Resideo smart thermostat shows E42 and cannot connect to Wi‑Fi

Safety first: This is a network issue; no high-voltage work is required. Do not open HVAC equipment panels.

Steps to troubleshoot:

• 1. Reboot network: Power cycle your modem and router by unplugging them for 60 seconds, then plugging back in. Wait until Wi‑Fi is fully restored.
• 2. Check other devices: Confirm that phones or laptops can obtain an IP address and browse the internet on the same Wi‑Fi network.
• 3. Verify SSID and password: On the thermostat, go to Menu > Wi‑Fi and forget the current network. Re-add the correct 2.4 GHz SSID and password. Avoid 5 GHz-only networks if your model does not support them.
• 4. Check router settings: Ensure DHCP is enabled on your router and that MAC address filtering or access control lists are not blocking the thermostat (look for the Resideo/Honeywell Home device in the client list).
• 5. Move or boost signal: If the thermostat is far from the router, consider moving the router, adding a Wi‑Fi extender, or changing the router channel to reduce interference.
• 6. Static IP (advanced): If your router allows, reserve an IP address for the thermostat's MAC address to avoid lease issues.

When to call a professional: If E42 persists after router reboot and reconfiguration, contact your ISP or Resideo support for network-level diagnostics.