Have a LG DLEX5170W electric dryer and it is not drying. On the display panel the "dry" lights up and at the same time the "cooling" lights up. This has never happened before. Help.

Usually the reason is the exhaust vent in the wall leading to the outside of the house is heavily restricted with lint or other debris. Pull the exhaust vent out of the wall and run the dryer to see if the load dries normally. If it does, you will need someone to clear out the exhaust vent to resolve the drying issue.
The cooling light is a display that tells you what will be happening during the cycle. When it flashes, the dryer is in the cooling mode and is continuing its operation but with no heat. That usually lasts about 10 to 15 minutes.
Good lucj

What happens on timed dry?

You may need to adjust the air flow on your dryer vent.
When an LG dryer is functioning normally---the time required to dry laundry is determined by the *Sensor* located on the drum side of the lint filter housing and by the main control board.
When laundry is near dry--any Time Remaining on the control panel that is *not* needed will be eliminated---except for the last 6 minutes.
The final 6 minutes of any LG automatic cycle is for cooling down the laundry before the dryer ends the cycle.
If a dryer *counts down* to the last 6 minutes but the laundry is still not dry--the computer will allow the dryer to run until the laundry has dried or defaults (not drying within the additional time allowed).
Dryer vents that are longer than 8 feet with more than (2) 90 degree turns--can affect drying performance.
Setting the DRY LEVEL selector to *Very Dry* will solve most dryability issues. In homes with abnormally long vents--even the longer run time in the *Very Dry* setting is usually not sufficient to dry laundry loads.
Reducing the length of the vent or starting a second cycle becomes necessary to fully dry the clothing.
If a second cycle was started---only the amount of time that is needed to dry laundry will be used before dropping to 6 minutes and then the cycle will end with dry laundry.
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LG gas dryers are more sensitive to venting length and air flow resistance. The gas burner can cause a rapid increase in temperature compared to an electric burner.
Rapid temperature rise in a gas dryer leads to premature burner shutdown (longer time required to dry laundry).
The gradual temperature increase of an electric element is more "forgiving" in homes where the dryer venting is less than ideal.
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Problem:
The dryer has an auto dry feature which never fully dries the clothes so we wind up manually setting the dry time to an hour
Solution:
LG gas dryers are VERY temperamental in long vent set-ups. If fabric softener sheets are used in the dryer--clean the two stainless steel strips of metal on the lint filter housing with a SCOTCHBRITE PAD--scrub back & forth several times to remove the waxy residue from the fabric softener sheets. Most,if not all manufacturers do *not* recommend using sheets in dryers that have a moisture sensor (electrodes).
However--using a SCOTCHBRITE pad keeps the wax off those sensors which can be seen if looking *into* the dryer drum and at the lint filter housing.
If only *liquid* fabric softener is used in the washer--clean the dryer sensors once a month. Liquid softener also contains wax which can slowly accumulate on the dryer sensors from the laundry washed in the washer.

LG DRYER SERVICE MANUAL

LG PARTS


If you need further help, reach me via phone at https://www.6ya.com/expert/dan_73bbd84fe1d95b61

Inspect the dryer venting from the rear of the dryer to the outside vent hood of the home.
If there's a restriction in the vent--the clothing in the dryer takes longer to dry.
When an LG dryer is functioning normally---the time required to dry laundry is determined by the *Sensor* located on the drum side of the lint filter housing and by the main control board.
When laundry is near dry--any Time Remaining on the control panel that is *not* needed will be eliminated---except for the last 6 minutes.
The final 6 minutes of any LG automatic cycle is for cooling down the laundry before the dryer ends the cycle.
If a dryer *counts down* to the last 6 minutes but the laundry is still not dry--the computer will allow the dryer to run until the laundry has dried or defaults (not drying within the additional time allowed).
Dryer vents that are longer than 8 feet with more than (2) 90 degree turns--can affect drying performance.
Setting the DRY LEVEL selector to *Very Dry* will solve most dryability issues. In homes with abnormally long vents--even the longer run time in the *Very Dry* setting is usually not sufficient to dry laundry loads.
Reducing the length of the vent or starting a second cycle becomes necessary to fully dry the clothing.
If a second cycle was started---only the amount of time that is needed to dry laundry will be used before dropping to 6 minutes and then the cycle will end with dry laundry.


LG gas dryers are more sensitive to venting length and air flow resistance. The gas burner can cause a rapid increase in temperature compared to an electric burner.
Rapid temperature rise in a gas dryer leads to premature burner shutdown (longer time required to dry laundry).
The gradual temperature increase of an electric element is more "forgiving" in homes where the dryer venting is less than ideal.

If you need further help, reach me via phone at https://www.6ya.com/expert/dan_73bbd84fe1d95b61

The cooling light stays on so that is not the issue. Check to make sure you have power in both hot legs of your 220v power outlet.

In normal operation as the dryer is drying the "drying" light flashes and the "cooling" light is on steady until the 5 minute cooldown at which time the "drying" light stays on steady and the "cooling" light flashes. click on this lg link
http://lgknowledgebase.com/kb/index.php?View=entry&EntryID=6253

AD is auto dry you may need to clean the sensor or you may have a fauilty one ...Auto Dry Loads dried using the Towels, Heavy Duty, Normal, Perm Press, and Delicates cycles will dry automatically at the selected temperature to the desired dryness level. The dryer uses sensing bars to sense the moisture level of the load as it tumbles through heated air. Auto Dry cycles save time and energy and protect fabrics. The estimated drying time will be displayed for about 5 minutes, then the outside of the display will light up in a circular track to indicate the drying time is automatically being determined by the sensors. When the load has reached the selected dryness level, it will continue to tumble, unheated, during a Cool Down period. This helps reduce wrinkling and makes items easier to handle during unloading. The estimated Cool Down time will be displayed. Drying time varies depending on size and dampness of load and fabric type. If the load is too small or almost dry, the sensing bars may not detect enough moisture to continue the cycle and the cycle may end after a few minutes. Select a Timed Dry setting to dry such loads. Room temperature and humidity, type of installation and electrical voltage or gas pressure can also affect drying time. For cycle selection turn the knob on the front of the dryer control panel. The light will illuminate next to the cycle selected. The following is a brief description for what cycles should be chosen for certain items and materials to be dried.

try cleaning real good. also the heating unit might be bad

The vent is clogged.  To make sure, disconnect the vent from the dryer and run the dryer normally.  If your clothes dry normally, then have a professional vent cleaning company take care of the vent.  
The cooling light will always stay on during the run.  When it is blinking, the heat has stopped and is in the cooling process.
If there is no heating, you have a problem with the high limit switch or something else electrical and needs to be addressed by a qualified LG technician.