How do i buy parts for ctb1821ar

There is not a defrost timer on a model PSC23NGN. If your model is the=> PSC23NGN and the refrigerator is not defrosting the problem is most likely the Defrost Heater and Defrost Thermostat that need replaced. The heater is item location 230 and the defrost thermostat is item location 240 in this photo=> HERE
The part numbers for the Defrost Heater are WR51X10055 Defrost Heater Assembly The part number for the Defrost Thermostat WR50X10028 Defrost Thermostat
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Lets see if the defrost timer being turned clockwise slowly will force defrost and cause things to get colder. On page 5 of the (click HERE=>ET4WSKXKQ02 Parts Manual ) you will see location 4? That is the 2188371 Defrost Timer

inside the fresh food com[partment at the top and behind the control panel will be the 2188371 Defrost Timer If forcing defrost solves the cooling problem then just replace the 2188371 Defrost Timer and if not we will need to check the 2176942 Defrost Heater Assembly and the 2192096 Defrost Thermostat Don't buy ba defrost thermostat until we discuss a different way of getting one to work should you need one (last resort because of cost) My focus is defrost timer and defrost heater at this point you will not need all the wires on the defrost thermostat should you need one just splice a simple defrost thermostat into the wiring harness. Below is a short movie on testing the defrost heater or click HERE Talk with me and we will get this refrigerator working, Sea Breeze
Refrigerator Defrost Heater Test
Refrigerator Defrost Thermostat Test

The first thing you want to check is if the fan is running. If not thats your problem, if it is your problem is one or more parts of the defrost system is malfunctioned and in return your problem arises. There are three parts to the system, the defrost timer or in some cases control board, defrost thermostat also known as a bi-metal, and the defrost heater. The defrost system is set up to operate as so. The defrost timer or control board will turn the machine off, in some cases every 8 hours, some 12. During this time period the defrost heaters, located behind the freezer wall and rest underneath the evaporator, yet wires to them run up the side of it, are energized and emit heat that raises upward to melt the ice that has formed on the evaporator coils. The bi-metal or defrost thermostat is at the top of the evaporator. When the heat from the heater starts to reach the bi-metal, it breaks the circuit turning the heater off. The whole purpose of this is because the freezer cool the refridgerator side by drawing air across the coils and blowing to the other side via the evap fan motor. In my opinion, if you change one part of the defrost system change them all because usually when one part fails another will follow, so save yourself the time and the food loss, the cost difference is minute.

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contact the manufacturer

I am pretty sure the model number you are referring to are HSS25IFMCC / HSS25IFMWW This model has no defrost timer like the typical defrost timer. The defrost time is determined by the Motherboard. If you are having defrost problems with your model it is most likely the defrost heater causing the problems. The defrost heater is part=> WR51X10055 Defrost Heater

Most times after manually defrosting the defrost heater on your model can be seen burned out
by just looking at the outer glass of the heater itself. This takes unplugging the refrigerator and removal of the rear panel inside and back wall of the freezer

Most times after manually defrosting the defrost heater on your model can be seen burned out Testing the defrost heater can be found here=> WR51X10055 Defrost Heater Assembly Once inside the freezer it is always a good idea to change the defrost thermostat, Click Here=> WR50X10054 Defrost Heater
I will be here should you have questions, Thanks Sea Breeze
Click Here=> HSS25IFMAWW Freezer Components parts 230 is the Defrost Heater and part 230 is the Defrost Thermostat

Defrost problem. Either the defrost thermostat, defrost heater or defrost control board needs replacement. The defrost heater has to have continuity. The defrost thermostat has to have about 0 Ohms while it's cold. If both test out ok - replace defrost control board.
Part locations: Defrost heater and thermostat: under your frosted cover; control board - left upper corner of fresh food section, under the damper cover, in the middle.
Part numbers:
240554502 - control board
5303918255 - defrost heater kit
Can't find the part number for the defrost thermostat

replace temp controll

RepairClinic.com is a fantastic online site for ordering appliance parts.

you must have a failure of one of your defrost componants. it could be defrost control(timer) or defrost thermostat or heater. if you have a multi meter you can check the heater for an open circuit..if not..this is where you need a technician. the defrost thermostats go out alot, but no sense in buying parts you don't need. its one of the three...

I would look at air flow as the cause. First, look at the inside back wall of your freezer. If you see a buildup of frost, you have a problem in your defrost system (timer, heater or thermostat). Second, listen for proper operation of your evaporator (freezer) fan. I have seen motors turning too slow or blades coming off the motor shaft.
You may also make sure the fresh food door closes properly, and the light goes out inside the fresh food compartment.