Bosch Washer Error Code 20

Error code 20 on a Bosch washer indicates unexpected heating activity, meaning the heating element is being activated during a cycle phase when no heating should be occurring. This condition points to a stuck or shorted heater relay on the power module. A relay that has welded contacts internally will allow current to flow to the heater continuously, regardless of the control signals sent by the main control board. The washing machine monitors heating periods carefully, and when it detects heater activation outside of programmed heating windows, it throws this fault code to protect the load and the appliance.

The two primary components involved are the NTC temperature sensor and the heater relay on the power module. The NTC sensor monitors water temperature and provides feedback to the control board. If the NTC is reading incorrectly, the board may misinterpret temperature data and falsely determine that unexpected heating is occurring. However, a more serious and common root cause is a mechanically or electrically stuck relay on the power module itself, which physically cannot open its contacts to cut power to the heating element.

Diagnosing this fault starts with a reset by cycling the machine off and back on. If the error persists, the NTC sensor should be tested for correct resistance values across the temperature range, as a drifting or failed sensor can produce misleading temperature readings. The heater itself should also be inspected for resistance and continuity to confirm it is not short-circuiting. If both components test within spec, the power module is the most likely culprit and will typically need to be replaced to resolve the stuck relay condition.