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.

What do 8 flashes mean on a Honeywell gas valve or ignition module?

On Honeywell S8910U and S89-series ignition modules, 8 flashes (slow blink) is a flame-sense or flame-current fault — the burner lit but the module is not reading enough microamps through the flame rod. Causes: dirty flame sensor, weak burner ground, cracked porcelain on the rod, or a marginal igniter holding flame at the wrong location. Clean the flame sensor with fine emery cloth, verify a solid ground from burner to chassis, and confirm flame current with a meter (target ≥ 1 2 µA DC depending on module spec).

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.

Are Honeywell and Resideo (Honeywell Home) the same brand?

No — they split in 2018. Honeywell Building Technologies kept commercial controls (gas valves, ignition modules, building management, industrial sensors). Resideo Technologies spun off and operates the residential thermostat and connected-home product line under the “Honeywell Home” trademark license. The hardware lineage is shared but support paths diverge: residential thermostat issues route to Resideo / Honeywell Home (1-855-733-5465); commercial gas-valve and ignition-module issues route to Honeywell Process Solutions or your distributor.

What does a flashing Cool On / Heat On icon mean on a Honeywell thermostat?

A flashing Cool On (snowflake) or Heat On (flame) icon on a Honeywell thermostat is the compressor protection delay — a 5-minute lockout that prevents short-cycling damage to the compressor or heat-pump reversing valve. It is normal behavior after a power interruption, after a setpoint change that requested cooling within minutes of a heating cycle, or after the system was switched off. Wait the 5 minutes; if the icon does not stop flashing and the system never starts, check for actual call-for-heat / call-for-cool wiring at the equipment.

Why is the Honeywell thermostat screen blank or unresponsive?

A blank Honeywell thermostat screen is almost always a power problem. Battery-powered models (RTH series): replace the AA batteries. Hardwired 24 V models: the most common cause is a tripped float switch on the AC condensate pan (interrupts R-power to protect against overflow), followed by a blown 3 A fuse on the indoor furnace control board, then an open low-voltage transformer. Pull the thermostat off the wall plate, jump R to G at the wall plate — if the indoor blower starts, 24 V is at the wall and the thermostat itself is the fault.

How do I reset a Honeywell Pro 8000 / VisionPRO thermostat to factory defaults?

On the Honeywell Pro 8000 / VisionPRO 8000 (TH8320, TH8321, TH8110), enter the installer setup: press and hold the SYSTEM button until the display changes (about 5 seconds), then press SYSTEM again until the installer setup screen appears. Function 0900 is “reset to factory defaults” — set it to 1 and press DONE. The thermostat reboots into setup mode where you re-enter the system type, equipment stages, and dealer info. The Wi-Fi credentials and Honeywell Home pairing must also be re-entered after a factory reset.

When should I call a technician for a Honeywell controls problem?

Call an HVAC professional immediately for any flame-rollout, gas-leak smell, soot at the burners, repeated ignition lockouts (3-flash on the S8910), or a gas valve that does not close on call-for-no-heat — these are CO and fire hazards. Wi-Fi / DHCP / app-pairing errors (E01, E02, E42, E99) on residential thermostats are network-side and rarely require a technician. If a thermostat passes the wall-plate jumper test but the system still does not run, the call is at the indoor unit (transformer, board, float switch), not the thermostat.

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.

hvac

Honeywell Troubleshooting Assistant

Get instant AI-powered help for Honeywell fault codes and equipment issues via WhatsApp. Our assistant is trained on official manufacturer documentation — available 24/7 for field technicians.

By Octavian Ghindea

Co-founder & CTO · Reviewed May 2026

What error codes does Honeywell display?

E01

Wi-Fi setup error — router not found during network search; verify 2.4 GHz broadcast and proximity

E02

Invalid Wi-Fi password — thermostat rejected by router authentication; re-enter case-sensitive credentials

E42

DHCP / IP-address allocation failed — router did not lease an address to the thermostat

E43

Thermostat could not reach the Honeywell Home / Resideo cloud server; check upstream internet connectivity

E99

Generic operation failure — disconnect from the wall plate for 30 seconds and reconnect to clear

1 flash

S8910 / S89 ignition module — airflow or pressure-switch fault before ignition trial

3 flashes

S8910 ignition lockout — failed to light after maximum retries; check gas supply, igniter, flame rod

4 flashes

S8910 — high-limit or flame-rollout switch open; check airflow and heat-exchanger condition

5 flashes

S8910 — flame sensed without a call for heat; gas-valve leak or shorted flame-sense wiring suspected

8 flashes

S8910 — low or no flame-sense current; clean flame sensor, verify burner ground, check microamp reading

Common questions about Honeywell equipment

What other HVAC brands does AskWhiz cover?

AskWhiz also covers troubleshooting for these manufacturers in the same industry:

Aermec Airquest Amana Belimo Bell & Gossett Bosch HVAC Braeburn Systems Bryant Carrier CIAT Climaveneta Comfortmaker