Honeywell Thermostat 175 Code: What 'AC Power Resumed' Means & What To Do

AC power resumed — Resideo's Lyric T6 Pro Wi-Fi table words it: 'AC power to the thermostat was restored after a power failure. You don't need to take any action.' The T6 Pro Smart guide's Alert Code Reference carries the same entry: 'AC Power Resumed — No action needed. Power restored after loss.'

Alert 175 is the odd one out in the Honeywell T-series table: it is not a fault, a reminder or a request — it is the thermostat logging an event that already ended. While power was gone, the thermostat could show nothing at all (a C-wire-powered model goes dark with the system; a battery-backed model rides the outage out on its batteries). When AC power returns, the thermostat restarts, notes that power was lost and restored, and files 175 in its alert list so the gap in operation is explained rather than mysterious. One normal behaviour follows it and is often mistaken for a second fault: heating or cooling does not restart instantly. Resideo documents a built-in compressor protection feature that prevents the compressor from restarting too soon after a shutdown, and states explicitly that this startup delay also applies after AC power loss — on the button-style T6 Pro the same protection shows as 'Cool On' or 'Heat On' flashing, and the manual's instruction is to wait 5 minutes for the system to restart safely, without damaging the compressor. The alert itself may sit in Menu > Alerts until you look at it; it is information, not a block. What 175 cannot tell you is why the power went away: a neighbourhood outage, a tripped breaker you reset, someone switching the system off at its power switch for other work, and a furnace door left ajar after a filter change all produce the same single line in the alert list. A single 175 after a storm is the end of the story. A 175 that keeps coming back, with no outage to explain it, is the thermostat witnessing an intermittent power problem — and then the alert stops being trivia and becomes the evidence.

What causes the 175 code on a Honeywell thermostat?

A utility power outage that has ended

The textbook 175: the grid dropped, the system lost its 24 VAC supply with everything else in the house, and power came back. The thermostat's wording — 'restored after a power failure' — assumes exactly this story, and Resideo's instruction for it is no action.

The breaker was tripped and reset, or switched off for other work

Any deliberate or protective interruption of the circuit feeding the HVAC system reads identically to an outage from the thermostat's side of the wire. A 175 that lines up with electrical work, a service visit, or a breaker you found tripped is explained — the thermostat is confirming the power came back, not reporting a new failure.

The system power switch was turned off and back on

Resideo's own troubleshooting for power symptoms names the heating and cooling system's power switch (the switch at or near the furnace/air handler) as a first-line check: a system switched off for a filter change, a repair or by accident, then switched on again, produces a clean power-loss-and-resume cycle and a 175.

The furnace/air-handler door was open, then closed

Resideo's blank-screen and system-not-responding troubleshooting includes 'make sure the furnace door is closed securely': on many air handlers and furnaces the door holds a safety switch that cuts power to the system (and with it the thermostat's supply) while the door is off. A filter change that ends with the door not quite latched gives you an outage no utility ever recorded — and, once the door is seated, a 175.

Repeated short interruptions (the pattern that matters)

One 175 is an event; a string of them is a pattern. Loose or failing power delivery anywhere in the chain — a breaker that keeps tripping, a switch losing contact, a door switch that lets go when the blower vibrates — shows up at the thermostat as repeated power-loss/resume cycles, each one logged the same way. Resideo does not give 175 a frequency threshold; the diagnosis is in the repetition itself.

Checks to run before you replace anything

1. Read the alert properly first: Menu > ALERTS (touchscreen models) or Menu, then +/− to ALRT, then Select (button models). If 175 is the only alert and the system is heating or cooling normally, Resideo's guidance is that no action is needed — note the date and move on.
2. Wait out the protection delay before judging the system: after power returns, 'Cool On'/'Heat On' flashing means compressor protection is holding the restart. Resideo's instruction is to wait 5 minutes for the system to restart safely. A system that starts within that window behaved exactly as designed.
3. Correlate the event: was there a storm, a neighbourhood outage, electrical work, a service visit, or a filter change at the air handler? A 175 with a matching household event is closed. A 175 with no matching event deserves the power-path checks below.
4. Check the three power points Resideo itself names for power symptoms: the circuit breaker (reset it if tripped), the power switch at the heating and cooling system (on), and the furnace/air-handler door (closed securely). These are the documented checks — do them in that order before suspecting the thermostat.
5. Check the thermostat's clock after the event: a related reminder, 171 (set the date and time), can follow a long outage on models that do not sync time automatically — a wrong clock corroborates that power was genuinely gone, and it needs setting for the schedule to run correctly.
6. Start a simple log if 175 repeats: date, time, whether the whole house lost power (clocks, microwave), and what the system was doing. Whole-house witnesses separate a utility story (not your equipment's problem) from a system-only story (your power chain's problem) faster than any single check.

How to fix honeywell thermostat 175?

1. Single 175, system running: nothing. Dismiss/acknowledge the alert in Menu > Alerts if your model asks you to, and let the system run. Resideo's documented action for 175 is 'You don't need to take any action.'
2. System hasn't restarted yet: wait the full compressor-protection window (about 5 minutes, 'Cool On'/'Heat On' flashing) before touching anything. Interrupting the delay by cycling power again just starts a new delay — and logs a new power event.
3. 175 after a filter change or service visit: reseat the air-handler/furnace door fully so its safety switch closes, confirm the system power switch is on, and confirm the breaker is on. That trio is Resideo's documented power chain, and the door is its most-missed link.
4. Breaker found tripped once: reset it fully (off, then on) and watch the system through a complete heating or cooling cycle. A breaker that trips again is not a thermostat problem and not a 'reset it again' problem — it is an electrical fault announcing itself (see Pro).
5. Recurring 175 with no house-wide outage: treat the power chain as the patient — breaker, system switch, door switch, and the 24 VAC supply at the equipment — and have it proven with a meter by a technician rather than swapping the thermostat, which is only the witness.
6. Do not 'fix' 175 by disabling alerts or pulling batteries to silence it: the alert is the only log the house keeps of these events, and a thermostat that stops reporting power loss has not stopped losing power.

The part that usually fails: No part — 175 is an event report, not a component failure

Nothing in the thermostat wears out to produce a 175. If 175s recur and the fault is found, the failed part lives in the power chain (a breaker, a switch, a door switch, a transformer) — equipment-side parts a technician identifies by measurement, never the thermostat itself on the strength of this code alone.

How to reset honeywell thermostat 175?

175 needs no reset procedure and has no reset button: it is an event log, not a latched fault. On touchscreen T-series models it sits in Menu > ALERTS until dismissed like other non-critical alerts; on button models it is read through Menu > ALRT. Clearing it does not affect the system, and it will simply be written again the next time power is lost and restored. The related restart behaviour is automatic: once AC power is back, the thermostat reboots on its own (settings and schedule are stored permanently in memory and do not need battery power, per the T6 Pro guide — only the clock can need attention, which is alert 171's job). If the display is blank rather than showing 175, power has not actually resumed — work Resideo's blank-screen checks (breaker, system power switch, furnace door, batteries where fitted) instead of waiting for an alert that cannot display without power.

Honeywell alerts do not clear like appliance fault codes, and the T-series splits them by kind. Maintenance reminders (181-189) are timers, and clearing one IS resetting its timer: on touchscreen models, replace/clean the item first, then touch 'Dismiss' on the alert screen — Resideo's table wording for every reminder is 'Reset the timer by touching the dismiss button after it is replaced'; on the button-style T6 Pro, press Menu, step to ALRT, press Select, then either snooze (SNZE, 7 days) or clear (CLER). Resideo's caution heads both flows: some alerts cannot be snoozed or cleared — those are the fault alerts (164, 168, 170, 173, 177, 178), which clear only when their cause is fixed or the dealer replaces the failed part, and Resideo's instruction when an alert will not clear is to call your heating and cooling professional. Informational alerts (171, 175, 210, 388) clear by doing the small thing they ask (set the clock, register the thermostat) or, for 175, by simply being read — power already resumed. VisionPRO 8000 (RedLINK) works the same way in words instead of numbers: Dismiss clears the alert message and turns off the red alert light, and each IAQ reminder has a Reset Timer control. VisionPRO IAQ error codes are diagnostic reports on a communicating system; they clear when the underlying condition (a failed sensor, a duplicate device, a silent bus) is corrected by the installer. No Honeywell platform here has a 'clear all codes' owner button, and a power cycle re-reports any standing fault rather than clearing it.

Why does my honeywell thermostat keep showing 175?

A returning 175 is the whole diagnostic content of this code. Read the pattern before the part: returns that match storms or neighbourhood flickers are the grid's story and end when the grid settles. Returns with no house-wide witnesses — the microwave clock keeps time while the thermostat logs outages — localise the loss to the HVAC power chain: a door switch that opens under blower vibration after a filter change, a system switch with a failing contact, a breaker weakening toward repeated trips, or a 24 VAC supply that sags under load. Each return also restarts the compressor-protection delay, so a system losing power several times a day spends its life waiting to restart and never quite heating or cooling — that comfort complaint, plus the log of 175s, is what to hand the technician.

How to stop 175 coming back

• A utility power outage that has ended: The textbook 175: the grid dropped, the system lost its 24 VAC supply with everything else in the house, and power came back. The thermostat's wording — 'restored after a power failure' — assumes exactly this story, and Resideo's instruction for it is no action.
• The breaker was tripped and reset, or switched off for other work: Any deliberate or protective interruption of the circuit feeding the HVAC system reads identically to an outage from the thermostat's side of the wire. A 175 that lines up with electrical work, a service visit, or a breaker you found tripped is explained — the thermostat is confirming the power came back, not reporting a new failure.
• The system power switch was turned off and back on: Resideo's own troubleshooting for power symptoms names the heating and cooling system's power switch (the switch at or near the furnace/air handler) as a first-line check: a system switched off for a filter change, a repair or by accident, then switched on again, produces a clean power-loss-and-resume cycle and a 175.

Retrieving stored fault codes on Honeywell thermostats

On T-series models an active alert puts the alert icon on the home screen (on the T6 Pro Z-Wave, the alert symbol and the alert NUMBER appear in the clock area). The full report lives behind Menu > ALERTS on touchscreen models — Menu > ALRT, then Select, on the button-style T6 Pro — where each alert shows its 3-digit code and its definition, and multiple alerts are paged through with the arrow keys. The same alerts are manageable in the Resideo app (select the thermostat, Settings, Manage Alerts) on connected models. VisionPRO 8000 (RedLINK) announces alerts differently: a red alert light and a plain-text message on screen — there is no numeric table to look up, the message is the diagnosis. VisionPRO IAQ hides its codes one layer deeper: an 'Err' in the time field of the home screen means the thermostat has detected an error on the communication bus, the code itself is read in the manual's Table 14 system (and via diagnostic recall), and the EIM's communication LED gives the bus's own verdict — rapid blinking is normal transfer, continuous blinking or always-off is a wiring problem, always-on means the EIM may need replacement. Practical rule across all lines: photograph the alert screen before dismissing anything — a dismissed reminder restarts its timer, and a fault alert that clears when its cause clears leaves no owner-visible history behind.

Other Honeywell thermostat error codes

See the full list on the Honeywell Thermostat error codes hub.

Frequently asked questions

Related guides

Facts on this page were checked against manufacturer documentation: manuals.plus · cdn-cms.f-static.net · manuals.plus · manuals.plus · dhb3yazwboecu.cloudfront.net · manuals.plus · manualzilla.com · safe-manuals.com · pimmedia.winsupplyinc.com