starts working normally. Once the extender is removed, the device only operates again after being heated.

- On some non-functional units, reducing the series resistors on the Ethernet data lines from 33 Ω to 10 Ω restores normal operation.

- We also suspect the possibility of counterfeit or non-genuine components in some products (PNG file attached). In three affected units, replacing the W5500 Ethernet controller with W5500 devices sourced directly from DigiKey resolved the issue completely. However, in one unit, replacing the W5500 alone did not solve the problem. The issue was only resolved after both replacing the W5500 and reducing the Ethernet data-line series resistors from 33 Ω to 10 Ω.

Attachments:

- [W5500.png](https://maker.wiznet.io/forum/getAttach.asp?aidx=381)

---

Source: https://maker.wiznet.io/forum/16093