Identify the cleaning system first. activeClean is pyrolytic and burns the cavity clean with everything removed. Where the cavity carries rough catalytic panels instead, those are the cleaning system and must never be scrubbed or sprayed.
Siemens is a BSH brand alongside Bosch and Neff, and its ovens share cavity engineering with both. That is the single most useful fact about cleaning one.
Same rules as its siblings
If your Siemens has a rough, dark, slightly porous panel on the rear wall where the side walls are smooth and glossy, that is a catalytic liner. It is the cleaning system rather than dirt, it is meant to look that way, and scrubbing or spraying it destroys the coating permanently.
Our oven cleaning guide explains what a catalytic liner is and how a saturated one is regenerated with heat rather than chemistry. The point specific to Siemens is that the same panel appears on Bosch and Neff ovens, so if you have a mixed BSH kitchen one rule covers all of it.
activeClean and varioClean
Siemens names its pyrolytic function activeClean. If yours has it, the door locks itself during the cycle, the cavity reaches around 500 degrees, and everything must come out first, including telescopic rails, which discolour permanently at that temperature.
varioClean is the lower-temperature assisted function on some models, which softens residue rather than removing it and is a help on light soiling rather than a rescue on heavy.
Parts cross-reference across BSH
Because Siemens, Bosch and Neff share engineering, components and manuals frequently cross-reference. If a Siemens-badged part is hard to find for an older model, the equivalent Bosch listing is worth measuring against, and the same is true in reverse.
Telescopic rails, and which way round they go
Siemens fits telescopic pull-out rails on much of its range, and they are the component most often damaged during an otherwise careful clean.
They unhook from the wire side rack, and the side rack itself then unclips or unscrews from the cavity wall. Note which way round each rail sits before it comes off, because left and right are not interchangeable and a reversed rail binds instead of extending. That fault is invariably blamed on the cleaning rather than on the reassembly.
Do not leave them in a strong degreaser for hours either. The sliding sections carry a light lubricant that a long soak strips out, and a dry rail stays stiff for the rest of its life. Wash them by hand and dry them.
What actually goes wrong
A scrubbed catalytic panel, believed to be baked-on dirt.
Telescopic rails left in during an activeClean cycle, which blues them permanently.
A rail refitted the wrong way round, which then binds and gets blamed on the clean.
A smell after a spotless clean, which is the fan chamber behind the rear baffle.
Should you do it yourself?
Do it yourself on the enamel areas, once you have established where any catalytic panels are.
Get someone in if a panel has already been scrubbed, or if the door needs to come apart. The Siemens cooktop above is usually the same age.