Model #PRDS364, this Thermador gas range began having a problem when the left burners would be turned down to the simmer position. Frequently, the flame would go out, causing the spark module to begin the ignition process again. This would happen over and over making the burners all but useless to simmer over. The other non-simmer burners seemed to be working well along with the oven so it wasn’t an emergency, but with a range of this pedigree, you expect everything to work.
The surface burners on this range use typical valves connected to a manifold to control gas flow to each burner. This along with a micro switch connected to the spark module for ignition, pretty much make up the range parts used to control the surface burners. The gas will flow much like water through the valves and into the burner assembly where it will combust with the assistance of a high temperature spark from the igniter electrode. Increase the gas flow and the burner flame will increase, while turning it down will lower the flame as you would expect. This type of system is very simple in design and operation making it very reliable.
One difference, however, is the burners that have a simmer mode. These burners have the ability to go from a very high BTU output, to a barely visible flame that can be used for such delicate operations as melting butter or chocolate. To get this low flame, a secondary valve is used, but it is not controlled by physically rotating a shaft. Instead, a potentiometer, or variable resistor is used in conjunction with a simmer control board to keep this low flame glowing.
As the knob is turned to the simmer position, the main valve will turn off and the low output valve will turn on. The output will vary depending on the position of the shaft as it rotates the potentiometer. The changing resistance is used by the simmer board to maintain the gas flow from the secondary valve. Since the problem was localized to both of the simmer burners, logic told me I was looking for something they had in common.
Because both burners were working, or not working as the case may be, the same way, I immediately suspected the simmer control which is located behind the lower panel under the oven door. I replaced the simmer board, along with both potentiometers as they should be changed as a set. With all the associated parts replaced, turning each burner to simmer resulted in a nice low flame, and it never went out. Not even once.