A dishwasher soil sensor, often called a turbidity sensor, checks how cloudy the wash water is and uses that reading to decide how much more washing, water and heat the load needs. It is the reason the auto or sensor cycle runs short one day and long the next, and why a dirty filter can make a good dishwasher clean poorly.
How it works, step by step
- You pick an auto or sensor cycle. ENERGY STAR's dishwasher page explains that a dishwasher with an auto-sensing cycle has a soil sensor that can detect whether the water coming off the dishes is dirty or clean.
- The first fill picks up soil. Water sprays over the dishes and drains back into the sump at the bottom of the tub, carrying food particles with it.
- The sensor reads the water. Most soil sensors are optical: a small light shines through a sample of water in the sump, and a detector measures how much light gets through. Cloudy water lets less light pass.
- The control adjusts the cycle. Heavy soil can mean an extra prewash fill, a longer or hotter main wash, or another rinse. Light soil lets the control shorten those stages.
- It checks again later. Readings during the rinses confirm that the water is clearing before the cycle moves on to drying.
What you'll notice because of it
- Cycle times that vary from load to load, even on the same setting.
- A time display that jumps partway through as the control adds or removes stages.
- Long cycles after big meals and shorter ones for lightly used cups and plates.
- Less water on light loads. ENERGY STAR lists soil sensors among the features that make newer certified models more efficient.
Where it can go wrong
A clogged filter
Symptom: long cycles, gritty dishes and cloudy water that never seems to clear. Food trapped in the filter keeps clouding the water, so the sensor keeps calling for more washing.
A coated or dirty sensor
Symptom: cycles that run long every time, or dishes that come out dirty on the auto cycle. Grease or mineral film on the sensor's window throws off its readings.
A failed sensor or control
Symptom: an error code, or an auto cycle that runs exactly the same way regardless of the load. Some models fall back to a fixed program when the sensor stops reporting.
Pre-rinsed dishes in a sensor cycle
Symptom: a short, light cycle that leaves stuck-on spots. If the water looks clean early on, the sensor can choose a lighter wash. ENERGY STAR notes that rinsing dishes first increases total water and energy use and recommends scraping instead.
Getting the most out of it
- Scrape food into the trash, but skip the full rinse at the sink.
- Clean the filter as often as the manual recommends. It is usually under the lower spray arm.
- Use the auto or sensor cycle for everyday mixed loads, since that is what it was built for.
- Run full loads when you can. ENERGY STAR notes that a dishwasher uses about the same water and energy whether it is full or half empty.
- Use the rinse or hold cycle for dishes that will sit for a while before the full wash.
When cycles stay long and dishes stay dirty even after a filter cleaning, the sensor or control may be at fault, and a dishwasher repair visit can sort out which. The soil reading also works hand in hand with water temperature, since the control may add heat for a dirty load; how a dishwasher heats its water covers that side.
Sources
Worth knowing: Varying cycle times on the auto setting are normal; they show the soil sensor doing its job.
Clean filter, long cycles and food still left behind? Call ProFix Miami at (786) 723-6874 to have the sensor and wash system tested.