Cleaning the glass

You have to dismantle the door if the the residue is on the inside, but I recommend using a paint scraping razor to clean the glass, avoid slopping large amounts of cleaning fluid on it

if the glass is sooty then cleaning it will not solve the problem , its not burning right causing the soot , needs to be serviced by a gas fitter.

The only way to clean this is to dismantle the door. Most brands you can remove the door and set it on a counter to dismantle. This can be very complicated, depending on the brand and model. Leaving the gravy inside the door won't hurt the range, and self cleaning usually will not clean this completely. It usually just bakes it on harder. Good Luck Appliance Specialists

It is a scanner problem....could be lamp, lens defect, or as simple as the lamp, and lirrors undeer the glass being very dirty, because the code refers to the 'white level"....a calibration that the machine does when switched on. It could be as simple as, the white plate which is underneath the left scale (beside the main contact glass) is dirty (or missing). Try cleaning this (2 screws), and while you're at it, try removing the main contact glass (3 more screws)and cleaning it as well as the scanner bulb, and mirrors (slide scanner to the right with the machine unplugged).
Good Luck

Try cleaning the glass on top of the machine. Otherwise you may have dust underneath the glass on the scanner.

Try cleaning the optics and scan lamp.Use normal glass cleaner
clean the underneath the copier glass.
If the problem is still there the developer probably need a change.
If you need any more assistance please get back to me.
Thank You

Scanners tend to be made not to be dismantled,

So this is a problem you will have to figure your self or buy a new scanner