Bosch [E12] fault codewhat it means and how to fix it

Bosch give E12 as the heating element being furred up, and Siemens word it as the appliance having detected a calcified heating element. The official remedy is two steps: descale the appliance, and operate it with the water softening system. That second step is the one that matters, because descaling without addressing the cause simply resets the clock. Every Bosch dishwasher has a built-in water softener with a salt reservoir, and it has a hardness setting that has to match your local water. The factory default is H04, which covers 16 to 20 degrees Clark. A great deal of southern and eastern England is harder than that, which means a large number of UK machines are running under-softened straight out of the box and quietly scaling up their own heating element. Bosch state the threshold plainly: water with a hardness above 9 degrees Clark must be softened to avoid damage to the appliance. On older machines with no numeric display this code shows as the salt refill indicator flashing, which owners reliably read as "needs salt" — they top it up, the light keeps flashing, and they conclude the machine is broken. A flashing salt light and a steady one are different messages.

1. 1

   Find your water hardness. Your water company publishes it for your postcode, and Bosch express the setting in degrees Clark, which is the unit UK suppliers use.

2. 2

   Check the machine’s hardness setting and correct it. Bosch’s scale runs H00 for very soft up to H07 for 39 to 62 degrees Clark, with H05 covering 21 to 26 and H06 covering 27 to 38. The factory setting is H04, so if you are in a hard water area it almost certainly needs raising.

3. 3

   Fill the salt reservoir with dishwasher salt. Bosch are specific: only special salt for dishwashers, not salt tablets and not table salt. Bosch UK add that table salt, sea salt and kosher salt can actually increase water hardness and damage the appliance.

   Start a programme immediately after filling. Bosch warn that salt spilled in the tub causes corrosion, and running a cycle straight away washes it out.
4. 4

   Descale the machine using a proprietary dishwasher descaler, following the product instructions with the machine empty.

   Do not use industrial or strongly caustic cleaners, and do not put aluminium items such as extractor hood grease filters through with them. Bosch warn that combination can cause an explosion.
5. 5

   Expect cycles to take slightly longer afterwards. Bosch note that the softener regenerates before the end of the main rinse and that this increases run time — that is normal, not a new fault.