, this built-in oven with microwave was functioning properly, but the cavity light in the microwave section would no longer operate when the door was opened. The customer found what he thought was the end of the bulb in the middle of the top, but couldn't figure a way to replace it. After enough looking around and removing screws, he gave in and called for service.
The light bulb in this microwave, like most others, will turn on when the door is opened, or the microwave is in use. Most bulb are not easily user changeable for safety reasons on built-in or counter top units, while many over the range units will have a bulb access door behind the vent grill. The bulb on this particular model is a halogen style, running on low voltage AC current provided by a step down transformer located behind the control panel.
When trouble shooting a no light situation, we must determine whether it is the bulb, the transformer, or the wiring that is the at fault. If you are playing the odds, go for the bulb, but I encourage people to do some basic checks so the correct part is ordered the first time. With the control panel tipped up out of the way, the output from the transformer can be checked for proper power. Be aware this unit has three separate transformers so you must check the correct output which is the two pink wires. A voltage of around 18vac when the microwave door was opened would be correct indicating the bulb is bad. If you don't have voltage, then either the transformer is bad, or the control board is not powering the transformer.
In the case of this unit, the bulb was bad and needed to be replaced. The process for getting to the bulb is rather involved and way past the scope of this post, but with the appropriate screws removed, I pulled the microwave out of the cabinet. The bulb is held in place with a metal clip and the end slips into a glass lens which is the part you see from the inside. Once the new light bulb was mounted back in place and connected to the harness, I turned the circuit breakers back on and found light again in the microwave. No more hot pockets in the dark for this customer.