A cooking appliance usually shows a heat problem well before it stops heating. The early signs are small, and most people explain them away. Writing down what you see in plain words is useful later, both for a repair call and for checking whether a contract's terms fit the fault.
What uneven heat looks like
Common early signs include a preheat that takes noticeably longer than it used to, browning that is darker on one side of the tray, a bake that finishes early on the top rack and late on the bottom, or a setting the oven reaches only after overshooting and then cooling below target. Each sign points to a different part of the system, so describe what you see instead of guessing at the cause.
Which parts are involved
In an electric oven, the heating elements and the temperature sensor work together, and a weakening element or a drifting sensor can both produce uneven results. In a gas oven, the igniter and the gas valve control the flame, and a slow or uneven flame is a sign to stop using the oven until a qualified person has looked at it. If you smell gas at any point, leave the room and contact your utility or the fire department from outside the home. That is a safety step, not a repair step.
Test before you call
An oven thermometer you already own can show whether the displayed temperature matches the actual reading. Place it on the center rack, preheat fully, and note the reading at the start and again after a few minutes. Record the gap, the setting, and what you were cooking. A large gap between the display and the thermometer points toward calibration or sensing. A steady wrong heat with a normal display points toward the heat source itself.
What to check in the paperwork
If the oven sits under a service contract, read which parts are listed, which items are excluded as wear, and whether a waiting period or a maintenance requirement applies to the oven. Ask whether the heating element and the sensor are covered separately, and whether a diagnosis visit is covered on its own. Keep the purchase record and any earlier service notes, since the contract may ask for them. The rules for a given state, including who regulates these contracts, are on that state's page on this site, which also says whether the state publishes a roster.