An oven that fails to heat at all, or heats far below the set temperature, is usually one of three common failures.
On electric ovens: the bake element is the most frequent failure. Inspect it with the oven off — a bake element with a visible break, bubble, or burn spot has failed and needs replacement. Bake elements are $50 to $120 parts and a 30-minute swap. The broil element and hidden bake element (in convection units) fail the same way but less often.
On gas ovens: the igniter is the most common failure. A weak or aged igniter fails to draw enough current to open the safety gas valve, so gas never reaches the burner. The visible symptom is an igniter that glows orange but never lights the burner. Replacement is typically $150 to $250 installed.
On both types: the oven-temperature sensor. A failed sensor sends the control board the wrong temperature and the board stops calling for heat. A multimeter resistance check at room temperature confirms the diagnosis — a working sensor reads approximately 1,080 ohms at 70°F.
On ovens with electronic control boards, a blown control-board thermal fuse or a failed board itself is the less common but possible failure path. These are diagnosed last because they're the more expensive replacement.