If your dryer stops mid-cycle with damp clothes, or runs long past dry, the moisture sensors are the first thing to check. They are two metal strips inside the drum, and a waxy film from dryer sheets stops them reading correctly. Cleaning them takes ten minutes.
Related Services: Dryer Repair
This guide covers how the sensors work, how to tell a dirty sensor from a failed one, how to clean them, and how to replace them if cleaning does not hold.
What the moisture sensors do
An auto-dry cycle does not run on a timer. Two metal strips inside the drum complete a circuit each time wet laundry touches both at once. Wet fabric conducts; dry fabric does not. The control board counts how often the circuit closes and ends the cycle when the rate drops far enough.
That is why a film on the strips breaks the cycle so completely. The board is not reading moisture, it is reading conductivity. Anything insulating the strips looks exactly like dry laundry.
Dirty sensor or failed sensor?
The symptom tells you which. A cycle that ends far too early with wet clothes is almost always a dirty sensor: the film prevents contact, so the board decides the load is dry within minutes. A cycle that runs indefinitely is more often a failed sensor or a wiring fault, because the board never sees the conductivity drop that ends the cycle.
Dryer sheets are the usual cause of the film. The softener transfers to the strips along with the fabric, and it builds up over months. If you use sheets regularly, clean the strips twice a year and you will rarely see the fault.
Cleaning the sensors
Step 1: Unplug and locate
Unplug the dryer. Look just inside the drum opening, usually below or beside the lint filter housing. You are looking for two parallel metal bars, each roughly the length of a finger, set into the plastic. On most Whirlpool, Maytag and GE dryers they sit low on the front bulkhead.
Step 2: Clean the strips
Rubbing alcohol on a cloth, three or four passes, until the metal is bright rather than dull. If the film has hardened, fine sandpaper or a brass brush will take it off. Do not use steel wool, which leaves fragments that bridge the strips and cause the opposite fault.
Step 3: Test it
Put a single damp cotton shirt in the drum and run an auto-dry cycle. The dryer should run several minutes and shut off shortly after the shirt dries. If it stops within two or three minutes, the strips are still insulated. If it runs on past a dry shirt, the sensor is not the problem and the fault is in the wiring or the board.
Replacing the sensors
Replace only if cleaning fails twice. The sensor itself is a $20 to $50 part on most models.
Step 1: Open the cabinet
Unplug the dryer and pull it clear of the wall. On most front-load dryers the top lifts after two screws at the rear, and the front panel comes off after that. The sensor wiring runs to the front bulkhead.
Step 2: Remove the old sensor
Each strip is held by a single screw and a wire harness connector. Photograph the harness before you unplug it. Remove the screw, unclip the harness, and withdraw the strip.
Step 3: Fit and test
Seat the new strip, refit the screw, reconnect the harness, and reassemble in reverse order. Run a full load of wet laundry rather than a single shirt for the test, because a real load is what exposes an intermittent connection.
When to stop and call someone
If cleaning and replacement both fail, the fault is upstream: the wiring harness, the thermistor, or the control board. A dryer that runs but never heats is a different problem entirely, usually a thermal fuse, a heating element or a blocked vent, and a blocked vent is the one that matters because it is a fire risk.
Frequently Asked Questions
How much does it cost to replace a dryer moisture sensor?
The part is typically $20 to $50. If you have us do it, the diagnostic is $100 and applies to your repair labour, with labour in the $225-$300 band for a typical dryer repair.
Why do dryer moisture sensors fail?
Most do not fail, they get coated. Fabric softener from dryer sheets leaves a waxy film that insulates the strips so they cannot read conductivity. Genuine failures happen, usually a broken wire at the harness rather than the strip itself.
How do I know if the sensor is bad rather than dirty?
Clean the strips first. If a damp cotton shirt still ends the cycle in under three minutes, the strips are still insulated or the sensor has failed. If the cycle runs well past dry, look at the wiring and the control board instead.
If you would rather not open the dryer
Book a dryer repair and a Red Seal technician will diagnose the sensor, the vent run and the heating circuit in one visit. The $100 diagnostic comes off your repair labour if you go ahead.