My AC Is Not Cooling: What to Check Before Calling a Technician
It's the most common breakdown during a heatwave: the air conditioner is running, but the air is no longer cold. Before panicking — and especially before calling out a technician — several simple checks can identify the problem. Some can be fixed in 5 minutes without any tools. Others require professional intervention, but knowing how to identify them will save you time.
Simple checks you can do yourself
Start by ruling out the most common causes. In the majority of cases, the issue is minor.
1. Check the filters
This is the number one cause of performance loss. A clogged filter blocks airflow: the unit runs, but air no longer circulates properly through the heat exchanger. Result: it blows lukewarm air.
How to do it: open the front panel of the indoor unit, remove the filters (they snap or slide out). If they are covered in dust or lint, clean them under warm water with a little mild soap, let them dry completely, and put them back in place.
Recommended frequency: every 2 to 4 weeks during periods of intensive use.
2. Check the settings
It may seem obvious, but it happens frequently:
– Is the mode set to cooling (snowflake icon) and not fan-only (fan icon) or heating (sun icon)?
– Is the remote control displaying the correct setpoint? (weak batteries = erratic display)
– Is the setpoint higher than the room temperature? (if the unit is set to 28°C and the room is at 27°C, it won't kick in)
3. Check the circuit breaker
If the outdoor unit isn't running at all (no compressor noise), the dedicated circuit breaker may have tripped. Check the electrical panel. Reset it and observe: if it trips again immediately, don't force it — it's a short circuit that requires a technician.
4. Check the outdoor unit
Go outside and inspect the outdoor unit:
– Is the fan spinning? If not, it's likely an electrical or mechanical failure
– Is the unit obstructed? Leaves, plastic bags, or other objects in front of the fins prevent heat dissipation. Clear any obstacles to at least 30 cm
– Is the noise normal? Rattling or metallic grinding sounds indicate a mechanical problem (fan, compressor)
Common faults that require a professional
If the simple checks haven't revealed anything, the problem is likely technical.
Low refrigerant level
This is the most common cause of gradual performance loss. Refrigerant is the "lifeblood" of the circuit: it carries heat from inside to outside. If the circuit has a leak, even a small one, the level drops and cooling capacity decreases.
Signs:
– The AC blows lukewarm or slightly cool air instead of cold
– Frost appears on the connection pipes (copper) or on the indoor heat exchanger — a paradoxical sign: low refrigerant causes a local pressure drop that freezes the moisture in the air
– Performance degrades progressively over several days or weeks
Faulty temperature sensor
The indoor unit uses a sensor (thermistor) to measure room temperature and control the compressor. If the sensor is displaced, disconnected or faulty, the unit "thinks" the room is at the right temperature and doesn't trigger cooling.
Sign: the unit starts then stops very quickly, or simply doesn't start the compressor at all.
Start capacitor
The compressor start capacitor (in the outdoor unit) is a wear part. When it weakens, the compressor can no longer start — the outdoor unit "tries" to launch (clicking sound) but fails.
Sign: repeated clicking from the outdoor unit, followed by shutdown. The compressor never runs.
Compressor failure
The compressor is the most expensive component. A compressor failure results in a total loss of cooling, with the outdoor unit silent or producing only a fan noise (without the characteristic compressor hum).
On a unit less than 5-6 years old, the compressor is often covered by the manufacturer's warranty. Beyond that, the cost of replacement (part + labour + refrigerant) may justify replacing the entire unit.
Refrigerant leak: the signs to look for
A refrigerant leak deserves special attention because it cannot be resolved by a simple "top-up." Recharging without repairing the leak is like filling a bucket with a hole in it.
How to spot a leak:
– Greasy oil traces on pipe fittings (the refrigerant circulates with lubricating oil)
– Frost localised at a specific point on the circuit
– Gradual performance loss (not sudden)
– Bubbling or gurgling noise in the indoor unit (the refrigerant is no longer circulating properly)
Important: refrigerants are regulated greenhouse gases. Their handling (recharging, recovery) is strictly reserved for professionals holding the required certification. Never attempt to recharge yourself.
When to call a certified technician
Call a professional if:
– The filters are clean, settings are correct, the circuit breaker is in place — and the air remains lukewarm
– Frost appears on the pipes or heat exchanger
– The outdoor unit won't start or produces repeated clicking
– You notice abnormal water leakage (beyond normal condensation)
– The unit emits an unusual smell (burning, chemical)
A qualified technician will perform:
1. A refrigerant pressure check
2. A circuit leak test
3. An electrical diagnostic (capacitor, sensor, circuit board)
4. A professional heat exchanger cleaning if needed
Preventing breakdowns: regular maintenance
The majority of mid-summer breakdowns could have been prevented with spring maintenance:
– Filter cleaning every 2-4 weeks (by the user)
– Annual professional maintenance: heat exchanger cleaning, electrical check, refrigerant circuit inspection
– Seasonal start-up: run the unit for 30 minutes in spring to check it's working before the first heat arrives — the worst time to discover a fault is the first day of a heatwave
Réseau Tubulure provides repair and air conditioning maintenance in Paris, 7 days a week. If your air conditioner is no longer cooling, contact us for a quick diagnosis.
Frequently asked questions
Why is my AC blowing warm air?
The most common causes, in order of likelihood: clogged filters (cleanable in 5 minutes), wrong settings (fan mode instead of cooling), low refrigerant (leak that needs professional repair). Less commonly: faulty sensor, worn start capacitor, or compressor failure.
Can I recharge the refrigerant myself?
No, for two reasons. First, handling refrigerants is legally reserved for certified professionals (mandatory certification). Second, a recharge without a leak detection is pointless — the refrigerant will escape through the same leak point. A qualified technician locates the leak, repairs it, then recharges the circuit to the exact amount specified by the manufacturer.
How much does an air conditioning repair cost?
Prices vary depending on the complexity of the intervention. Check our pricing page for detailed estimates tailored to your situation.
Contact Réseau Tubulure
Need a plumber or air conditioning specialist in Paris? Réseau Tubulure responds quickly throughout Paris and Île-de-France.
