The Hayward HeatPro LO error is one of the most misdiagnosed heat pump codes in the field — mostly because it has two completely different causes that look identical on the display. LO can mean the ambient temperature is simply too cold for the unit to operate, or it can mean the refrigerant circuit has a low-pressure condition that needs professional attention. Telling them apart is the key diagnostic skill.
Pool heat pumps extract heat from the outside air and transfer it to pool water. Below a certain ambient temperature (typically 50°F / 10°C for standard HeatPro models, 40°F for titanium-equipped models), the refrigerant in the evaporator coil can't absorb enough heat from the air to maintain adequate suction pressure. The low-pressure cutout switch trips, and LO appears on the display.
This is completely normal behavior. The heat pump is protecting its compressor from damage. Check the current air temperature. If it's below the unit's minimum operating temperature, LO is expected. The unit will clear automatically when temperature rises and no service is needed.
If the LO error appears on a warm day (above 55°F) with good airflow over the evaporator, the problem is a refrigerant-side issue. Low refrigerant charge (from a leak) is the most common culprit, but a blocked expansion valve or a refrigerant restriction can produce the same symptom. This is not a DIY repair — refrigerant work requires EPA Section 608 certification.
Quick test: If LO appears when ambient temperature is 70°F+, it is almost certainly a refrigerant issue, not an ambient temperature issue. Call a refrigeration tech.
| Condition | Ambient Cause | Refrigerant Cause |
|---|---|---|
| Air temp during LO | Below 50°F (10°C) | Above 55°F (13°C) |
| Evaporator coil | Iced over / frosted | May ice partially |
| Clears with warm temp | Yes, automatically | No — persists |
| Compressor noise | Normal (or silent) | May short-cycle |
| Fix required | None — wait for warmth | Refrigerant recharge by certified tech |
The evaporator needs unrestricted airflow to function. If leaves, debris, or enclosure walls are blocking the fan intake or exhaust, the unit can't extract enough heat from the air even in warm weather, dropping suction pressure. Clear 18 inches of clearance on all sides. Inspect the coil fins for debris accumulation — rinse with a garden hose from inside the coil outward.
Over time, cottonwood seeds, dust, and debris clog the evaporator fins, reducing airflow and heat transfer. This is especially common in residential areas with cottonwood trees. Annual coil cleaning (gentle rinse only — do not use pressure washers which crush the delicate aluminum fins) is part of proper heat pump maintenance.
Less commonly, the low-pressure cutout switch itself fails in the open position, displaying LO even when refrigerant pressure is normal. A refrigerant tech can verify this by measuring refrigerant suction pressure directly during operation. If pressure is within spec but the switch reads open, replace the switch.
Any time LO appears in warm weather and does not resolve after checking airflow and coil cleanliness, you need a licensed refrigeration technician. Signs of refrigerant issues:
Warning: Do not add refrigerant to a pool heat pump yourself. Refrigerant work on sealed systems requires EPA 608 certification. Improper charging can damage the compressor and is illegal without certification.
You can reference the full Hayward HeatPro error code list — including LO, HI, FL (flow), and EO — in SplashLens, offline at the equipment pad.
SplashLens covers Hayward HeatPro, AquaCal, Pentair UltraTemp, and more. Offline, free, no account needed.
Open SplashLens Free →LO on the Hayward HeatPro heat pump means either the ambient air temperature is below the unit's minimum operating temperature (usually 50°F), or the low-pressure refrigerant switch has tripped due to a refrigerant issue.
Most Hayward HeatPro models are rated for ambient temperatures above 50°F (10°C). Below this, the refrigerant evaporator can ice up and the low-pressure switch trips. Some premium models operate to 40°F.
If the cause is low ambient temperature, yes — the LO error will clear automatically once air temperature rises above the unit's minimum operating threshold. If the error persists above 55°F, a refrigerant or sensor issue is likely.
Yes. A refrigerant leak causes low suction pressure, which trips the low-pressure safety switch and displays LO. This requires a licensed refrigeration technician to diagnose and repair.
Yes. LO on the HeatPro display corresponds to a low refrigerant pressure condition monitored by the low-pressure cutout switch in the refrigerant circuit.