What does Honeywell E01 mean?

Error E01 appears during Wi-Fi setup when the thermostat cannot detect the router — most commonly because the router lost power mid-setup or is out of wireless range. The thermostat broadcasts a probe for the selected SSID and receives no response, causing the setup to fail at the router-discovery stage.

What does Honeywell E02 mean?

Error E02 appears during Wi-Fi setup when the thermostat rejects the router credentials you entered. The thermostat attempted to authenticate with your home router but received an access-denied response because the password did not match.

What does Honeywell E42 mean?

Error E42 means your router did not assign an IP address to the thermostat during Wi-Fi setup. The thermostat connected to the wireless network but the router's DHCP service failed to complete the address allocation handshake. Without an IP address, the thermostat cannot communicate with Honeywell servers or the app.

What does error code E01 mean on a Honeywell thermostat?

E01 on a Honeywell Wi-Fi or Lyric thermostat is a Wi-Fi setup error — the thermostat could not find your wireless router during the network search step. Confirm the router is powered on, broadcasting on the 2.4 GHz band (Honeywell thermostats do not connect to 5 GHz), and within range. Move the router closer if needed, then restart the Wi-Fi setup wizard from the menu. If the SSID is hidden, enter it manually on the network selection screen.

What does E42 mean on a Honeywell thermostat?

E42 means the Honeywell thermostat connected to the Wi-Fi network but the router did not assign it an IP address — a DHCP failure. Log into the router admin page (typically 192.168.1.1) and verify DHCP is enabled with available addresses in the pool. Power-cycle the router for 30 seconds, then restart Wi-Fi setup on the thermostat. The unit will request a fresh DHCP lease and complete registration once an address is granted.

How do I clear E99 on a Honeywell thermostat?

E99 is a generic Honeywell thermostat error indicating the unit cannot complete an operation — a Wi-Fi connection drop, a server-side time-out, or a stalled firmware update. The recommended fix is to disconnect and reconnect the thermostat: pull it from the wall plate for 30 seconds (cuts 24 V power), reseat it firmly, and let it boot. If E99 returns within minutes, factory-reset and re-pair to the Resideo / Honeywell Home app.

How do I read Honeywell S8910 ignition module fault codes?

The S8910/S8910U has a single status LED on the module body. Codes are slow flashes separated by a long pause: 1 flash = airflow / pressure-switch fault; 2 flashes = pressure switch closed when it should be open; 3 flashes = ignition lockout (failed to light after retries); 4 flashes = limit / rollout circuit open; 5 flashes = flame sensed without a call for heat (gas valve leak suspected); 6+ flashes = internal module fault, typically requiring replacement. The wiring diagram on the appliance door is the authoritative chart — wording varies slightly by furnace OEM.

Quick Answer

Honeywell controls report faults three ways depending on the product family. Thermostats (T6/T9/T10, Lyric, Pro 8000) display numeric E-codes — E01, E02, E42, E99 — on screen. Gas valves (VR8204/VR8304) and S8910 ignition modules blink slow LED codes — 8 flashes typically means a flame-sense fault. Identify the part first, then match the chart.

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By Octavian Ghindea

Co-founder & CTO · Reviewed May 2026

What error codes does Honeywell display?

E01

Wi-Fi setup error - router not found

E02

Invalid Wi-Fi password

E42

IP address issue (router DHCP problem)

E99

General error - disconnect and reconnect

Common questions about Honeywell equipment

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