The unit runs, the indoor fan blows, nothing cuts out and no error shows, but the air is not cold or only a little. Unlike "runs a few minutes, then cuts out", the compressor keeps running here: the fault is usually in the refrigerant circuit. The table below lists the causes technicians found for this symptom.
A summary of 90 public questions with this symptom that got a fix in technicians' groups (2021–2026). 83 name one main cause, 21 of them confirmed by the poster as the fix; in 7 the answer only lists possibilities, so they are not in the table.
Causes technicians found, by number of cases
Typical repair cases
Low refrigerant, leak
- The air conditioner only cools half of the indoor unit, and the temperature is not deep (23-24°C). The poster is asking a technician for guidance on how to fix it and is willing to pay a fee. – Check the pressure and voltage first, then recover the refrigerant, evacuate the system, and recharge the correct amount by weight. If the indoor unit still cools only half, check whether the expansion valve is clogged, verify the refrigerant type, and check the heat dissipation of the outdoor unit.
- The air conditioner stopped cooling after the technician charged it with refrigerant about 1 month ago; the poster asks what the cause is and how to fix it. – Check where the unit is losing refrigerant by looking for oil traces around the flare nuts, pipes and joints, then remove the unit and pressure-test with nitrogen or submerge it in water. Once the leak is found, braze it again or replace the pipe, evacuate the system, and then charge the correct amount of refrigerant.
Blocked pipe, capillary, strainer, moisture
- Toshiba: After installing the "To clear 40" air conditioner for the customer, the indoor unit makes a loud hissing sound like a gas leak while running, and the suction line shows light condensation. The customer suspects moisture… – The hissing and suction line condensation are usually caused by a kinked or pinched suction/liquid line, plus moisture entering when the gas valves were opened to start the unit. Clean the unit, carefully inspect the piping and all brazed joints, then recover the refrigerant, evacuate thoroughly, and recharge with new…
- Daikin: A Daikin cassette indoor unit rated 4HP has a fan that runs normally and the unit is not dirty, but the machine still has a fault. A new technician asks for the cause. – Check the refrigerant line and the liquid separator (bầu tách lỏng) for blockage, while measuring the refrigerant pressure and checking for kinked pipes. If the fan runs normally but the unit is not cooling, prioritize clearing a blocked liquid separator or correcting an incorrect refrigerant charge.
Electronic expansion valve (EEV), reversing valve, valve coil
- Daikin: A Daikin mono-block air conditioner has run out of refrigerant: the system was evacuated and recharged, but overnight the gas pressure dropped again, and the poster is asking for advice on the cause. – The most likely cause is a leak at the Schrader valve (core) or at the gauge manifold hoses. Check by re-tightening or replacing the Schrader valve, re-testing the gauge set, then leak-test with soapy water or by immersing the joints in water, and monitor the pressure before evacuating and recharging again.
- The air conditioner runs in cooling mode, but the outdoor unit is cold while the indoor unit is hot. The pressure keeps rising and the current is high, causing the unit to cut out on protection. The poster asks for the… – The unit is running the refrigeration cycle in reverse, most likely because the reversing valve is faulty or stuck. Check with a meter whether the reversing valve coil connector has voltage. If there is voltage but the valve does not switch over, replace the valve. If there is no voltage, check the control board. If…
Faulty compressor
- Panasonic: Panasonic R32 air conditioner has sufficient refrigerant and ramps up normally, but cooling is poor; the refrigerant line is 15 m long. Please advise how to fix it. – Recover the old refrigerant, evacuate thoroughly, and charge the correct refrigerant quantity stated on the unit. If the work was done correctly but cooling is still poor and current is rising, check the suction pressure (close the service/compression valve); if it does not drop to 0, the compressor has lost its…
- Samsung: Samsung inverter air conditioner still does not cool enough after replacing the compressor (outdoor unit) with the same model; after recharging gas, the compressor starts and then trips out. – Check the new compressor by measuring the winding resistance at each terminal against the old specification, and check whether the piping or heat exchanger is clogged. At the same time, check the control board, filter capacitor and DC 300V bus voltage, because a faulty board or intermittent IC in the outdoor unit can…
Overcharge, wrong refrigerant
- The air conditioner's cooling is weak. The outdoor unit has been cleaned thoroughly, but the cause is still unclear. Technicians, please diagnose. – Check the temperature difference between the supply air and the return air. If cooling is weak, recover the old refrigerant, evacuate thoroughly, then recharge the correct amount of refrigerant by weight. If it is still weak, clean the indoor unit and check for other causes such as liquid flooding. (confirmed by the poster)
- Panasonic: A Panasonic air conditioner (labeled National) runs for nearly 1 hour on maximum speed and does not cool; the air coming out remains icy cold. When the fan is reduced to low speed, it cools. The poster asked for guidance… – A high discharge pressure (145 psi) and the unit not cooling indicate refrigerant overcharge or poor heat rejection, not a refrigerant shortage. Recover the refrigerant, evacuate the system thoroughly, then charge the correct amount listed on the unit's nameplate. Also inspect the outdoor unit and fan to rule out heat… (confirmed by the poster)
Installation (long or high pipe run, kinked pipe, unit not level)
- Multi-split system: 1 outdoor unit, 3 indoor units. When all 3 indoor units are running, it works well, but when one 9000 BTU indoor unit is turned off, the other two indoor units do not cool and frost forms on the… – Newly installed multi-split units often fail due to incorrectly wired terminal block (domino) connections or wrong indoor unit addresses. Re-check the wiring of each indoor unit according to the terminal block (domino) connections, set the correct address for each indoor unit, then test-run each indoor unit.
- Samsung: Samsung mono 1 HP air conditioner, R22 refrigerant, pressure 75, weak cooling: the liquid line feels slightly cool, the suction line is cold and sweating, and the owner asks what the cause is. – The unit is clogged in the piping and coil by oil that has accumulated because there is no oil trap, which reduces cooling. Blow the piping and indoor coil clean with dry nitrogen, clean, inspect, and top up the oil if it is short, then evacuate and recharge the refrigerant to the manufacturer's specified weight.
Overall
- A low charge and a blockage (pipe, capillary, strainer, moisture) are about as common – both give a low pressure but need very different repairs. Use the liquid and suction pipe temperatures and where frost or condensation forms to tell them apart before charging.
- A low charge means a leak: charging without fixing the leak only lasts a few weeks.
- An overcharge or the wrong refrigerant also cools poorly – charge the type and amount on the unit's label.
Note: this is technicians' field experience, not a manufacturer's document. Working pressure differs by refrigerant, model and outdoor temperature – compare with the exact model's documentation.
Safety: switch off both the indoor and the outdoor unit before opening a cover or touching a board; the capacitors on the outdoor board stay charged for a while after power-off - wait for the board's lamps to go out and measure before touching. Refrigerant and high-voltage work belongs to a certified technician.