I woke one morning to find the suction line on my York YCJF48S41S2A (with blower assembly AHR48D3XH21A) frozen and realized I hadn't cleaned my coils in two years. I cleaned both the evaporator coil and condenser coil, put in a new contactor, new run capacitors on the blower and compressor, cleaned the blower motor, checked the charge (using the subcooling since I have a TXV valve) and measured 11 degrees difference between the temperature of the liquid line and that of the saturated temperature of the outdoor coil. Lastly I checked and replaced the filter (which I do monthly) and it was clean.

The blower fan blades were quite dirty. They appeared to have some sort of rust build up on the blades. I cleaned each blade with a toothbrush and coil cleaner. After completing all of the above and the unit ran with no problems for over a month until the outdoor temperature dropped below 60° one night. I woke the next morning to the outdoor unit running, the indoor blower not running, the suction line frozen outside and inside, but the evaporator coil not frozen at all.

I'm at a loss. Anyone got any ideas that can point me in the right direction?

  • Great question but I don't really have an answer. If I were checking this unit I would expect the relay that turns on the inside air handler (blower) has failed or the one for the outside unit (condenser) has welded contacts. You can try placing a wooden dowel on each of the relays and lightly tapping the other end of the dowel and see if it cycles off. If so that would be the bad relay. From the wiring diagrams and not knowing your system that should be the problem. Another thought be sure the fan switch is off, if that solves the problem it is miswired.
    – Gil
    Commented Oct 28, 2022 at 19:13
  • 3
    I know this sounds opposite, but a low coolant will produce frozen lines.
    – Traveler
    Commented Oct 28, 2022 at 19:42
  • Welcome to the site Thomas. Please register and merge your accounts so you have access to edit, comment, and accept answers on your own question. Note that posting responses as answers can result in an automated lock that can prevent others from assisting you.
    – BMitch
    Commented Oct 29, 2022 at 13:22

2 Answers 2


Bottom line: The system is probably undercharged, but if properly charged it needs to have Low Ambient Pressure switch kit S1-2LA06700424 installed.

Since the suction line can only freeze moisture in the air one the outside of the copper tubing when the refrigerant gas inside the tubing is below 32°F, the next question is whether the evaporator and suction line temperatures are the same when running. Ideally they are, practically there’s always a small pressure drop along the tube and a small temperature difference.

A very low suction line temperature can be caused by an obstruction in the tubing causing a large pressure drop and a large temperature drop. A suction line filter drier installed after a burn out that should have been removed after a few days is one example. I’ll assume that’s not the case.

It is not inconsistent for a system with a functioning TXV to have a frozen evaporator or suction line, it’s just undesirable in an air conditioner.

A TXV restricts refrigerant delivery to achieve a (reasonably) fixed evaporator superheat, not a fixed temperature difference between the liquid line and the suction line. A TXV has a port that samples the suction line pressure and a temperature sensing bulb and capillary that communicate the suction line temperature to the valve as a pressure. The difference between the suction line pressure and the liquid refrigerant pressure at the same temperature plus a small amount spring pressure opens the TXV valve, if there’s too little difference it closes.

Superheat is the temperature difference between the suction line and the condensing temperature of the refrigerant at the pressure of the suction line at that moment. At zero superheat the entire evaporator is boiling liquid into gas maximizing the evaporator surface area in use. A small amount of superheat ensures that liquid is not returned to the compressor inlet.

Normally an AC is charged so that the suction line pressure is high enough to keep the line above freezing in typical operating conditions. If the system needs to run when the outdoor ambient temperature is low, then it has to have some specific adaptation to prevent freezing. If the AC system is properly charged and the suction line is freezing the system needs a low ambient adaptation. This is usually something that reduces the airflow across the condenser to raise the liquid line temperature. Some large systems also use a thermally controlled valve to allow some refrigerant to bypass the condenser to achieve the same result.

The York YCJF48S41S2A is rated for cooling operation with the outdoor ambient at or above 50°F. At 60°F your system should not require the Low Ambient Pressure switch kit S1-2LA06700424. If your estimate of the nighttime low temperature was from a weather service instead a thermometer in your yard, then it helps to know that on clear still nights the low temperature varies locally and the weather report isn’t a reliable guide.


I'm at a loss. Anyone got any ideas that can point me in the right direction?

youtube !

If youtube is taboo, check out https://culpeperhomeservices.com/what-causes-the-suction-line-to-freeze-up/

a youtube search of proper syntax will provide the best answer with video and explanation by actual AC service tech type people! You simply cannot get that here. That will give you the best education on the topic given your scenario to where you can make a judgement to know if step 1 of cleaning coils and checking filter restriction was the cause or if you need a tech with refrigerant and gauges to work on the system. A system slightly low on refrigerant can cause that condition, as well as operating the system in too cold (you said 60°F) an environment (generally rule of thumb is not less than 55°, and 60° is close, there's a sensor to prevent compressor operation in a outside too cold situation). The evap coil not being frozen at all is not conclusive evidence of anything. If the Tx valve is stuck open and not functioning properly, in addition to low outside temps, I think may result in an icing up suction line. There's a systematic process to determining the cause, which other than going to hvac college is best learned by watching educational vids on youtube on the subject. What stands out most is you saying running AC at 60.0°F outside... for starters don't do that.

  • Most of your answer, with some formatting to make it readable, seems like it would be really useful. Just telling someone to search YouTube isn't useful. Linking to one or two specific videos (with a summary here in case the videos disappear) is absolutely useful. Telling someone to "search YouTube" is as useful as telling someone to "search the internet" without providing any additional context. The presumption is that the OP has done some sort of searching prior to asking here. (Of course, not everyone has...)
    – FreeMan
    Commented Feb 13, 2023 at 16:46
  • Helping out by, at a minimum, providing the "proper syntax" for said search would be helpful. Otherwise it's like someone has asked you for directions on how to get to Carnage Hall, and you've said "walk". Not much help.
    – FreeMan
    Commented Feb 13, 2023 at 16:48

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