An oven that won't heat usually has a dead heat source — a burned-out bake element on an electric oven, or a weak igniter that won't open the gas valve on a gas oven.
Electric and gas ovens fail differently. On an electric oven, the bake element glows to make heat and visibly fails. On a gas oven, a hot-surface igniter must glow hot enough to open the gas safety valve — a weak igniter is the most common gas no-heat cause even when it still glows.
Check these in order. The first accounts for most cases.
The bottom bake element develops a break or blister and stops heating. You'll often see a visible split or no orange glow.
A gas igniter weakens with age. If it glows but takes too long, it never gets hot enough to open the gas valve — so the oven never lights.
A bad sensor (RTD) feeds the control wrong readings, so the oven won't call for heat or shuts off early. Many models throw an F3-type code.
A failed relay on the control board, or broken igniter wiring, cuts power to the heat source.
Follow these in order. Stop as soon as the problem clears.
Switch the oven breaker off. For a gas oven, also locate the shutoff. Let everything cool before working.
Look for a blister, split, or dark spot on the element. With power off, test it for continuity; an open element won't glow and must be replaced.
Restore power and start the oven. Watch the igniter under the floor panel: a healthy igniter glows bright and the gas lights within ~90 seconds. If it glows weakly and never lights, replace the igniter.
With power off, check the oven sensor's resistance against spec (typically ~1080 ohms at room temp). A reading far off means a failed sensor.
An F3/F1-type sensor or keypad code points to the sensor or control board rather than the element/igniter.