The inverter unit runs, but the compressor's current and frequency never rise and the room cools weakly. The board sets the compressor speed from the room temperature, the pipe temperatures and its protection limits, so one wrong reading or one blockage keeps the unit at low speed. The table below lists the causes technicians found for this symptom.
A summary of 72 public questions with this symptom that got a fix in technicians' groups (2021–2026). 67 name one main cause, 21 of them confirmed by the poster as the fix; in 5 the answer only lists possibilities, so they are not in the table.
Causes technicians found, by number of cases
Typical repair cases
Temperature sensor
- LG: The LG air conditioner does not cool deeply. After running for about 2.8-3 hours, it no longer ramps up the fan/compressor speed. – Check the indoor unit temperature sensor by unplugging the probe: if the unit ramps up, the sensor is faulty and must be replaced. Also check the fan, the outdoor unit fan capacitor and the discharge valve to rule out other causes.
- Daikin: A domestic Daikin 2 HP unit runs for about 10 minutes, then the gas pressure rises from 150 to 200 and the unit loses its cooling. The poster asks whether anyone has encountered this condition and whether the cause could… – Check the gas distribution valve and the piping, close the hex valve partially to see whether the unit ramps up. At the same time, check the outdoor unit temperature sensor (clamp sensor) and the indoor control board by swapping them to rule them out, then charge refrigerant to the correct pressure.
Outdoor board (power stage, IPM, inverter)
- Mitsubishi: Mitsubishi air conditioner does not ramp up (no compressor speed increase) and shows no error code; the poster asks for advice on how to fix it. – Check the outdoor unit board (inverter board) and the power module, since a unit that runs but does not ramp up on a 2-way cycle is usually a board fault. Also check the refrigerant (gas), the expansion valve, and clean the indoor unit before reaching a conclusion.
- Aqua: Aqua inverter air conditioner does not ramp up, draws 2.4A and keeps the same frequency no matter what temperature it is switched on at. Need advice on how to fix it. – The fault is in the voltage divider of the temperature sensor on the outdoor unit control board (the yellow component at the outdoor unit). Check the 5.6kΩ resistance of the voltage divider, and replace or repair the board if it is incorrect. After that, the unit will ramp up normally. (confirmed by the poster)
Electronic expansion valve (EEV), reversing valve, valve coil
- Daikin: Daikin air conditioner has refrigerant pressure dropping from 220 psi to 160 psi, cannot ramp up compressor speed, and the compressor trips out when refrigerant is added. – Check the contactor and the expansion valve body (or reversing valve), since low pressure and failure to ramp up are usually caused by these components. Recover the refrigerant, re-measure the pressure, and test the valve coil with a magnet; if the valve body is still faulty, replace the valve body and check the piping…
- The air conditioner makes a 'clunking' noise when running at 1.0 (frequency), does not ramp up in frequency, and does not cut out; the board has been tested and is OK. Asking colleagues for advice on the cause. – The unit stays stuck at a low frequency of 1.0 Hz and does not ramp up in speed because the expansion valve (mechanical valve) is jammed. Check and deal with the stuck valve, such as cleaning it or tapping it gently and then retesting; if that does not work, replace the valve. After that the unit runs stably. (confirmed by the poster)
Low refrigerant, leak
- Daikin: The Daikin air conditioner has leaked refrigerant. After evacuation and recharging, the unit does not ramp up, and the current only reaches about 3.3–4 A before shutting off. – The unit fails to ramp up due to insufficient refrigerant after the leak and evacuation. Evacuate for longer than 1 hour (check moisture level), then charge liquid refrigerant by weight, allowing for line losses. Check discharge pressure; it should reach above approximately 28 kg before the compressor ramps up. If the…
- The air conditioner was installed in a new location and ran for one day before losing cooling. The technician reported low refrigerant, no compressor ramp-up, and that the gas must be replaced. The poster asked for… – Loss of cooling after installation is usually caused by a leak in the refrigerant line or a flare fitting, which leaves the system short of gas. The remedy is to check for leaks, recover the refrigerant, evacuate the system for a sufficient time, then recharge the correct amount of refrigerant according to the unit's…
Blocked pipe, capillary, strainer, moisture
- Daikin: Daikin air conditioner running at 3.5-4A but not cooling: the liquid line is frosting, the suction line is cold, and the air blowing out is 27-28°C even after cleaning and replacing the coil. – Check whether the unit reaches maximum compressor speed and the correct pressure, then check the indoor unit sensor (error U0) and clean both the indoor and outdoor units. Next, recover the old refrigerant, evacuate thoroughly, and recharge refrigerant to the unit's specified charge quantity. If it still does not cool,…
- LG: LG inverter air conditioner has sufficient refrigerant, but it does not ramp up in frequency when running, and the current fluctuates from 3.3A down to 2.7A; the issue has not been resolved yet. – Recover the refrigerant, remove the indoor unit and blow out the blockage with nitrogen or aromatic solvent, then reinstall and evacuate thoroughly. Recharge the exact refrigerant amount using a scale and check the current while the unit runs. If the fault persists, check the fan capacitor, the indoor unit fan and the…
Compressor or fan capacitor
- Daikin: Daikin air conditioner does not cool and the compressor does not ramp up speed; suspected cause is insufficient refrigerant or a faulty capacitor. – The unit does not cool and does not ramp up because of a faulty capacitor. Check and replace the capacitor with the correct type. After replacement, reapply thermal paste to the related components. (confirmed by the poster)
- Daikin: A Daikin air conditioner outdoor unit runs but does not ramp up (increase compressor speed); the poster asks an expert for guidance on how to fix it. – Measure the capacitance of the outdoor unit capacitor with a multimeter; if it is below 20uF, replace the capacitor. Also check the compressor discharge temperature sensor, the contactor, and the expansion valve. If necessary, recover the refrigerant, evacuate the system, and recharge with the correct amount of… (confirmed by the poster)
Overall
- Sensors come first: an indoor coil, outdoor coil or discharge sensor out of range, or clamped loosely, holds the unit at low speed – measure the resistance and check the clamp.
- A stuck electronic expansion valve or a faulty valve coil looks like a low charge – check the valve before adding refrigerant.
- The outdoor board and the capacitors on it (power stage, supply filter) come next – usually after sensors and the charge are ruled out.
Note: this is technicians' field experience, not a manufacturer's document. Normal frequency, current and pressure differ by model and running conditions – 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.