Moisture Sensor

Some higher-end dryer models may contain a "moisture sensor" or "dryness control" which retards advancing of the timer. Such a device uses an electronic circuit board in conjunction with sensor strips inside the drum which come into actual contact with the clothing as it tumbles. When contact is made with wet clothing, a short is created between the two strips allowing power to flow to the electronic control board which then acts upon that information either allowing the timer to advance or stopping it. The sensor strips themselves are not fancy electronic semiconductors, just plain pieces of metal.

More advanced moisture sensor controls may count the number of times this short occurs over a given period and act upon that information appropriately. More 'hits' means more wet clothing in the dryer in which case the control may delay advancing of the timer for longer, to allow more time for them to dry. Less 'hits' would mean fewer or not as damp clothing in the dryer, in which case advancing of the timer may not be retarded as long.

A common problem that can occur is a permanent short in the sensor circuitry making the control 'think' there is still wet clothing in the drum. In such a case the timer may never advance to the off position, even if the clothing is actually dry. This 'short' may be in the wiring from the control to the sensor strips or right at the sensor strips like a coin or other metal object stuck against them.

Another common occurrence is that the sensor control never perceives the contact with wet clothing. A open circuit in the wiring to the sensor bars or the bars unable to make full contact with the wet clothing because of a lint or other buildup on them. The residue from dryer fabric softener sheets are often also a cause of this. In either case, the control will determine the clothes are dry and allow the timer to advance to the end of the cycle before they actually are dry.

Of course, a problem in the electronic control board itself could mimic either of those scenarios.