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
Kill power before you open anything. Unplug a 120 V gas dryer at the cord. For a 240 V electric dryer, pull the cord and confirm dead at the terminal block with a meter; the cord is easy to reach behind the machine and there is no reason to work it live. Verify at the terminal block, not by trusting the breaker label.
Gas dryers: close the manual shutoff at the appliance connector before disconnecting anything. Leak-test every joint you break with soap solution when you restore gas. Never test with a flame. If you smell gas and cannot find the source, shut the gas off, ventilate, and get the gas utility out.
Lint is the fire hazard, and it is the reason most of these calls exist. Lint accumulates in the cabinet, around the heater housing, in the blower, and along the duct. Vacuum the cabinet interior every time you have it open. A restricted vent raises operating temperature until the hi-limit is doing the job of the operating thermostat, and that is the condition that starts dryer fires.
Never reset a tripped hi-limit and walk away. A manual-reset thermostat trips for a reason, almost always airflow. Reset it, then find and fix the restriction. Resetting alone is a callback at best and a fire at worst.
Venting rules that matter:
- Rigid or semi-rigid metal duct only. No plastic and no foil accordion flex.
- Duct joints taped, never screwed. Screws catch lint and build a plug.
- Terminate outdoors with a backdraft damper. Never into an attic, crawlspace, soffit, or garage.
- No screen at the termination; screens load with lint.
- Keep the run as short and straight as practical; every elbow costs equivalent length.
Gas dryers exhaust combustion products. A disconnected or blocked vent on a gas dryer puts moisture and carbon monoxide into the house. Treat a disconnected gas dryer vent as an immediate safety issue, not a housekeeping note.
Electrical: the terminal block ground strap configuration differs between 3-wire and 4-wire installations. Get it wrong and the cabinet can be energized. Confirm the correct arrangement for the receptacle present, and do not install a 3-wire cord on a circuit that has a separate ground available.
Do not run a dryer with the front panel or drum belt access open to "watch it work." The drum, belt, and blower wheel are all exposed and moving.
Two-person lift on stacked units. A dryer coming off a washer is a back injury and a broken machine waiting to happen.
Customer safety education before you leave: clean the lint screen every load, get the duct cleaned periodically, and never run the dryer while away from home if the vent condition is questionable.
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)