Empty the Water Tank
Condenser dryers do not run when the water tank is full to prevent damaging the electrics. If the water light is on, the most likely cause is a full water tank. Pull the tank out of the washer – the location varies by model – and empty it down the sink. The light should go off when you slide the tank back into position.
Clean the Condensor
Dirt in the condenser coils could be the cause of a false "tank full" warning. Open the hatch on the front of the dryer – it's often near the floor – and pull out the condenser coils. Brush off any dust or debris accumulated around the coils and in the condenser's slot. Slide the coils slide back into position. The dryer should begin to work when the cover is closed if this was the problem.
Check the Float
A special floating sensor tells the dryer's computer when the tank is full of water. If the float is wedged in the "up" position, the computer will receive an erroneous "tank full" message. The float is usually located behind the rear panel near the pump. Unplug the appliance, remove the rear panel screws and disconnect the pipes leading the pump to access it. Clean away any dirt from around the float and make sure it is able to move up and down. Reassemble the dryer before reconnecting the power and running a test load.
Check the Piping
A blocked pipe leading to or from the pump could be the cause of the errant warning light. Take the back off the appliance, as outlined above, and disconnect the rubber pipes leading to the pump. If the dryer is kept in an unheated garage, the lines might have frozen. Lint could also be the cause of the blockage. Use pipe cleaners to flush out any obstructions and reconnect the pump and the dryer.