Oven Thermostat Faults Guide
Oven Temperature Problems?
Please read our Important Safety Notice before undertaking repairs.
This article refers to traditional oven thermostats, as fitted to most Ovens. Unfortunately some manufacturers are now beginning to replace Thermostats with less reliable printed circuit board (p.c.b.) systems.
See Our Printed Circuit Board Help Guide
How Oven Thermostats Work
Oven thermostats are calibrated to regulate the oven cooking temperature, thermostats have a capillary tube or "phial" with a probe on the end containing a gas, the probe is located inside the Oven. The gas inside the capillary tube expands and contracts, responding to the oven temperature, thus switching the oven element on and off, thereby maintaining the selected temperature.
Most oven thermostats will have just two electrical terminals and an earth terminal. (see photo). Thermostats can sometimes have more than two electrical terminals, when a function / selector switch is incorporated into the thermostat assembly. Always make a wiring diagram BEFORE removing any leads!
A typical (two electrical contacts) type oven thermostat.
Where is the Oven Thermostat Located?
Oven thermostats are usually located behind the control panel, the thermostat shaft may fit directly to the temperature control knob, or it may locate and push fit into the rear of a selector switch, if so, gently prise the two apart to separate. Most capillary tubes are about 1200mm long, some can be longer.
Make sure you are looking at the right thermostat, there is usually one for the top oven and another for the bottom oven. Look at the control knob markings to confirm which is which.
Just so you know the difference, grill elements and hotplate elements are controlled by regulators, also known as simmerstats; these contain a tiny internal heater and a bi-metallic strip which switches on and off internally. Regulators don't have a capillary tube.
Front view of a typical oven thermostat.
If your oven is Overheating, it suggests that the oven thermostat is at fault, as it's the thermostat that regulates and determines the oven temperature. Overheating suggests that everything else should be working OK. Ideally the oven temperature should be checked using a heat resistant oven thermometer, to confirm that the temperature is too high.
Phone us on 01202 738440 to purchase a purpose made oven thermometer.
Oven thermostats can become inaccurate, internal contacts can become pitted, or can weld together, or if the capillary tube (phial) fractures, the thermostat would then be unable to switch off, the thermostat indicator light would stay on, with the oven element heating continuously. This fault could then blow a thermal overload safety cutout (T.O.C.) (see photo below) if one is fitted to your oven.
Oven Not Getting Hot Enough?
However,if your oven is not reaching the correct temperature, it could still be the oven thermostat at fault, but not necessarily so! The fault could be caused by a faulty selector/function switch, a faulty T.O.C., an intermittent wiring fault, etc. Any one of these faults could stop the Oven from reaching the required temperature. Elements usually work, or don't work, but always inspect and test the oven element . Please see our Oven Element Faults Guide
A typical Oven Thermal Overload Cutout. (T.O.C.)
T.O.C. temperatures vary so the correct replacement must always be obtained.
Temperature Overload Cut Outs.
An oven thermostat overheating fault, may in turn cause a thermal overload safety cutout (T.O.C.) to fail, but not all ovens have T.O.C.'s. These are usually located at the rear of the oven, accessible by removing the exterior rear oven panel.
When operating correctly, there should be continuity through the two T.O.C. electrical contacts. If it has failed, there would usually be no continuity. (Open circuit). Always test the T.O.C. with one lead disconected. An intermittent fault could also occur, so check the T.O.C.for any signs of burning or overheating. T.O.C.'s are usually "one shot" devices, so if faulty, it would need to be replaced. See Our Important Safety Notice
T.O.C.'s can fail due to overheating faults, but can also just fail for no particular reason. Once a T.O.C. fails, this would usually cut the electrical feed to the oven element, and sometimes to the fan oven motor as well. This will not always be the case. Sometimes the thermostat light can still operate, but there will be no supply to the element, if the T.O.C. has failed.
We don't supply T.O.C.'s, or oven thermostats on the A.S.A.P. site, because there are so many variations, and it's also very important that faults are correctly diagnosed.
If you live in the Bournemouth and Poole area,however, we may be able to help! Please bring in your Make, Full Model and Serial Numbers, Finding Cooker Model Numbers, your old thermostat or T.O.C., also look for any manufacturer's reference names and numbers printed on your thermostat. We do keep several types of oven thermostats and T.O.C.'s on the shelf.
A Few Important Tips!
Always make a wiring diagram BEFORE removing any leads! Why not take some photographs.
When fitting a new thermostat take care not to damage the capillary tube (phial)!
Make sure that the capillary tube (phial) can't come into contact with any electrical contacts!
See our Important Safety Notice BEFORE undertaking any repairs.
See Our Other Oven and Cooker Other Help Guides:
Oven Element Faults
How To replace A Fan Oven Element
Finding Cooker Model Numbers
Fan Oven Motor Faults