All air vents block after an hour or so in summer and winter

It sounds like the evaporator is freezing up. The clue is this takes about an hour. The second clue is your dandy observation of the air temperature of 45 degrees. That is way too low.

It is a wonderful coincidence that pressure follows temperature almost exactly with refrigerant R-12, and it is close to the same for other refrigerants. All AC systems regulate the pressure in the low side of the system to around 40 pounds which closely equates to 40 degrees. The main function of AC systems is to remove the high humidity in the air. That is where the comfort comes from. Cooling the air is secondary. That humidity condenses on the evaporator, then drips onto the ground just behind the right front tire. I suspect you are not going to see that water puddling on the ground. When the system doesn't regulate properly, the evaporator gets too cold, then the condensed water freezes to it, then blocks air flow. After being turned off for an hour or two, that ice melts, then you get air flow again. To verify this is what's happening, observe if you start seeing water dripping onto the ground more than an hour after you stop the engine.

With 80 degree air blowing through a 40 degree evaporator, you should be real happy to see 60 degree air from the vents. That is actually chilly and too cold for comfort. Have your mechanic look for things that cause too much refrigerant to be flowing. The typical things to look for include a sensing bulb filled with gas, and attached to the evaporator, but it has worked loose or is no longer in contact with it. As the evaporator cools, that gas is supposed to cool and contract. That gas is connected through a tube to the flow control valve. When the evaporator warms up, that gas expands and pushes on the valve to open it an let more refrigerant flow in.

Your system uses a temperature sensor on the evaporator in place of the gas-filled bulb. This is an electronically-controlled system instead of mechanically-controlled, so there's a lot more things that can go wrong. The expansion valve could be stuck open. That sensor could have fallen out of contact with the evaporator. The controller could be keeping the compressor running constantly.

Observe if the compressor is cycling on and off. There are some systems that use a variable-displacement compressor that never cycles off. That eliminates the thump each time they cycle on. Your system uses a standard compressor that should be cycling off periodically.

Another way to verify this is the cause of the problem is to switch the AC off and on in ten-minute intervals while you're driving on a long trip. While it's off, there will be time for the ice to melt off the evaporator. This can happen in winter too because all AC systems run, at least in "defrost" mode, to remove the humidity before the air is blown onto the cold windshield where it would condense and cause fogging.

Wow. Fantastic explanation and write up. That makes so much sense! Thank you for taking the time to write this.

You were able to troubleshoot it without even seeing the car! The dealership could only try this while the car was in this state. The first time I brought it in they just did static discharge.then told me to bring it in while it is doing it! This took a year because most long trips I arrived home after business hours.

This time they did find the Freon level low and recharged it (They never check this last summer, grrr). And the evaporator was frozen. No leaks when they did UV oil check.

I thought the evaporator was frozen because there was no air flow, not vice versa!
So I guess the car came from the plant low on Freon! Unreal. 5 years of frustrations on long trips which should never have happened.

I feel confident that it is fixed thanks to your explanation. The dealership does not realize they fixed it. They said to keep monitoring and keep them posted.

Thanks again.

Sounds like you have a good dealership to work with. Most are, but they don't make the news.

My first reaction was that can't be a low-charge problem, but I had to think longer. With the correct amount of refrigerant, the evaporator in the dash will be close to half-full of liquid and half-full on top with vapor. It's at that point where it has to absorb a real lot of heat to turn the liquid to vapor, and that is where we want it to get cold. When the system is partially-low on charge, the evaporator might only be a quarter-full of refrigerant, so the area it gets real cold will be down lower, but the air will still be getting cold, like normal. My next question would be "where is the temperature-sensing switch located?" If it is up higher where the evaporator is warmer, the sensor might indicate more cooling is needed. That could keep the compressor running longer rather than cycling off at the right time.

As a side note, too much refrigerant is just as bad as too little. The point where it turns from liquid to vapor is where it gets cold, and we want that to be in the dash. With too little refrigerant, it will vaporize and get cold in the hose under the hood leading to the evaporator, and with too much refrigerant, cooling could take place in the hose going back out to the compressor. Cooling under the hood doesn't help with comfort inside the vehicle. The bigger worry is if there's too much liquid in the system, it could slosh into the compressor and destroy it. Compressors can only compress vapor, not liquid, so at a minimum, the compressor would lock up and the belt or clutch would squeal intermittently. In really bad cases it could break a piston in the compressor. I'm only mentioning that because many of us think if some is good, more is better, but that doesn't apply here.

One more thing to look at is if the two plastic caps are on the low and high-side ports. Even some people who do a lot of AC work aren't aware the valves in those ports will leak out the refrigerant over time. The purpose of the valves is only to hold the refrigerant in while the hoses are disconnected and the caps are screwed on. Don't get excited if you see a little foaming, or bubbling of green leak-detector dye at a port. That will stop leaking when the cap is installed. They have a rubber seal in them for that purpose. If a cap was inadvertently left off years ago, that would explain the low refrigerant.

I'll keep my fingers crossed the problem has been solved. Please keep us updated.

Similar problem. 2010 Honda Ridgeline RTL Vents closing off gradually with AC on while fan continues to run. Shutting AC off for a while while driving doesn't seem to correct problem, but stopping for an hour or so seems to. Don't know about heat because I don't use it long enough here in Florida to notice it. Also, the compressor must be one of those that runs constantly as when switched off what little escapes vents is cold. Would the same remedy likely work for me?

Thanks

Unfortunately, the only way to know if the system has a full charge of refrigerant is to look in a sight glass on the receiver / drier under the hood, but those are only found on older Chrysler products. If you see vapor bubbles flowing through the glass, it's low on charge. You can add refrigerant just until those bubbles stop appearing.

Ford used a sight glass too in the 1980s and later, but they do not work. There will still be vapor bubbles in the glass when the system is fully charged.

For all other brands, the only way to know an AC system is fully charged is to recover what is in there now, then pump in the measured amount called for. As I mentioned, that will cause the evaporator in the dash to be filled halfway with liquid and half with vapor. There are people who will tell you they can determine the state of charge by watching the two gauges connected to the low and high-side service ports, but that can't be relied on either. When the system is at rest, the pressure throughout it will be very close to ambient temperature. That's just a lucky coincidence with refrigerant, but it works in our favor.

Suppose it's 80 degrees outside today. You will find the low and high-side pressures in the system will be close to 80 psi, but only when there is some liquid and some vapor in there. If you bleed off some of that pressure, logic would dictate the pressure should drop, and it will, . . . at first. Stop here and think of the pressure cap on the radiator, or the pressure cooker you cook food with. Water boils at 212 degrees, but cooling systems can get much hotter than that. By using a pressure cap, the boiling point of water increases three degrees for every pound of pressure. Most caps hold up to 15 psi on the system. That increases the boiling point by 45 degrees to 257 degrees. This is also why the coolant comes gushing out when the cap is removed while the engine overheating. That coolant might be at 230 degrees, for example, but when the pressure is removed, it instantly vaporizes, expands hundreds of times, and blows out as steam.

Similarly, refrigerant boils at a real low temperature, somewhere around minus 50 to minus 80 degrees, as I recall. It stays in liquid form when it's under pressure. Back to our story, if there's 80 psi in the system, with some liquid and some vapor, if you bleed off some vapor, the pressure drops. Some of the liquid turns to vapor and expands as it does. That drives the pressure right back up to 80 psi. If you bleed off some more vapor, more of the liquid turns to vapor, and the pressure goes right back to 80 psi. That will keep on happening until the point is reached when there's no more liquid left to turn to vapor. It's at that point the pressure will drop when more vapor is released, just like letting air out of a bicycle tire. If you go by the gauge readings, you'll find normal pressures even when most of the refrigerant has leaked out. All that's needed with the system running is to have enough liquid left that there's liquid flowing through the expansion valve. That can leave the evaporator empty of liquid. The "cold" develops back where the liquid is turning to vapor. If that is under the hood, the evaporator won't cool the air blowing through it.

If this doesn't help, consider starting a new question specific to your vehicle. First, unlike on other sites where anyone can chime in to confuse the issue, this became a private conversation between just two or three people. This question has gone "off the list" of questions waiting for a reply, so none of the other experts are going to see your addition. That might not get you the help you need. When you start a new question, you're likely to get help from someone more qualified or familiar with your model.

The second problem has to do with how these get categorized so others, like you, researching a similar problem can search by system, (heating and AC), or by brand and model. Your new question will be found by others when searching for Honda solutions. This can help others in the future.

If there's no need to start a new question, please keep me updated on what you find or what the solution turns out to be.