Heater displays E21; heat pump mode shuts down on cold days.

Safety First: Turn off power before accessing sensors.

• Verify ambient temperature: Check actual outdoor temperature. Many Rheem heat pump pool heaters lock out below a certain temperature (e.g., ~45°F / 7°C).
• Locate ambient sensor: Find the outdoor air temperature sensor on or near the unit. Inspect for physical damage or sun exposure that could skew readings.
• Test sensor: Measure resistance and compare it to Rheem's ambient sensor chart. Replace if out of range.
• Use gas mode if hybrid: On hybrid models, switch to gas heating when the ambient temperature is too low for efficient heat pump operation.