Oven Temperature Accurate Cold But Overshoots When Hot Sensor Vs Relay Decision Tree

Why this matters

An oven that calibrates dead-on cold but overshoots its setpoint once the cavity is hot points at exactly two failure modes, and choosing wrong wastes a part and a trip. Either the temperature sensor (RTD probe) is drifting nonlinearly as it heats, so the control thinks the cavity is cooler than it is and keeps the element on, or the bake relay on the control board is welding closed under sustained load and the element never shuts off. Both produce burnt food and customer-reported "runs hot," but the cure for one is a sensor and the other is a board. Because the fault only shows at temperature, a tech who measures the sensor cold and calls it good will keep coming back. The discipline here is to force the oven to operate range and read both the sensor curve and the relay state hot, not cold.

Symptom presentation

The owner reports food burning at moderate setpoints, or an oven thermometer reading 40 to 80 degrees F above the dial once preheated, while a cold-start calibration check looks fine. On electronic controls the displayed temp may match setpoint while the actual cavity climbs past it. A welded relay presents as a cavity that keeps climbing well past setpoint and only stops when the high-limit trips or the door is opened. A drifting sensor presents as a steady offset that grows with temperature and the element cycling normally but to the wrong target.

Quick checks

Place a calibrated oven thermometer or thermocouple at rack center. Set 350 degrees F, let the oven fully preheat and cycle three or four times, then read actual versus set. Note whether the bake element glows continuously (relay or sensor under-reading) or cycles on and off but to a high average (sensor offset). Listen for the bake relay clicking off at setpoint. If it never clicks off and the element stays lit past setpoint, lean toward a welded relay immediately.

Isolation tree

Branch 1, read the RTD sensor hot. The standard oven RTD reads about 1080 to 1090 ohms at room temperature and rises with heat, roughly 2 ohms per degree F, so at 350 degrees F it should read near 1700 ohms (confirm against the model's resistance chart). Measure the sensor cold, then again at temperature by pulling the connector at the back and reading with the oven hot. If the hot resistance is lower than the chart for the actual cavity temperature, the control under-reads and overdrives the element: replace the sensor. A sensor that tracks the chart correctly hot exonerates the sensor.

Branch 2, check the bake relay state. With the oven calling for heat and at setpoint, verify the bake relay opens. Easiest field test: at setpoint, the element should de-energize. Clamp the bake element lead; current should drop to zero when the control commands off. If the element keeps drawing current past setpoint with a good sensor reading, the relay contacts are welded closed and the board must be replaced. Tapping or cycling power may temporarily free a marginally stuck relay, which is a clue not a fix.

Branch 3, harness and connections. A high-resistance sensor connection adds ohms in series, which the control reads as higher temperature and would cause under-heating, the opposite of overshoot, so it rarely explains this symptom. Still inspect the sensor plug at the rear for corrosion or a backed-out pin, since an intermittent there can mimic sensor drift.

Branch 4, calibration offset. Some controls allow a user calibration offset; confirm it has not been set to a large negative value that masks then unmasks at temperature. Reset offset to zero before judging the sensor.

Branch 5, sensor location and contact. A sensor probe bent against the cavity wall or pushed too far into a hot corner reads local heat rather than average cavity temperature, producing an apparent overshoot at the rack. Confirm the probe sits in its bracket at the intended position and is not touching a rack or the element shield. Reposition a displaced probe before condemning it, since a good sensor in the wrong spot reports a misleading temperature that the control faithfully chases.

Confirming diagnosis

Distinguish the two primary branches by the relay behavior at setpoint. Sensor fault: element cycles off at setpoint but the average cavity temp is wrong because the target is wrong; the hot resistance read is off-chart. Relay fault: element does not de-energize at setpoint; the cavity climbs past target regardless of an in-spec sensor. Confirm a sensor fix by re-running the 350-degree soak and seeing actual within about 25 degrees F of set across multiple cycles. Confirm a relay fix by watching the element de-energize cleanly each cycle.

Remediation

For a drifting sensor, replace the RTD probe, route the lead clear of the element, and reverify with a thermocouple at temperature. For a welded bake relay, replace the control board (the relay is not separately serviceable on most platforms) and confirm clean element cycling. Never bypass a high-limit thermostat to keep a unit running; if the high-limit has been tripping due to a welded relay, replace the board and verify the limit resets and functions.

References

• UL 858, Standard for Household Electric Ranges, temperature-limit and control requirements.
• DOE 10 CFR 430 Subpart B, Appendix I, oven test procedure (cavity temperature measurement method).
• GE/Whirlpool Range Tech Sheet, oven RTD sensor resistance-versus-temperature chart and bake-relay diagnostics.
• ANSI/AHAM standards for household electric ranges, performance and temperature accuracy.