An oven that says 400 and bakes like 340 is one of those faults people live with for years. It is also one of the easier ones to diagnose properly, and in Boston's older housing there is a specific set of causes worth knowing.
First, find out whether it is actually wrong
Get an oven thermometer — a real dial or probe one, not an infrared gun, which reads surface temperature and will mislead you. Put it on the middle rack, set the oven to 350, and wait a full twenty minutes after the preheat beep. Home ovens cycle around a setpoint, so read it a few times across ten minutes and take the range.
Twenty-five degrees low, steadily: that is a calibration matter, and most ovens let you offset the calibration yourself from the control panel. Check the manual before paying anyone.
Seventy degrees low, or wildly swinging, or slow to preheat: that is a fault.
Electric ovens
The bake element. It fails visibly more often than not — a blister, a break, a bright spot. If the broil element glows and the bake element does not, that is your answer and it is an inexpensive part.
The temperature sensor. A thin probe in the oven cavity, usually on the back wall. It drifts, and when it drifts it reads high, so the oven turns the heat off early. This is the classic "runs cold" fault on an otherwise healthy oven.
One leg of the 240-volt supply. Same failure as the dryer. On a range, the classic symptom is that the surface elements work perfectly and the oven does nothing, or the bake works and the broil does not.
The circuit itself. This is the Boston part. A lot of triple-deckers and older two-families were wired for a much smaller kitchen load and then had ranges swapped in over the decades. A modern range wants a dedicated 40 or 50 amp circuit. If your oven works but trips the breaker on a long bake with the self-clean or the broiler, get an electrician to confirm what circuit it is actually on before you replace parts.
Gas ovens and the igniter
Most gas ovens do not have a pilot light any more; they have a glow-bar igniter that has to draw enough current to open the safety valve. As the igniter ages it still glows — it just draws less — and eventually it glows without ever opening the valve.
That produces the exact symptom that confuses everyone: you can see it glowing, and there is no flame. People conclude the igniter is fine because it lights up. It is not. A weak igniter is the single most common gas oven fault there is.
Practical rule: if the burner takes more than about a minute and a half to light after the igniter starts glowing, the igniter is on its way out. Replace it before it strands you.
The specifically-old-building problems
- No clearance. Wall ovens installed into cabinetry that was built around them, with the cooling vent path blocked. The control board sits above the door and cooks itself. This shows up as intermittent electronic faults in an oven that is otherwise healthy.
- Range hoods that recirculate. In historic districts where an exterior vent was never permitted, the hood filters grease and puts the heat and moisture straight back into a small kitchen. Not an oven fault, but it changes how the room behaves.
- Old gas piping and undersized supply. Treat this one as a stop sign rather than a step: undersized or corroded supply piping is a combustion risk, and correcting it is licensed gas-fitting work requiring a manometer to prove the delivered pressure. If the oven flame drops noticeably when the range top is lit, or the burner will not hold, that is a supply question for a licensed gas fitter.
- Floors that are not level. A range that leans forward bakes unevenly, cakes slope, and everybody blames the oven. Check with a level across the racks.
Self-clean: use it, but not the night before Thanksgiving
Pyrolytic self-clean runs the cavity extremely hot, and that heat is what finishes off marginal components — door latch motors, temperature sensors, control boards, and sometimes the thermal fuse on the cooling circuit. This is not a reason never to self-clean. It is a very good reason not to run it for the first time in two years on the evening before you need the oven.
What is worth repairing
Elements, igniters, sensors, thermostats, door hinges and latches are all ordinary, sensibly-priced jobs on almost any oven. Control boards on high-end built-in ovens are where the arithmetic gets uncomfortable, and on a built-in wall oven the replacement conversation also involves the cabinetry, because modern cutout dimensions rarely match a thirty-year-old opening.
We will tell you which side of that line you are on before we take anything apart.
