Clean filter light stays on LG TD-C70045E Dryer

Check all the thermostats for continuity.

Try unscrewing the water hoses and cleaning or replacing the filter screens where they screw into the unit and also the valves.

do you mean a condenser dryer?, as steam is water doesn't dry anything to well, if it is a condenser dryer the check filter or clean filter light should always come on towards the end of the cycle (varies between models some have both clean filter and empty tank lights on at end of cycle just as a reminder to do so and some don't have any lights), it is simply a reminder for you to clean the filter after each use unless this is something that has only just recently happened

remover the filter and run it see if that helps or remove the vent from the back

Clean the vent all the way to the outside.
The dryer will take 4-5 loads to reset the filter sensor.

Mahe sure the vent is not smashed behind the dryer.

On the LG TD-C70045E the reset button for the heater is located inside the back air-way cover. This is easy enough to remove and the button is obvious once the cover is off. The button will 'click' satisfyingly if it has tripped. Don't forget to switch off and unlug first...

Fluff has blocked the drain hole into the water drawer.Try locating the hole and using a small bore plastic tube **** the debris out of the container.

is the dryer getting hot?
if not turn the power off and pull the dryer out and take the back off and push the red button in at the back

There is an access flap below the door on the bottom left.  Open the flap turn blue levers, take plastic cover off (there may be a bit of water there).  Clean the inside.  Problem solved

if not turn the power off and pull the dryer out and take the back off and push the red button in at the back