Heats On Test But Not During Cycle
Why this matters
A dryer, dishwasher, or oven that heats perfectly when you force the heater on in service mode but produces no heat during a normal cycle is a fault pattern that points almost exactly at its own cause, yet techs routinely misread it. The heater clearly works, so the heating element gets cleared, and the board takes the blame. But the split between test-mode heat and cycle no-heat tells you the heater and its high-voltage path are fine; the heat call is being withheld or cut during the cycle by a control input, a thermostat, or a sensed condition that test mode bypasses. This tree converts that split into a targeted diagnosis instead of a parts swap.
Symptom presentation
In service or test mode, the heater energizes on command and the element glows or the cavity warms. Under a normal cycle, the element never energizes, or energizes briefly then drops out, and the load comes out cold or damp. On a dryer the drum tumbles with cold air; on a dishwasher the heated-dry and the wash-water heat never happen; on an oven the cavity stays cold in bake but the element fires when forced. The motor and other functions run normally. The heater is provably capable, so the cycle no-heat looks mysterious.
Quick checks
Understand what test mode bypasses around the heat circuit. Forcing the heater in diagnostics usually energizes the element directly, ignoring the cycle's heat-demand logic, the thermistor feedback loop, and sometimes the cycling and high-limit thermostats' role in modulation. A normal cycle requires the control to call for heat based on cycle state and sensor feedback, and requires every series safety to be closed under running conditions. Confirm the cycle selected actually calls for heat (not air-fluff, not a no-heat eco option). Confirm airflow on a dryer: a restricted vent lets the high-limit cut heat during a real run while a short forced test never trips it. Pull the stored fault from the normal-cycle attempt.
Isolation tree
The heat path works; the heat call or a running-condition cutout does not.
Does the element fire at all during the cycle, or fire then drop out?
- Never fires during the cycle. The control is not calling for heat, or a series device is open under running conditions. Check the heat-demand logic and the sensor that gates it.
- Fires then drops out partway. A running-condition cutout is opening: a thermostat tripping on temperature or a safety opening on a condition that builds over time (airflow, runaway heat).
Element never fires in-cycle.
- Cycle calls for heat but element stays off. Read the thermistor the control uses to decide heat demand. A thermistor reading falsely hot tells the control the load is already at temperature, so it never calls for heat. Test mode ignored the thermistor and fired anyway. Suspect a thermistor reading high (low resistance) or a shorted sensor.
- A series thermostat or thermal cutoff is open under running load. A cycling thermostat or thermal fuse that is open only when warm passes a brief test but blocks the cycle.
Element fires then drops out.
- Drops out as the cavity or drum warms. Suspect a cycling or high-limit thermostat opening early, often driven by a real airflow restriction (clogged dryer vent, blocked dishwasher airflow). The short forced test never reached the trip temperature; the real cycle does.
- Drops out on a sensed-condition fault. A thermistor that drifts during the run, or a control losing the feedback signal, cuts the call.
The consistent logic: forced test energizes the element open-loop; the real cycle depends on a heat call and on safeties that respond to running conditions, and that is where the fault lives.
Confirming diagnosis
Confirm by reading the heat-demand chain live during a normal cycle. Monitor the thermistor resistance and the cycle's heat-call output as the cycle runs. If the thermistor reads falsely hot (resistance too low for the actual temperature) and the control therefore never calls for heat, the thermistor is the fault, not the heater. If the heat call is present but no voltage reaches the element, a series safety (cycling thermostat, high-limit, thermal cutoff) is open under running conditions; measure across each while warm to find the one dropping line voltage.
For the fire-then-drop-out case, run the full cycle with airflow monitored: if the high-limit trips because the vent is restricted, restoring airflow keeps the element energized. Confirm by clearing the restriction and re-running. A heater that then sustains through the cycle confirms the airflow cutout, not the element.
Remediation
- Thermistor reading falsely hot: Replace the thermistor and repair its harness so the control sees true temperature and calls for heat.
- Open series safety when warm: Replace the failed cycling thermostat, high-limit, or thermal cutoff, and find why it opened.
- Airflow restriction tripping the high-limit: Clear the dryer vent or the airflow path; a thermal-cutoff that blew indicates the restriction is severe and a fire risk.
- Confirmed control fault: Only after the sensor and safeties verify good, replace the control board.
A dryer high-limit or thermal cutoff that opens during a normal cycle is responding to restricted airflow and runaway heat, a fire hazard. Never bypass a thermal safety to keep the heater on; clear the vent restriction and replace any blown thermal cutoff. Heat-circuit work involves 240 V; lock out the breaker before re-terminating.
References
- UL 2158, Standard for Electric Clothes Dryers
- UL 749, Standard for Household Dishwashers
- UL 858, Standard for Household Electric Ranges
- NFPA 70 (National Electrical Code), Article 422 Appliances