, this built-in double oven had been working fine for the customer, and then both ovens stopped heating altogether. Figuring I already knew the answer, I asked if the oven had recently been self-cleaned. And as expected, a cleaning cycle had been started on both ovens, and it hasn't worked since. An easy fix for me, but it started with a quick lesson for the customer.
How it Works
Built-in ovens are very dependant on insulation and proper airflow around the oven liner and electrical components to keep everything cool and to prevent any possibility of overheating. During a normal baking cycle, the outside oven temperature is usually well contained within the insulation blanket, and any excess heat is removed by a cooling fan, or through normal convection currents where cool air is drawn in under the oven and exhausted out the top. During the self-cleaning cycles, the oven cavity temperature will rise in excess of 700 degrees F, which translates into higher temperatures outside the oven, making the cooling fan and good airflow even more important to prevent any possible damage to the surrounding cabinetry.
Most manufacturers of cooking products install a thermal fuse or thermal overload device (TOD) in series with the heating elements to act as a safety cut-off in the event the outside of the unit begins to experience excessivly hot temperatures. Looking at the strip circuit of this oven, we can see once the TOD opens the circuit, current can no longer flow resulting in the elements no longer heating.
Verify the Failure
Because the thermal cut-off is designed to be the weak link in the circuit, they are very succeptable to failure. But if you find that a TOD has failed and needs to be replaced, it is important that you verify the failure was just due to age and not the result of something else causing the failure. For example, a slow or intermittantly operating cooling fan will not provide proper cooling and exposes the TOD and everything else to higher temperatures. Blocked or clogged air vents (under and above the oven doors) can limit the fans ability to move air to keep things cools. Sagging door hinges, or loose door seals can result in hot air from the oven getting pulled in by the cooling fan, and hot air doesn't work very well for cooling. By looking at the other components that can cause a failure and eleminating each of them as the problem, we can feel confident that the TOD has just failed and needs to be replaced
I pulled the oven out from the cabinet enclosure and removed the rear panel to get access to the TOD. It is possible to do a continuity check of this component without removing the oven, but because the odds are high that it needs to be replaced, I cut to the chase and pulled the oven. A quick continuity and voltage check with my meter verified my prediction that both oven TOD's had failed. Installing a new TOD behind each oven had this unit back up and heating in no time.