--- title: "Your Oven Has Not Broken, It Has Drifted: Testing a Thermostat" description: "How to test a La Cornue oven thermostat for drift: the forty-minute probe test, and why a corrected oven changes your recipes." url: "https://la-cornuemiami.support/blog/when-an-oven-thermostat-has-drifted/" date_modified: "2026-09-03" Category: "Troubleshooting" --- # Your Oven Has Not Broken, It Has Drifted: Testing a Thermostat ## La Cornue oven thermostat: the most quietly expensive problem on this appliance Not expensive to fix. Expensive in wasted food, wasted afternoons and three years of a cook slowly rewriting recipes around an oven that has stopped telling the truth. La Cornue's own troubleshooting section names it in one sentence, and the sentence is unusually diagnostic for a manual: *if turning the temperature down using the oven control knob has not worked, or has only worked for a short time, then you may need a new thermostat.* It adds that it should be fitted by a service technician. That is the whole official answer. What follows is how to find out whether it applies to you. ## Two numbers, not one An oven has two behaviors that people both describe as "running hot", and they are different problems. **Overshoot.** On the way up, a thermostat lets the oven pass the setting before it cuts. A vaulted oven with real mass can spike to 425 °F on the way to 350 and then settle. That is normal, it is not a problem, and it is why food put in the moment the oven reaches temperature can catch. **Settled temperature.** What it holds once it has stopped climbing. This is the number that matters. An oven that settles at 390 with 350 on the dial has drifted, whatever it did on the way there. One reading tells you neither. Reading every ten minutes for forty tells you both, and the pattern is the diagnosis. ## Do the free checks first All three are in the maker's own troubleshooting section, all three produce the same complaint, and all three cost nothing. **Level.** A dish of water on the middle shelf should sit at one depth all over. Deeper at the back means the back of the range needs raising or the front lowering — the legs are adjustable across a specified range. We find ranges that were slid into place, connected and never leveled. **The door seal.** Close a strip of paper in the door and pull it out at eight or ten points around the perimeter. Where it grips, the seal works; where it slides out freely, it does not. On a vaulted oven the door is part of the cooking mass, so a leaking seal presents as an oven that will not hold its setting. **Tray size and loading.** Nothing bigger than the specified size, placed centrally, and room left between two loaded shelves. An oven that fails any of these does not have a thermostat problem yet. ## Then measure it properly Thermometer on the middle shelf. Set **350 °F**. Read at ten, twenty, thirty and forty minutes. Write the numbers down. Then do it again at **450 °F**, because an oven can be accurate in the middle of its range and short at the top — and that is exactly where roasting complaints come from. An oven with no headroom left at high heat is a real problem even if 350 is perfect. On a **dual-fuel** range, do both sides. Each cavity has its own thermostat and they drift independently, so a range can carry one perfectly calibrated oven and one thirty degrees out. Whichever one you bake in is the one you know about. ## Calibration, or a part Some thermostats on this make can be adjusted and some cannot. Even where adjustment is possible, a thermostat that has drifted a long way has usually drifted because it is wearing rather than because it was set wrong — so adjusting it buys a season and then you are back. What a good report gives you is which of the two applies, the reason, and the before and after figures. That last part is not bureaucracy. It is the only record of what changed in your kitchen. ## The warning nobody gives you An oven that has run thirty degrees hot for three years has a cook who has adapted completely: shorter times, lower settings, recipes annotated in pencil. Put the oven right and every one of those adaptations becomes an error in the other direction. Cakes that were perfect are now underdone. The roast that took two hours takes two and a quarter. We say this before doing the work rather than after, and we leave the measurements so there is a record. It is a fortnight of recalibrating the cook, and then you have an oven you can trust for another decade. ## One thing that is not the thermostat Uneven cooking. A vaulted oven is graded by design — radiant heat from a heavy sealed dome, hottest towards the center and back — and the maker's own notes ask you to turn a large item during cooking. An oven can be perfectly calibrated and still not bake evenly on two loaded shelves, and no thermostat will change that. Calibration is about *what temperature*. Distribution is about *where*. They have different pages and different repairs. --- **[Read full article: Your Oven Has Not Broken, It Has Drifted: Testing a Thermostat](https://la-cornuemiami.support/blog/when-an-oven-thermostat-has-drifted/)**