Ask a mechanic: When your air conditioning leaves you heated

Dear Ask a Mechanic,

The air conditioning in my 2016 GMC Sierra was blowing warm air and my mechanic replaced the condenser. The air worked fine for three months and then it started blowing warm air again. He said this time it was the compressor, and that more than one component can cause problems with the air conditioning. Is that true? If it is, why didn’t he find the problem the first time? – Hot and bothered

First, a little background. While there have been plenty of changes along the way, the basic principles behind automotive air conditioning are the same as when the feature was first introduced as an option in some 1940 Packards.

All automotive air conditioning systems use refrigerant to effectively transfer heat from the passenger compartment to the outside. Your fridge works the same way. Refrigerants have changed over the years, as we’ve come to recognize their negative environmental effects. Ozone damaging R-12 (commonly known as freon) was replaced by R-134a in the early 1990s, and since the mid-2010s, most automakers have switched to R-1234yf.

Regardless of which is used, during operation considerable pressures are generated. On a hot day, this can be upwards of 350 pounds per square inch.

Unlike your fridge, the numerous components that make up your vehicle’s air conditioning system are exposed to substantial temperature changes, continual vibration, damage from road debris and bugs, and corrosion from salt and moisture.

It also has multiple connection points, and each is a potential source of leaks. Newer systems typically use smaller quantities of refrigerant, so even a minor leak can influence the performance of your air conditioning. There are also mechanical and electrical parts that must function properly to ensure cooling.

In your Sierra’s case, you didn’t mention whether the compressor needs to be replaced because it’s leaking or because of a mechanical or electrical issue. It clearly worked after the initial repair. Assuming there was a leak in the condenser – a common issue in those pickups – that would have made it difficult to detect a much smaller leak elsewhere in the system. With the condenser working properly again, it may have aggravated other leaks in the system, or just made them more evident.

Alternately, since the refrigerant circulates a small amount of oil to lubricate the compressor, operating it while the refrigerant was low due to a leak might have damaged it mechanically. It could even have suffered, if the condenser leak was left unrepaired long enough, from internal corrosion from moisture.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.