Uncle Harry's Mobile Repair Wizard

This problem has numerous potential causes but by far the lock mechanism is most common.  

1. Problems with the lock mechanism.  Setting the computer (or selector switches on old designs) to "clean" either starts the locking motor or allows you to more the latch to lock. Once the oven senses that the door is truly locked it begins heating up the two elements. However, any damage to the lock system linkage or a bad micro-switch will halt the process. The lock is the primary suspect. If the unit is a double oven try the second one. (See "oven locked up" for more info.) A careful visual inspection plus checking out the switches will usually reveal the problem. Access may require considerable effort. 

2. Customer error. If a customer seldom uses self-cleaning they may forget all the steps necessary to correctly start the process. This may seem a bit silly and it may get a bit awkward to teach a customer how to use their own oven but it happens. Check the unit slowly and carefully before looking for a faillure. Always reset the breaker and try a few things like bake and broil to get familair with a strange model. Some complex designs are very difficult to set and require many steps.

3. Door switches. Some designs have a door switch/es that checks door position before enabling self-clean. Old Thermador's had a switch hidden in the side wall which was operated by the hinge arm. Other designs have rods that push remote switches. Be certain to look over the door jam for any potential devices and check them out.

4. Obviously the computer itself can be the culprit but this is seldom the case. If the readout seems to be working it is usually waiting for feedback from a check switch. 

5. Double Ovens can seldom be cleaned simultaneously and have check switches that prevent you from doing so. A bad check switch will stop both from cleaning. These interlock check switches are part of the lock mechanisms and can often be located on the schematic.