Dryer Diagnosis Reference
Why this matters
Dryers fail in predictable ways: no heat, no tumble, takes two cycles to dry, won't start, makes noise. About 80% of "won't dry properly" calls are actually a clogged vent restriction, not an internal failure - and missed vent restriction causes house fires. The diagnostic flow is straightforward once you know the components, and the most common service item (vent cleaning) is often a 30-minute upsell on what the customer thought was an equipment problem.
Electric vs gas dryers
Electric dryer:
- 240 V, 30 A circuit (typical)
- Heating element: nichrome wire coil, 4000-5500 W
- Element typical resistance: 9-13 Ω
- Most common in modern homes
- Hookup: 4-wire cord (NEMA 14-30R)
Gas dryer:
- 120 V single-phase
- Burner: glow-bar igniter + gas valve (similar to gas furnace)
- Igniter typical resistance: 50-400 Ω depending on type
- Hookup: 120 V outlet + 1/2" gas line + 4-wire ground (older 3-wire grandfathered)
Both share the same drum, motor, belt, blower, vent, controls.
Major components
Drive motor - direct-drive or pulley-drive. Spins the drum and (often) the blower wheel.
Drum belt - wraps around the drum and around the motor pulley + idler tensioner. Common failure point on older units.
Idler pulley - spring-loaded tensioner for the belt. Bearings wear over time.
Drum rollers (back of drum) - support the back of the drum. Wear over time = squealing or thumping.
Blower wheel - moves air through the heater and out the vent. Sometimes shares the motor shaft, sometimes belt-driven.
Heating element (electric) OR gas burner (gas):
- Electric: nichrome resistance heater
- Gas: igniter → flame → heat exchanger
Thermal fuse - one-time-use safety device. Opens at over-temperature (typically 150-200 °F). Common failure when vent is restricted.
Hi-limit thermostat - adjustable / resettable thermal switch.
Operating thermostats - cycling thermostats that control normal heat cycle.
Door switch - disables motor when door open.
Start switch / control board - initiates and times the cycle.
Moisture sensor (modern dryers) - two metal bars in the drum; detect resistance based on moisture in clothes contacting them. Drives "auto-dry" cycle.
Common failure modes
Won't start at all
Door switch failed: Most modern dryers refuse to start with door open. Test: open and close door several times; listen for the door switch click. If no click, replace switch.
Thermal fuse open: Common after a vent-restriction event. Test continuity across the thermal fuse (typically located on the blower housing or near the heating element). Open = blown; replace AND fix the underlying restriction.
Start switch failed: Push-button or knob. Test continuity in the start position.
No power: Verify 240 V (electric) or 120 V (gas) at the outlet. Check the breaker (electric - two-pole breaker; check both legs).
Control board failed: Modern dryers with electronic controls. Diagnose by elimination after testing simpler items.
Won't tumble (drum doesn't spin)
Broken belt: Most common cause on units 8+ years old. Pull the front panel or top panel and inspect.
Failed idler pulley: Sometimes the belt is intact but the idler can't tension it. Spin the pulley by hand - should rotate freely without grinding.
Motor failure: Bearings seize, windings open. Test resistance across motor windings (typically 2-6 Ω).
Drum bearing failure: Drum doesn't spin freely by hand even with belt off. Replace drum support bearings.
Stuck object in drum: Loose change, bra wire, kid's toy - wedged between drum and outer housing. Pull the drum.
Won't heat (drum spins but no heat)
Electric dryers:
Open heating element: Test resistance across the element. Open (OL) = burned out; replace. Visible break in the coil sometimes obvious.
Hi-limit thermostat tripped: Reset (manual reset thermostats) or replace.
Operating thermostat failed open: Test continuity at normal temperature; should be closed.
No 240 V power: Check both breaker legs. One leg out = motor runs (uses 120 V) but heater doesn't (needs 240 V).
Failed timer or control board: Diagnose last.
Gas dryers:
Igniter not glowing: Test resistance (50-400 Ω depending on type). Open = bad igniter.
Igniter glows but no flame: Gas valve solenoid failed OR no gas supply. Listen for the valve click; check that gas is on at the supply valve.
Flame ignites but goes out: Flame sensor / thermocouple weak; safety system shuts off gas.
Vent restriction: Heat builds up, hi-limit cuts power to igniter as safety.
Takes two cycles to dry (the most common call)
This is almost always vent restriction, not equipment failure.
Diagnosis:
- Run a normal dry cycle with damp clothes
- Time how long it takes
- Compare to nameplate cycle time
- 50%+ longer = vent restriction
Vent restriction causes:
- Lint buildup in flexible duct (typical 3-5 years for any home; sooner with high-use)
- Bird nest at exterior vent termination
- Crushed flex duct (improperly routed; concealed in wall cavity)
- Long flex-duct run with multiple 90° elbows (each elbow ≈ 5 ft equivalent)
- Indoor venting (not allowed for gas dryers; only with proper indoor-venting kit for electric)
Cleaning procedure:
- Disconnect dryer from outlet, gas, and vent
- Pull dryer forward
- Pull vent off back of dryer
- From inside the house, vacuum / brush the vent line
- From outside, remove the vent termination cover; clean it out
- Reconnect with new foil tape (NOT screw-attached - screws inside the vent catch lint and cause buildup)
- Verify airflow at the exterior termination (place your hand at the vent during operation; strong warm air = OK)
Long-term:
- Recommend cleaning every 1-2 years (heavier use = more frequent)
- Consider upgrading flexible duct to rigid (4" smooth-wall) where possible
- Sell as ongoing service
Drum makes noise
Squealing: Idler pulley bearing failure OR drum support bearings (rear of drum). Open and inspect.
Thumping (rhythmic): Drum rollers worn. Replace rollers.
Grinding: Foreign object trapped between drum and outer housing. Pull drum, retrieve.
Rattling: Loose mounting, foreign object loose in drum.
Specific brand notes
Whirlpool / Maytag / Kenmore: dominant residential brands. Common failures: thermal fuses (vent-related), drum rollers, idler pulley.
GE: drive motor failure more common in older models.
Samsung: electronic control issues; sometimes complex diagnostic codes.
LG: moisture sensor sensitivity; sometimes auto-dry cycles run too short or too long.
Bosch / Miele: premium European brands; longer service life, fewer common failures; specialty parts.
Safety considerations
References
- Manufacturer service manuals (Whirlpool, Maytag, Kenmore, GE, Samsung, LG, Bosch)
- UL 2158 (Dryer safety standard)
- IRC M1502 (Clothes Dryer Exhaust requirements)
- NFPA 54 (Fuel Gas Code) - gas dryer connections
- IPC for venting (commercial)