<<<LG dryer DLEX7177RM, customer complains that for about three months, the unit has been leaven clothes damp.>>>
<<<Even in the manual settings.>>>
A strong indicator of a partially blocked vent system (in the home).
If the venting goes into the wall---and is not accessible---it's best to recommend that the customer contact a professional vent cleaning company.
I use a Whirlpool dryer vent tester---very effective tool.
Another possibility---though slim---is that the Heater Housing (box) may have been knocked out of position.
This would cause poor drying results in both Sensor Dry and Time Dry cycles.
Verify that the Heater Housing *round end* is fully inserted into the rear duct.
Moisture Sensors (2 metal strips on the lint filter housing) do not wear-out and therefore are not likely at fault---unless damaged/pulled out/snagged by something in the laundry during a cycle.
I've yet to find/encounter a failed Heating Element---so that is a low possibility.
Nevertheless---it wouldn't hurt to verify that both elements are "good" with a simple continuity test.
I've also never had a single Thermistor failure (on Blower Housing)---check the ohms anyway---at room temperature/77F----should be at or very near 8.63K ohms.
Confirm that the Door Gasket is not missing/damaged/torn (low problability).
If the vent system was found to be undeniably 100% clear/okay---I'd replace the THERMAL LIMITER and THERMOSTAT on the Heater Housing. There was an old Service Bulletin on this once----a Serial Number would help.
Let's assume that the customer made a slight error---and the dryer *actually does* dry well in the Time Dry Cycle (I've seen this more than few times)---clean the surface of the Moisture Sensor Bars on the lint filter housing with a ScotchBrite Pad.
If the customer uses Fabric Softener Sheets---advise the customer to---clean the Moisture Sensors after *each* load in which a Fab Soft Sheet was used in the dryer.
This removes waxy build-up from the surface of the sensors (wax build-up causes premature ending of the cycle by about 10 to 15 minutes).
The Moisture Sensor is *not* used during Time Dry mode.
Verify correct voltage at the wall receptacle. Long shot,I know---but let's rule-out something odd---like finding that the actual voltage is 208/209 instead of the required 220 volts
Good Luck ! Let us know what you find