A/C doesnot work when its hot outside

2006 Honda Civic, 150k miles

A/C doesnot work when its hot outside. When the A/C does work it cools well.

When I take the car out of garage the A/C cools well but as I start driving and if the weather outside is hot, the A/C stops cooling after some time. And it will then not cool again till I park the car in a garage and the engine has cooled down.

If the day temperature is generally high and the garage is hot then the A/C will not cool right from the beginning.

Please advise.

You need to go to a good independent AC shop and have the problem isolated. There are several reasons that your AC will not cool on a hot day. You will save money by having the problem properly isolated and repaired.

More than likely it’s low on refrigerant.

+1 to missileman’s post. Trying to fix this yourself will only lead to frustration. An AC shop will have the expertise and equipment to do the job properly… and the license to work with the refrigerant.


I am 609 licensed, meaning I can legally service automotive AC systems

However . . . most of the guys I’ve worked with over the years were/are not

But it’s a very easy test to pass, so I don’t know what the big deal is

Presumably, there is a general lack of awareness


Db, I’m inclined to think lack of awareness isn’t nearly as common as lack of willingness.

For the OP’s edification, we’re talking about the EPA’s regulations for handling refrigerants containing HFCs.

My comment about licensing was in reference to this, but as db pointed out not everyone out there is actually licensed. However if you take it to a shop, that becomes their problem and not yours.


In all seriousness, many, perhaps most, of my colleagues had no idea you’re supposed to be 609 licensed in order to work on automotive ac systems

The shop owners probably know about the regulations, but I think in some instances they don’t bother to have their guys become certified

I tend to be cynical . . .

With A/C problems what really needs to be known are both the low and high side pressues; at idle and at elevated RPMs.

I could make a wild guess that the evaporator is freezing up and that can go back to pressures.
With the A/C inoperative and after being parked, wait about 15 minutes and look underneath the car on the passenger side to see if there is a considerable puddle of water or condensation drip.
That could point to a frozen evaporator.

Regarding licensing, there was a story in the paper here about 15 years ago about a fleet mechanic in Kansas City who was about to be hammered for 5 years in prison and a 100 grand fine.
Someone ratted him out for illegally servicing the A/C on a company van. I never heard a follow-up story on what finally happened but I’d think 5 years in the pen and a 100 grand is a bit much; especially considering that many murderers don’t even get whacked that hard.

AC systems work – not by cooling something directly – but by transferring heat from one place to another. In the case of a car, the heat is transferred from a heat exchanger in the passenger compartment to a heat exchanger in the engine compartment. And that heat has to be released to the ambient air, otherwise the whole process quickly comes to a halt.

Since heat flows faster if the temperature differences are greater, as the ambient temperature increases, it becomes harder and harder to transfer that heat in the engine compartment heat exchanger fast enough to the outside air. The engine compartment fans are there to facilitate this heat transfer, so part of this diagnosis is for your AC shop to check that those engine compartment fans are working like they should. Sometimes they’ll just stop rotating b/c they are broke, but they can appear to be working but not rotating as fast as they should too.

Db, it isn’t cynical if it’s true.

George, excellent explanation, but allow me to suggest it be reworded to say that the heat in the passenger compartment is transferred BY the heat exchanger to the expanded, cooler refrigerant in the AC system. The refrigerant is then recompressed, and the heat from the passenger cabin along with the heat created by compressing the refrigerant is then transferred by the heat exchanger under the hood to the ambient air. The cooled refrigerant is then expanded to drop its temperature and sent back to the passenger cabin heat exchanger to once again absorb the cabin heat.

Its principle is simple. It relies on the principle that expanded matter absorbs heat (it’s cold) and by removing the heat from the compressed refrigerant (matter heats up when compressed) and then expanding (chilling) it, it will by using a heat exchanger absorb the heat energy from the cabin.

It might help readers to understand that cold does not travel, heat does, “heat” being a measurement of atomic activity. For me that helps to understand the process of heat being transferred from the cabin to the refrigerant and the refrigerant to the outside air. That helps clarify the entire process. Cold isn’t added to the cabin air, heat is removed.