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Q: Is there any simple way to diagnose why my AC won't blow cold?

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It worked fine last summer. This summer I turned it on and it only blows hot air... so we got a can of refrigerant and was going to charge it but the Guage reads full. I don't hear the compressor coming on but when I had to have a new computer out in it the guys at the shop told me the ac compressor was good. Any ideas?

My car has 161165 miles.
My car has an automatic transmission.

A: Hello. I will help you with this issue. If ...

Hello. I will help you with this issue. If the a/c compressor is NOT running (and thus NOT creating suction in the refrigerant piping on the low side) and you apply a gauge to measure system pressure on the low side, the gauge will give a relatively high pressure reading (the reading will indeed appear as "full" in the DIY refill systems) even if the system is nearly empty. What you have to do is put a manifold gauge set on both the low and high sides of the A/C system.

If the compressor is not running, briefly force it to run by energizing the clutch. As the compressor kicks in, quickly read the pressures (don't run it long because if system is low on refrigerant, that means the oil in the system is not circulating either and you will ruin the compressor by running it without adequate oil circulating). If the system is low in refrigerant, the low side might read as low as a vacuum on the gauge, and the high side pressure will be way too low (out of spec.), like maybe 80 or 100 PSI when it should be 180 to 250 PSI, depending on ambient temperature.

The best thing to do is have a Technician perform these diagnostic tests (refrigerant is dangerous; you will be blinded if liquid refrigerant sprays in your eye and if enough liquid refrigerant happens to spray on your skin you can be permanently injured). YourMechanic, being mobile, can come right to your door and take care of it. If the system is low, you might have a leak.

I say "might" because automotive systems are not like the nearly (or actually) hermetically sealed refrigeration systems that you are used to at home. Those systems are NOT supposed to leak at all during normal operation. Auto systems though generally "seep", due to the type of sealing technology (o-rings, etc.) that is used and the harsh operating environment and so although the amount that seeps is very small, over 12 years you WILL lose a certain amount of refrigerant.

If you are lucky, the reason it is low now (if it is low) is due to the normal loss that occurs over many years. If you are not lucky, you have a leak, the magnitude of which can vary quite a bit depending on where it is. In any event, anything is repairable and YourMechanic can both diagnose the current failure, identify any leaks that are significant and ultimately do whatever is necessary to get your A/C system functioning properly. Finally, your car uses a refrigerant that is designated as R134a.

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