Does ERR_TUNNEL_CONNECTION_FAILED mean the website is down?

Almost never. The error happens at the proxy hop, before Chrome ever talks to the site. If the site loads on your phone over mobile data, or a direct HTTP check from outside your network returns 200, the site is fine and the fault is your local proxy configuration.

What's the difference between ERR_TUNNEL_CONNECTION_FAILED and ERR_PROXY_CONNECTION_FAILED?

ERR_PROXY_CONNECTION_FAILED means Chrome couldn't reach the proxy at all — wrong address, wrong port, proxy process dead. ERR_TUNNEL_CONNECTION_FAILED means Chrome reached the proxy fine, sent it a CONNECT request, and the proxy failed to open the tunnel to the destination. The first is 'I can't find the proxy'; the second is 'the proxy won't connect me through.'

Why does it only happen in Chrome, or only when my VPN is on?

Because the broken piece is a proxy that only Chrome (or only that VPN) routes through. Another browser using a different proxy setting, or your system with the VPN off, skips the failing hop entirely. That's a strong hint the destination is healthy and a local proxy is the problem.

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ERR_TUNNEL_CONNECTION_FAILED Fix

ERR_TUNNEL_CONNECTION_FAILED is your proxy refusing the CONNECT tunnel, not the site. Check 3 things: proxy, site status, DNS. Free instant check, no sign-up.

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Problem

Chrome shows ERR_TUNNEL_CONNECTION_FAILED with the message “This site can’t be reached.” The page never loads, but only through this browser or this network path.

Symptoms

The error appears on HTTPS sites specifically, and often on all of them at once.
The same URL loads fine on your phone over mobile data, or on a machine that isn’t behind the same proxy or VPN.
It started right after you connected to a corporate network, turned on a VPN, or installed an extension or “web accelerator.”
Turning off the system proxy or VPN makes the error disappear.

What ERR_TUNNEL_CONNECTION_FAILED Actually Means

When Chrome is configured to use a proxy and you request an HTTPS URL, it can’t just hand the proxy your encrypted bytes — the proxy would have no idea where to send them. So Chrome asks the proxy to open a raw pipe using the HTTP CONNECT method: it sends CONNECT example.com:443 HTTP/1.1. A working proxy opens a TCP connection to example.com:443, replies 200 Connection Established, and then gets out of the way so Chrome can run its own TLS handshake end-to-end through the tunnel.

ERR_TUNNEL_CONNECTION_FAILED is Chrome saying: I sent that CONNECT and never got a usable tunnel back. The proxy answered with a non-200, or hung up, or timed out, or couldn’t reach the destination itself.

The key word is tunnel. This is not the site rejecting you and it’s not a TLS problem with the certificate. The request died one hop early, at the proxy, before your browser and the destination ever spoke to each other. That single fact tells you where to look: everything between you and the proxy, not the site.

Top 3 Causes

A proxy is configured that shouldn’t be, or is dead - A leftover system proxy from an old VPN, a PAC (auto-config) script pointing at a server that no longer exists, or adware that quietly set a proxy. Chrome dutifully sends CONNECT to a machine that isn’t answering.
The proxy is alive but refuses this tunnel - A corporate proxy you’re no longer on-VPN to reach, a proxy that requires authentication (it answers 407 instead of 200), or one whose policy blocks the destination host or port 443. The proxy is up; it just won’t connect you through to this target.
A local TLS-inspection layer broke - Antivirus “HTTPS scanning,” a VPN client, or a filtering extension inserts itself as a local proxy to inspect traffic. When that component crashes, updates, or loses its upstream, every HTTPS request through it fails at the tunnel.

Diagnose with DechoNet

HTTP Check to reach the site directly from outside your network. DechoNet’s servers don’t use your proxy, so if the check returns a normal status code, the site is healthy and the failure is 100% local to your proxy path.
DNS Lookup to confirm the hostname still resolves to a real address — a dead PAC script or hijacked resolver can be part of the same misconfiguration.

Resolution Checklist

Run an HTTP Check on the failing URL. If it returns 200 (or any real status), stop blaming the site — the problem is your proxy.
Open your OS proxy settings. If a proxy or auto-config script is set and you don’t know why, turn it off and retry.
Disconnect any VPN and retry. If the error clears, the VPN’s proxy or split-tunnel config is the culprit.
Temporarily disable antivirus HTTPS/SSL scanning and any request-modifying extension, one at a time.
If you rely on a corporate proxy, confirm you’re on the network/VPN that can reach it and that your credentials aren’t being rejected (a 407 behind the scenes).
Flush DNS and reset the network stack only after the above — it rarely fixes a tunnel error but clears stale state.

When to Escalate

Escalate to your IT/network team if the proxy is corporate-managed. A CONNECT refused by policy, an expired proxy credential, or an upstream the proxy can’t reach are server-side problems you can’t fix from the browser.
If the error survives with every proxy and VPN off and a clean Chrome profile, capture a chrome://net-export log while reproducing — that shows whether Chrome is still routing through a proxy you haven’t found.