Baking that used to work and now does not is rarely the recipe. An oven drifts, and it drifts slowly enough that people adjust their cooking around it for a year before they call anybody.
Prove it first
Put an oven thermometer on the middle shelf, set 350, and leave it forty minutes. Read it three times over the following twenty. You are looking for two separate things: how far off the average sits, and how wide the swing is between the high and the low reading.
A twenty-degree swing is normal. An oven that reads 310 at its warmest, or one that goes from 320 to 400 and back, is telling you something.
Off by a constant amount
That is calibration, and on most ovens built in the last fifteen years you can correct it yourself from the control panel. The manual calls it offset or adjust. It usually allows plus or minus thirty-five degrees.
If the offset is already at its limit and the oven is still wrong, the temperature sensor has drifted. It is an inexpensive part and it is the single most common thing we replace in a wall oven.
Swinging wildly
That is a different problem. Either the sensor is intermittent, or one of the two elements is only partly working. In a gas oven it can also be the igniter: a weak igniter takes too long to open the valve, so the oven overshoots and undershoots around the setting.
Hot on one side
Check the door first. A hinge that has dropped, or a seal that has hardened, lets heat out at one corner and nothing inside the oven will compensate for it. Look at the gasket the whole way round with the door shut against a piece of paper.
What not to do
Do not run self-clean to see whether that fixes it. Self-clean drives the cavity past 800 degrees and is the single most reliable way to kill a control board on an oven that is already struggling.
Call (847) 555-0142.
