What E:28 Means
The aqua sensor is an optical turbidity sensor that shines light through the wash water to judge how dirty it is, which is how Bosch's automatic programmes decide whether to extend a wash or add a rinse. E:28 means that sensor is not reporting sensibly. The documented causes are a sensor that is faulty, missing or simply dirty, and a faulty harness feeding it, with cleaning where it enters the sump listed as one of the fixes.
Common Causes of E:28
- Grease or scale film over the aqua sensor window
- Sensor failed or left out after a previous repair
- Harness between the sensor and the power module faulty
🔧 One Safe Check You Can Try
Clean the sump area thoroughly after removing the filters, which is where the sensor window sits and where the greasy film that blinds it collects. A hot cycle with a dishwasher cleaner afterwards is worth running before assuming the sensor has failed.
⚠ Why You Shouldn't Go Further Than That
- The sensor window is flush with the sump wall and scratches easily, and a scratched window reads permanently dirty even after cleaning.
- Bosch's chart lists a missing sensor as a cause, which means confirming it is actually fitted, and that is a strip-down rather than a look.
- Sensor and harness give the same reading from the display, so the part is only worth buying after the wiring has been checked.
- Working under the sump means being beside the heat pump connections on a machine that has to be fully isolated first.
Opening a panel to inspect or repair something yourself doesn't automatically cancel your manufacturer's warranty — but it can risk accidental damage, and some manufacturers do have specific service conditions. Check your product's warranty terms, or ask us, before deciding.