Some kind of pressure release and fog came from my 2008 Honda Civic. What could it be?

So my car wouldn't start this morning, mot even the power locks. AAA came out and said it looks to only be a dead battery but, with the car still running, there was a loud sound like a pressure being released and a fog came out from under the car. My wife said it smelled like freon from the air conditioner. What could that be?

I have no idea what it was. Freon is pretty much odorless, I don't know what she is basing that on. Does the A/C work?

The only item under the hood under pressure if you don't have a NOS system it would be the refrigerant from the Accumulator on the Air Conditioning system, It is possible that the Condenser was punctured it looks like your radiator, or you had a line defect, puncture. If Your A/c is running you should feel it kick on and the engine will either rev or loose a tad bit of power when doing thirty MPH is a good speed to test that theory. HVAC MA

Freon has no odor. Sounds like the AC compressor is locked up or possibly the water pump bearing is locked up or the alternator is locked up. Any one of those and the belt will start to drag over the pulley and smoke and stink.

CactiJoe is on the right track. The A/C compressor (or less likely the water pump) seized and burned off the S-belt releasing smoke and a burnt rubber smell. Subsequently the alternator was not turning and therefore not charging the battery.

Don't drive it. Batteries do not just go dead all of a sudden by themselves. More likely is you broke the water pump belt and over heated it while discharging the battery. The smoke was likely a blown head gasket. But if you can't locate the source of the fog, your only alternative is to have it towed to a shop. About all you can do is check for oil in the water, low water, or water in the oil. A cooling system pressure test requires the pump and pressure gauge. Freon is odorless, so that is not it.

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A coolant leak, most not all AAA drivers are WRONG. Open the hood look for green liquid (notfreon) also look for a broken fan belt.

There are a several possibilities in this. The most likely of them are: a blown radiator hose, a blown/leaking radiator, and a blown heater coolant line. Very remotely possible and similar to these is a blown/leaking heater core in which case it is extremely likely that vapor/mist would be inside the car from defroster vents or heater vents.

Having said all that, as someone mentioned previously, freon refrigerant has no odor. Some people can detect a very very slight odor of it but at best it is almost not noticeable at all.

The only time that freon leaking will produce visible vapor is if the leak is huge such as if a hose or connection completely fails allowing the system to vent over just a few seconds. In this case the vapor will be white or slightly possible barely tinged with a color: usually red, possibly yellow or very remotely some other color.

These color tinges are probably not normally noticeable but they would be from various dyes sometimes put into AC systems to help determine the source of small leaks.

As mentioned in previous answers a locked up alternator or water pump could be the source of a burning smell.