A stain on an oven door is often not on either of its two glass surfaces but between the two panes, so wiping the inside or outside will not remove it, and the cleaning cycle cannot reach it. Steam enters through the vent along the bottom of the door, condenses on the cooler outer pane, and leaves grease behind. The solution is to reach through that gap or remove the glass.
An oven door is not made of a single pane of glass. To keep heat inside, two panes—and, in most models, three—are stacked with narrow air gaps between them. These gaps provide insulation, but they are not sealed: a vent at the bottom edge of the door opens into the space between the panes. Air heated while the oven is running flows in and out through this vent. A stain between the two panes that the user can see has traveled in through this opening, which is why no wiping motion can reach it. This is why people wipe the glass over and over and, when the stain does not move, think the glass is scratched.
The stain forms through a simple physical process. Steam rising from food in the oven carries tiny droplets of grease. When this steam enters the gap between the panes through the lower vent, it meets the coldest surface there; the outer pane is noticeably cooler than the inner one. The steam condenses on that surface, the water evaporates, and the grease it carried remains on the glass. Each time food is cooked, another thin layer is added; over time, the layers build up into a yellowish haze. The pyrolytic cleaning cycle cleans the oven cavity but cannot reach between the panes.
The easiest approach, and enough for most stains, is to reach through the lower vent. When the oven is cool, open the door and locate the narrow gap along its bottom edge. Wrap a thin, soft cloth around a long ruler with a rounded tip or a flexible rod, and secure it with a rubber band. Dampen the cloth slightly, insert it through the gap, and wipe across the glass from right to left. Replace the cloth as it gets dirty. Do not use a sharp metal tool for this job; it can crack the glass at the edge, and the crack may grow when the oven heats up.
If the stain is thick, remove the glass. In many home ovens, the inner pane has no screws and sits in just two retainers; fully open the door, then slide the pane upward to release it from its slot. On some models, first remove the entire door by lifting the safety latches on the hinges. Before removing the glass, do two things: mark the order and orientation of the panes, and consult the manual for that model. If the panes are reinstalled in the wrong order, the door may not close fully and the oven can start losing heat.
Use a gentle cleaning method. Lay the panes on a flat cloth and wipe them with hot water and dish soap or a baking soda paste; leave the paste on stubborn grease for a few minutes. Razor blades and metal scrapers can strip the coating, especially on glass with an anti-reflective coating. Do not reinstall the panes until they are completely dry, or moisture left between them will fog up the glass the first time the oven heats. To prevent buildup, cook greasy foods in a covered dish or under foil. After reinstalling the door, check that it closes fully.