On a T6 Pro Smart the alert appears as a small symbol and a number in the clock area of the home screen. Press Menu, go to Alerts and select it to read what the number means, then snooze or dismiss it; some alerts cannot be snoozed or cleared until the cause is dealt with. Touchscreen models such as the T9 skip the numbers and write the alert out in words, so a Wi-Fi module error on a T9 and a 168 on a T6 are the same fault. In the table below, a call-now rating means book a service visit: nothing on a thermostat alert list is a gas or carbon monoxide warning.
Three messages get searched as error codes and are not faults. A flashing Cool On, or a flashing Heat On if you have a heat pump, is the built-in compressor protection timer holding the outdoor unit off after a shutdown, and the T6 Pro Smart shows the word Wait for the same thing. The installer sets that timer anywhere from off to five minutes, so the rule is simple: if the message is still flashing after five minutes and nothing has started, stop blaming the timer and check the equipment. A battery warning (BATT, Lo Batt or Replace Batt, depending on the model) is a reminder with roughly two months of notice built in.
A blank or rebooting screen is the one that sends people shopping for a thermostat they do not need. A Wi-Fi thermostat runs on 24 volts from the furnace, so a dark screen usually means the furnace has cut that supply: the furnace switch or breaker is off, the blower door is not pressing its safety switch after a filter change, the low-voltage fuse on the furnace board has blown, a float switch on the air conditioner drain has opened, or there is no C-wire carrying constant power. Our blank thermostat screen guide walks through those checks in order. When the thermostat is fine, the repair is at the furnace, and that is a furnace service call rather than a thermostat swap.
These numbers come from the T6 Pro and T6 Pro Smart manuals published by Resideo, the company that makes Honeywell Home thermostats, and codes vary by model: many older Honeywell models show no alert numbers at all. A thermostat has no legend printed inside it, so the user guide for the model number on the back of the thermostat is the authority. Where the fault turns out to be in the furnace, the diagnostic label inside the furnace panel wins over this page. To have it sorted in one visit, see our Honeywell thermostat installation page or contact ZK Mechanical.
Honeywell thermostat codes, decoded
| Code | What it means | Safe? | Try first | When to call |
|---|---|---|---|---|
164 | Heat Pump Needs Service. The heat pump has energized the thermostat's L terminal to report a fault of its own; the thermostat is only passing the message on. | call now | Check that the outdoor unit is clear of snow and its breaker is on. Switch to Em Heat if the house is cooling down. | Book a heat pump diagnostic. The fault is outdoors, not on the wall. |
168 | Wi-Fi Radio Error. The wireless module inside the thermostat is not working, so app and remote features are unavailable. Heating and cooling still run from the wall. | keep running | Pull the thermostat off its wall plate, or switch the furnace breaker off for one minute, then let it restart. | If 168 is still there after the power cycle, the thermostat needs replacing. |
170 | Internal Memory Error. The thermostat's memory has failed and its settings can no longer be trusted. | call now | Nothing to repair at home. Note the model number on the back of the thermostat. | Book a replacement. Resideo's instruction for this alert is to contact the installer. |
171 | Set the Date and Time. The clock has been lost, and the program schedule cannot run without it. | keep running | Set the date and time from the menu, or reconnect Wi-Fi so the thermostat can set itself. | Not needed, unless the clock keeps resetting, which points to a power supply problem. |
173 | Thermostat Temperature Sensor Error. The sensor built into the thermostat has failed, so it cannot measure the room it is controlling. | call now | None. Watch the room temperature with a separate thermometer until it is replaced. | Book a thermostat replacement. This alert means the thermostat itself is finished. |
175 | AC Power Resumed. The 24-volt supply came back after being lost. It is a notice, not a fault. | keep running | Dismiss it and check that the clock and schedule survived the outage. | Only if it shows up when there was no hydro outage. Then the furnace is dropping power to the thermostat. |
177 | Indoor Temperature Sensor Error. A wired remote indoor sensor is disconnected or its wiring is shorted. Only appears where a wired sensor was installed. | keep running | Check that nobody has knocked or unplugged the remote sensor. | Yes, to have the sensor and its wiring tested and the setup matched to the sensor type. |
178 | Outdoor Temperature Sensor Error. A wired outdoor sensor is disconnected or shorted. | keep running | Look for a damaged sensor wire outside, often near the outdoor unit. | Soon on a heat pump or dual-fuel system, where the outdoor reading can decide which heat source runs. |
181 | Replace Air Filter (1). A maintenance timer the installer set by fan run time or calendar days has expired. The thermostat cannot see the filter. | keep running | Replace the furnace filter, then press dismiss so the timer starts again. | No call needed. |
182 | Replace Air Filter (2). The second filter timer, used where a system has a second filter or media cabinet. | keep running | Change the second filter and dismiss the alert to reset its timer. | Not for the alert itself. |
184 | Replace Humidifier Pad. A maintenance reminder for a furnace-mounted humidifier wired to the thermostat. | keep running | Swap the evaporator pad and dismiss the reminder. | Only if you would rather have the pad changed during a tune-up. |
388 | Register Online for Remote Access and Outdoor Temperature. The thermostat is on Wi-Fi but has not been registered to an account in the app. | keep running | Open the Resideo app and finish registering the thermostat. | No. This is an account step, not an equipment fault. |
399 | No Internet. The connection to the internet has been lost. The thermostat keeps controlling the house from the wall. | keep running | Check whether other devices are online, then restart the modem and router. | No. A technician cannot fix your internet service. |
400 | No Wi-Fi Signal. The thermostat can no longer see the wireless network it was joined to. | keep running | Wait for it to reconnect, or pick the network again in the app if the router or password changed. | Not needed for heating or cooling. |
508 | Wi-Fi Not Configured. The thermostat has never been connected to a network, or the connection was reset. | keep running | Follow the app's steps to join the thermostat to your Wi-Fi. | No, unless you want the setup done for you. |
405 | Low Battery, on battery-capable T6 Pro models. The home screen also shows BATT. | keep running | Fit two fresh AA alkaline batteries. Do it inside two minutes and the clock will not need resetting. | No call needed. |
407 | Critically Low Battery. The backlight has already been shut off to save power and the display is down to the battery warning. | keep running | Replace the AA batteries today, before the thermostat stops switching the furnace. | Only if new batteries do not bring the screen back. |
Cool On or Heat On flashing | Compressor protection. The thermostat is making the compressor wait a few minutes after its last shutdown before it restarts. | keep running | Leave the setting alone and wait five minutes. The message stops flashing when the compressor starts. | If it flashes for much longer than five minutes and the outdoor unit never starts. |
Wait | The same minimum-off timer, as the T6 Pro Smart words it. A call for cooling or heat pump heating arrived while the timer was still counting. | keep running | Nothing. Changing the setpoint again only restarts your patience, not the timer. | Not for the message alone. |
Blank screen | No power reaching the thermostat. On a 24-volt install this is nearly always a furnace-side problem, not a dead thermostat. | call now | Check the breaker, the furnace switch, that the furnace door is closed securely, and the batteries if your model has them. | If all four are fine. A blown board fuse or an open float switch needs finding, not bypassing. |
The codes that usually mean a real repair
Blank or rebooting screen: the furnace has stopped sending 24 volts
The thermostat is usually the victim here. In summer the common cause is a float switch on the air conditioner's drain pan opening because the condensate line is blocked, which cuts thermostat power on purpose so the pan cannot overflow. Clearing a condensate drain or replacing a trap runs $150–$350, and a new condensate pump installed is $250–$450. A blown low-voltage fuse on the furnace board is the other regular, and it blows for a reason, typically a thermostat wire shorted somewhere along its run. Diagnostic call: $90–$150, counted toward the repair when you book it.
No C-wire: a thermostat that reboots, drops Wi-Fi or shows 175 repeatedly
The T6 Pro Smart needs constant 24-volt power, and that means a C-wire. Many 1950s and 1960s bungalows were wired with two conductors for heat only, and plenty of later houses have a spare conductor in the wall that was never landed on the furnace board. Sometimes the fix is connecting that spare wire at both ends; sometimes a new cable has to be pulled. The kit has no flat price for that because the cable route decides the labour, so you get a written quote first. Our smart thermostat installation service covers the wiring and the setup in one visit.
170 or 173: the thermostat has failed internally
A memory error or a failed temperature sensor is not repairable, and Resideo's own guidance for both alerts is replacement. What changes the price is the system behind the wall plate: a single-stage furnace with air conditioning is a straight swap, while a heat pump or dual-fuel setup has to be configured for staging and changeover or it will run the wrong heat source. If you are choosing between brands, compare how Google handles the same job in our Nest thermostat error code guide. We quote the thermostat and the labour in writing before anything comes off the wall.
Licensed technicians, a written price before the repair, no overtime charges.
Common questions
Why is Cool On flashing on my Honeywell thermostat?
It is the compressor protection timer, and it is working as designed. Restarting an air conditioner or heat pump too soon after it stops can damage the compressor, so the thermostat holds it off for a few minutes and flashes Cool On while it waits. Give it five minutes. If the words go solid and the outdoor unit starts, there is no fault to chase.
My Honeywell thermostat screen is blank. Is the thermostat dead?
Usually not. Honeywell's own troubleshooting list for a blank display is the circuit breaker, the power switch for the heating and cooling system, the furnace door being closed securely, and fresh batteries where the model takes them. Three of those four are at the furnace. If they all check out, a blown control board fuse or an open drain safety switch is the likely cause.
Does alert 168 or 399 stop my furnace from heating?
No. Both are connectivity alerts. With 399 the thermostat has lost the internet, and with 168 its own Wi-Fi radio has stopped working, so you lose the app, remote changes and anything that relies on online weather. The thermostat still reads the room and still switches the furnace from the wall. A 168 that survives a one-minute power cycle does mean the thermostat should be replaced.
How do I clear an alert number on a Honeywell T6?
Press Menu, scroll to Alerts and press Select to see which alerts are active. From there you can snooze a reminder or dismiss it, and dismissing a filter or humidifier pad reminder restarts its timer. Some alerts cannot be snoozed or cleared from the menu. Those stay until the underlying problem is fixed, which is the thermostat telling you the number is not a nuisance.
Why does my Honeywell thermostat show words instead of a number?
Different models report differently. The T6 family uses alert numbers, the T9 touchscreen writes the alert out in plain words, and older battery models simply flash Lo Batt or Replace Batt when the cells are low. That is why a list for one model can miss on another. Match the model number on the back of your thermostat to its own user guide, and browse the other makes we cover in the brand guides.