Machine sucking soil in from grate

This thread was marked as solved. Here's a summary of the problem and the fix, generated from the discussion below.

Bosch Classixx 1200 Express with grit-like debris, leak and noise

The problem

A Bosch Classixx 1200 Express washing machine began leaking slightly during a cycle, then after a couple more washes made a noise described as gravel hitting the drain pump cover. Small grit-like pieces were found at the drain pump. The landlord reportedly said the machine was a write-off because debris had damaged the tub, caused leaking and affected the electrics.

Likely cause

The accepted reply said it should not be possible for soil or water to be sucked back in through the waste pipe if the machine was plumbed in correctly with a U-bend trap and air break. Based on the later description that the debris was brittle, broke apart easily, looked calcified and some pieces were too large to have passed through the drum holes, the strongest supported diagnosis was internal build-up/deposits such as limescale or other accumulated residue rather than grit being sucked in from the outside drain. The exact cause of the final damage was not independently confirmed.

What fixed it

No repair was documented. The discussion mainly ruled out back-suction from an outside grate if the drain installation was correct, and suggested the debris was more likely long-term internal deposits. Exact repair was not confirmed.

Checks to try

• Drain pump inspected and small grit-like bits found
• Debris examined and described as brittle/calcified
• Observation that some pieces were too large to fit through the drum holes
• Forum advice that correct plumbing should include a U-bend trap and an air break