Why does my car A/C stop cooling after a while?

Why it works at first, then stops

The A/C compressor is driven by the engine through an electromagnetic clutch. Sensors switch it off when pressure is too low or too high, or when the evaporator (the small radiator behind the dashboard) gets cold enough to freeze. A system that cools at first is mostly working, so the fault is usually in one of those protections or in the clutch.

The most common causes

• Low refrigerant from a slow leak: pressure drops as the system runs, and the low-pressure switch cuts the compressor. Leaks are common at the condenser (behind the grille, hit by stones) and at hose joints.
• Compressor clutch problems: as the clutch coil heats up, a worn coil weakens, or a worn clutch develops too wide an air gap, and the clutch stops engaging. Often it works again once things cool down.
• A frozen evaporator: ice blocks airflow, so the air coming out goes from cold to weak. Usually a faulty temperature sensor or too much moisture in the system. A blocked cabin air filter makes it worse.
• Condenser or radiator fan not running: in traffic, pressure climbs too high and the system shuts off. It cools again on the highway.
• Overcharged system: too much refrigerant from a DIY top-up can make it shut off on high pressure.

Quick checks you can do

• With the engine running and the A/C on, look at the front of the compressor: the centre of the pulley should spin. Note whether it stops when the air turns warm.
• Check that the radiator or condenser fan runs when the A/C is on.
• Does it cool again after you switch it off for ten minutes? That points to icing or a hot clutch.
• Replace the cabin air filter if it hasn’t been done in a year or two. It’s cheap and often a DIY job.
• Avoid DIY refrigerant cans with sealant: they can clog the system and contaminate a shop’s recovery equipment.

What the repair usually involves

A shop connects gauges or a scan tool to read pressures while it runs, then leak-tests with dye or a detector. Refrigerant handling requires certification in Canada and the US. The fix might be a new O-ring, a condenser, a clutch air-gap adjustment or clutch replacement (on some cars the clutch is sold only with the whole compressor), or a sensor.

Recharging without finding the leak usually only buys a few weeks.

What to tell your mechanic

• How long it cools before it turns warm, and whether it recovers after a rest.
• Whether it’s worse in traffic or on the highway.
• Whether you saw the compressor clutch stop spinning.
• Any recent refrigerant top-up, front-end damage or radiator work.