Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Rheem Furnace 3 Flashes — What It Means
Three flashes on a Rheem furnace diagnostic LED signals a pressure switch fault — specifically, the draft pressure switch failed to close within the expected time after the induced draft motor started. The pressure switch monitors that the inducer is pulling adequate combustion airflow through the heat exchanger before the gas valve is allowed to open. When the switch stays open, the board blocks ignition as a safety measure.
Common Causes
- Blocked or restricted flue/vent pipe — Bird nests, ice, leaves, or debris blocking the PVC or metal flue exhaust pipe prevent the inducer from building enough negative pressure to close the switch.
- Failed induced draft motor — A motor that runs slowly or not at all cannot generate sufficient draft, keeping the pressure switch open.
- Defective pressure switch — The switch diaphragm can rupture or the contacts can fail, causing the switch to stay open even with adequate inducer operation.
- Cracked or disconnected pressure switch hose — The small rubber or silicone hose from the inducer housing to the pressure switch must be intact; a crack or disconnection prevents the switch from sensing draft.
Step-by-Step Fix
- Inspect the flue/exhaust pipe — Go outside and look at the vent termination. Clear any blockage (ice, bird nest, debris). Inspect the entire pipe run for disconnected joints or damage.
- Listen for the induced draft motor — When a heat call starts, the inducer should come on first. If you hear nothing, or it sounds sluggish, the motor or its capacitor is likely failed.
- Inspect the pressure switch hose — With power off, locate the rubber hose connecting the inducer housing port to the pressure switch. Flex it gently and look for cracks. Replace if cracked; reconnect if loose.
- Test the pressure switch — With the inducer running, use a manometer or a jumper wire (temporarily bridge the switch terminals) to confirm the switch is the fault. If the furnace lights with the switch jumped, replace the pressure switch.
- Reset and verify — Remove any jumpers, restore normal wiring, power cycle the furnace, and run a full heat cycle to confirm the switch closes and the burner stays lit.
Parts Often Needed
| Part | Notes |
|---|---|
| Draft pressure switch | Match Rheem part number; pressure rating varies by model |
| Pressure switch hose | Standard 1/4” ID silicone or rubber tubing |
| Inducer motor capacitor | Test before replacing the full motor assembly |
| Induced draft motor | Replace if capacitor checks good but motor is slow or noisy |
Technician Tips
Modern furnaces use LED flash codes on the control board to diagnose failures. Before calling for service, count the number of flashes between pauses and check your owner’s manual for the code chart. The most common furnace error — 1-4 flashes usually means ignition failure — is often caused by a dirty flame sensor. The flame sensor is a $15-30 part that takes 10 minutes to clean. Remove it with a 1/4” nut driver, gently scrub the metal rod with emery cloth or a dollar bill (yes, a dollar bill — it’s the right abrasiveness), and reinstall. This fixes about 60% of “no heat” service calls.
DIY vs Pro: Error codes related to pressure switches (usually 2-3 flashes) often indicate blocked vents or condensate drains — safe to check yourself. Codes related to gas valves or ignition modules (4+ flashes) are best left to a licensed HVAC technician.
Need Replacement Parts?
Before ordering, match the furnace part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
If the flue is clear, pressure switch and hose are intact, and the inducer motor runs at speed but the switch still won’t close, have a licensed HVAC technician check for a failed heat exchanger or incorrect pressure switch calibration — both require specialized tools to diagnose safely.
See Also
- Rheem Furnace 8 Flashes Error Code — Causes & Fix
- Rheem Furnace 5 Flashes Error Code — Flame Sensor Fault Fix
- Rheem Furnace 9 Flashes Error Code — Causes & Fix
- Rheem Water Heater Error Codes — Complete Guide