Location of defrost timmer - Frigidaire FRS23W3A Side by Side Refrigerator

DEfrost timer is placed beside compression unit, behind refrigerator.

it will be the defrost timer. If you manually forward the defrost timer it will click into defrost mode. This will melt the ice. The defrost termination switch will stop the heater once all the ice has melted.
If defrost does not happen then it is a faulty defrost termination switch

There is a problem with the defroster circuit. You already mentioned the timer was replaced. You need to consult your manual for a diagram ( or search at applianceparts.com) to see where the defroster thermostat, thermal fuse, and defroster heater itself are located at, or where the wiring is run.

I suspect your thermal fuse / defroster thermostat has failed.

if you are talking about the defrost control, it is located inside the refrigerator probably close to the light at the top behind a plastic panel. There is normally a hole where you can insert a screw driver to advance the timer.

if you are talking about the defrost heater, it is located inside the freezer at the back wall behind a panel and under neath the evaporator coils along with the termination thermostat.

depends on the brand, most likely the defrost thermostat.
heaters rarely go bad, timers usually will not advance causing the coil to frost over.
if you turn the timer to the "defrost" position and wait for 20 minutes. after a couple of minutes it should have the heater get hot and then frost should drip onto the heater and make a "hissing" or "sizzling" sound. if it does make these sound, then replace the timer.

check the defrost timmer if it is working you need a defrost heater you can manually turn the defrost timmer to check the heater the timmer turns off the compressor and on the heater 2 or 3 times a day the timmer is a box with a round knob sticking out of it turn the knob tell it clicks look under the front for it or in the back most refrigerators have a wiring diagram on the back or by the compressor

DO u have frost build up in the freezer ? If so you have a defrost problem.. defrost thermostat, timmer or heaters. You can by-pass the thermostat cut it and tape the wire together and put it into defrost with the timmer by clicking it to the right until it clicks the heater you need to ohm out .. the defrost timmer will be good most of the time if it comes out of defrost ... ref. will start running again when u test the defrost thermostat here is a diagram that may help
http://www.searspartsdirect.com/partsdirect/retrieveSubComponentPartsAction.action?diagramPageId=00011&componentDescription=AIR%20FLOW%20AND%20CONTROL%20PARTS&documentId=00016472&modelNumber=1068562782&productCategoryId=0160000&brandId=0582&modelName=REFRIGERATOR&backToLink=Return%20to%20Sub%20Components

Defrost heater or defrost timmer not working.

The timer has a rotation 24/7 every 6 to 8 hrs. depending on timmer set up it goes into defrost... there is No way to adjust when it will defrost.

No model # .. Im assumeing its frost free.. so that means a defost problem.. heaters, defrost themostate or timmer.