ServiceNomad RV · Knowledge
Dometic AC LO/PS Low Pressure Error
The LO/PS (Low Pressure) warning on some Dometic RV air conditioners indicates that the refrigerant pressure has dropped below the safe operating threshold. This error is designed to protect the compressor from running in a low-refrigerant condition, which can cause overheating and catastrophic failure. Understanding what causes low pressure helps you determine whether the issue is a refrigerant leak, airflow restriction, or sensor fault.
What Low Pressure Means in AC Systems
RV air conditioners operate on a closed refrigerant loop. The compressor pumps refrigerant through the system, and proper pressure is critical for efficient heat transfer. If pressure drops on the low (suction) side of the system, it indicates either insufficient refrigerant in the system or restricted airflow preventing proper heat absorption. The pressure switch or sensor detects this condition and shuts down the compressor to prevent damage. Low pressure can also occur if the system is overcharged (counterintuitively) or if there's a blockage in the refrigerant circuit, such as a clogged filter-drier or kinked line. A technician will use pressure gauges to measure actual system pressures and compare them to factory specifications.
Refrigerant Leaks and Loss
The most common cause of low pressure is refrigerant loss due to a leak. RV air conditioners experience significant vibration and flexing, which can cause cracks in copper tubing, loose fittings, or leaks at brazed joints. Even a small leak over several seasons can deplete enough refrigerant to trigger the LO/PS error. Common leak points include the compressor fittings, evaporator coil connections, and condenser (outdoor coil) tubing. A technician will inspect the system for oil stains (which indicate refrigerant leaks), use electronic leak detectors, and may inject UV dye to locate hard-to-find leaks. Once the leak is repaired, the system is evacuated, recharged to proper specifications, and tested. Simply adding refrigerant without finding and fixing the leak is a temporary fix at best.
Airflow Restrictions and Heat Absorption
Low pressure can also result from inadequate airflow over the evaporator coil. If the coil cannot absorb heat from the return air, the refrigerant won't evaporate properly, causing low suction pressure. This is often caused by dirty air filters, clogged coils, or blower motor issues. The system may even ice up if airflow is severely restricted, further reducing heat transfer. A technician will inspect and clean the evaporator coil, replace air filters, and verify the blower motor is operating at full speed. They'll also check that return air vents are not blocked by furniture or bedding inside the RV.
Pressure Switch and Sensor Faults
In some cases, the pressure switch or sensor itself is faulty and triggers a false LO/PS error even though system pressure is normal. These switches can stick, corrode internally, or fail electrically. A technician will test the switch with a multimeter and compare actual refrigerant pressures (measured with gauges) to the switch's cut-out point. If pressures are normal but the switch indicates low pressure, the switch needs replacement. Dometic uses specific pressure switches for different models, and they must be matched correctly to ensure safe operation.
⚠️ Safety Notes
- Never attempt to add refrigerant yourself—refrigerant handling requires EPA certification, proper tools, and knowledge of system pressures.
- Do not bypass the low-pressure switch—this critical safety device prevents compressor damage.
💡 Prevention Tips
- Inspect the AC annually for signs of refrigerant leaks (oil stains, ice buildup, or hissing sounds).
- Keep air filters clean and evaporator coils clear of debris to maintain proper airflow and system pressures.
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