C-wire power connector
Common fix for smart thermostat power, Wi-Fi, and delayed-start problems.
Honeywell T10 Pro / RTH9585 Thermostat Error Codes — Quick Reference
The Honeywell T10 Pro (THX321WFS) and RTH9585 Wi-Fi smart thermostats display error codes on screen when a sensor or communication fault is detected.
| Error Code | Meaning | Quick Fix |
|---|---|---|
| E1 | Indoor temperature sensor fault | Check wiring, replace if needed |
| E2 | Outdoor temperature sensor fault | Check outdoor sensor connection |
| E3 | Floor sensor fault (radiant) | Check floor sensor wiring |
| E9 | Condensate line full / wet switch | Check and clear condensate drain |
| Cool On Flash | Short cycle protection | Wait 5 minutes — normal protective delay |
| Aux Heat On | Auxiliary heat active | Check outdoor temp and heat pump operation |
Most Common Errors
E1 — Indoor Temperature Sensor Fault
The thermostat cannot read its internal temperature sensor. This is usually an internal failure — the T10 Pro’s built-in sensor has failed. Try resetting the thermostat:
- Remove the thermostat from the wall plate
- Remove one AA battery for 30 seconds
- Reinstall and check if E1 clears
If E1 persists, the thermostat unit needs replacement — the internal sensor is not separately serviceable.
E2 — Outdoor Temperature Sensor Fault
If you have an outdoor temperature sensor connected to the T10 Pro (accessory sensor), and it fails, E2 appears. Check:
- The sensor cable connection at the thermostat’s accessory terminal
- Sensor wiring for damage (especially if running outdoors through walls)
- Sensor resistance — a 10K NTC thermistor reads approximately 10,000 ohms at 77°F (25°C)
E9 — Condensate / Wet Switch
The condensate drain line is full or a wet (water) switch has tripped. This protects against water damage from an overflowing drain pan. Fix:
- Locate the condensate drain line and pan
- Clear the drain line — use a wet/dry vacuum at the end of the drain line
- Check for algae or debris clog in the drain trap
- After clearing, reset the thermostat to clear E9
Cool On Flashing
“Cool On” flashing on the display is NOT an error — it is a short-cycle delay (typically 5 minutes) to protect the compressor. Wait for the flash to stop and normal cooling to begin.
T10 Pro vs RTH9585 Differences
| Feature | T10 Pro | RTH9585 |
|---|---|---|
| Smart Response | Yes | No |
| Wi-Fi | Yes | Yes |
| Remote sensor | Up to 6 | No |
| Humidity control | Yes | Basic |
| Display | Color | B&W |
Wiring Error at Installation
If error codes appear right after installation, verify the wiring:
- R/Rh/Rc — 24VAC hot
- C — 24VAC common (required for Wi-Fi operation)
- Y — cooling
- W — heat
- G — fan
A missing C wire causes power issues and can cause erratic behavior or error codes.
Parts Often Needed
| Part | Notes |
|---|---|
| Replacement thermostat | For E1 internal sensor failure |
| Outdoor temperature sensor | For E2 if accessory sensor is damaged |
| Condensate pan tablets | Prevent algae growth causing E9 |
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Jump to Fix
- E1 → Reset thermostat → If persists, replace thermostat
- E2 → Check accessory sensor cable → Measure sensor resistance → Replace sensor
- E9 → Clear condensate drain → Vacuum drain line → Check drain pan → Reset
When to Call a Pro
If the thermostat wiring is correct but errors persist after replacement, contact an HVAC technician — the fault may be in the equipment (condensate sensor, refrigerant issue).