Thermostat shows E18 and may indicate 'Sensor calibration error' or 'Temperature/humidity out of range'.

Steps:

• Compare readings: Place a reliable room thermometer and hygrometer near the thermostat and compare readings. If the Rheem thermostat is off by more than a few degrees or 5–10% RH, calibration may be needed.
• Check environment: Ensure the thermostat is not exposed to drafts, direct sunlight, or heat sources that could skew readings.
• Recalibrate (if option exists): Use the thermostat's installer or advanced settings to apply an offset to temperature and/or humidity readings.
• Firmware update: Connect to Wi-Fi and update firmware via the Rheem app to correct any known calibration issues.
• If E18 persists and readings are wildly inaccurate: The internal sensor package is likely failing; replace the thermostat.