Part 1 - Your oven thermostat - how it works
Your thermostat is in charge of regulating the heat in your oven. It does this by sensing what the current temperature is. And then turning the heat on or off depending on whether your oven needs to be hotter or colder to maintain the temperature you set.
Part 2 - Your oven thermostat light - not coming on or stays on all the time
Your oven's temperature indicator light is usually one of the first signs that something might be up. Other than a blackened or massively undercooked pot roast, of course. The light works like this:
When you turn on your oven, the light will come on to show the oven is heating. Once the temperature you set has been reached, the light should go out.
- Oven thermostat light not coming on: this shows you straight away that there's a problem with your thermostat. It could just be a case of your oven thermostat light not working. But it's much more likely that your thermostat is broken.
- Oven thermostat light stays on: this is likely showing you that the problem is with your oven's heating element. The light never goes off because the oven won't ever be able to hit the temperature you set with a broken element.
Part 3.1 - How to test a thermostat (simple version)
You can do a simple test yourself to see if your thermostat is in full working order:
- Buy an oven-safe thermometer and put it inside your oven.
- If your oven has a see-through section in the door, for ease make sure you can see the thermometer reading through it.
- Turn your oven on and set it to 350 degrees.
- Give your oven time to heat up. Half an hour should be ample.
- Check the thermostat reading (preferably without opening the oven door)
- Repeat this process a few times to make sure you've got a solid average reading.
Now. What does the result look like?
- Temperature is off by a couple of degrees: your thermostat is in good shape. Nothing to worry about. (If you had to open the door to measure it, it can be as much as five degrees off and still be fine.)
- Temperature is off by more than a few degrees: it's time to call for oven repair in London, or wherever else you happen to be based. Or to proceed to a more advanced version of the test…
Part 3.2 - Testing your thermostat (more advanced version)
It's worth noting at this point that the following test will require you to dismantle parts of your oven. If you're not comfortable doing that, call in a professional.
Important safety notice: always make sure that your oven is unplugged before you start. Or that it is disconnected at the circuit breaker or fuse panel. You really do not want it to be powered while you're playing around with it. The results can be fatal. Once your oven is unplugged, open the door and take out all the racks and trays. Then follow these steps:
1) Locate your thermostat
To the uninitiated, this is probably the most difficult part. It's most likely to be behind the back panel, which you'll need a screwdriver (Phillips head) to remove. But if not, check under the exhaust hood, below the control panel or beneath the backsplash. Its location will depend on what type of oven you have.
With the back panel off, look for a rectangular metal plate with two screws in it. It's probably edging towards the bottom right of your oven's back. Follow the wires which lead from that metal plate to a plastic plug. That's your thermostat.2) Remove it
You'll probably need some needle nose pliers to pull the wire leads from the terminals connecting it to the metal plate. Or if you have plastic plugs on your 'stat, you might just be able to pull them free. You can then unscrew the two screws on the back of the thermostat sensor and carefully extract it.3) Use a multimeter to test it
You'll need to set your multimeter to the lowest ohm rating. Or, if your device is an older one which doesn’t have one of those, to the 2k or 4k setting. Touch one multimeter probe to each of your device's terminals. Is it reading zero? If so, your thermostat is broken. If it's as close to zero as possible without actually being zero, it’s fine. A good reading would also be 1000-1100. If you're getting a reading much higher or lower than that, you've got a problem.
Step 4 - (Most likely) Talk to the professionals
You can get replacement thermostats from many places. But do be 100% sure that you've chosen the correct one for your make and model of appliance. To be absolutely confident that there's no trouble with the replacement (or that the problem isn’t with your heating element or another part of your oven) it's worth contacting a professional.
With the value of your oven being so high and oven thermostat repair cost usually so low, it's pretty much a no-brainer.