Modern ovens are inherently different from old ones. "I want you to come out and calibrate my oven; the temperature is off." was once a common service call. Not anymore. Today a temperature error is catastropic, not minor. A sensor never drifts off 25 or 50F like the old thermostats did. A bad sensor shows an error code, provides no heat or goes sky high. Consequently when a customer complains that her modern oven is off 35 Deg, I am immediately suspicious that something else is afoot.
Most frequently the cause is customer error. I often try to handle such a complaint over the phone by asking a lot of questions and suggesting that the customer buy a thermometer at the grocery store and test it herself.
I always suggest that they preheat longer or perhaps start using a pizza stone to even out the heat. A calibration call takes close to an hour and is usually just a waste of time.
For More Even Heating Leave a Pizza Stone Permanently In The Oven
There are a few rare instances where such a complaint is legit:
1. Old designs with capillary tube thermostats will drift to the low side as they age and lose their fluid. Most of these are either on the junk pile, parts are not available or just too old and expensive to fix. Don't even consider trying to calibrate one; it will not hold an adjustment because it is failing.
Gas Thermostat
Some relatively new "pro type ovens" still use the old style thermostat shown above. If the oven is new the situaltion changes. They can actually be calibrated by adjusting the center screw. A few oven knobs are adjustable too:
However, following is a chart showing what really happens to the oven temperature over time. It takes oven 30 minutes for an oven to come to equilibrium. Customers are very impatient and don't understand how much the temperature varies.
2. A brand new oven may not be set up correctly. For instance, it may be reading out in Centigrade instead of Farenheit. Go through the Owner's Manual carefully.
3. A few early ERC designs required an "offset" be calibrated during setup or a new ERC installation. I have seen this in JennAir and it requires a full callibration. This process is not at all obvious and is described in the owners manual.