Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Rheem Furnace Error Codes — Quick Reference
Rheem furnaces signal faults with a blinking LED on the control board. Open the lower access panel (or look through the sight glass) and count the blink sequence. Rheem also makes Ruud furnaces — the codes are identical. The flash pattern repeats continuously; count the blinks in one cycle.
| Code | Meaning | Quick Fix |
|---|---|---|
| 1 flash | System lockout — retries exceeded | Reset; check igniter and gas supply |
| 2 flashes | Low-pressure switch open | Check inducer, hose, drain trap |
| 3 flashes | High-pressure switch open | Check high-pressure switch hose (2-stage) |
| 4 flashes | Open high-limit device | Replace filter; check airflow |
| 5 flashes | Flame without call / roll-out tripped | Gas valve or roll-out issue |
| 6 flashes | Inducer motor fault | Check motor speed; replace if needed |
| 7 flashes | Low flame signal | Clean flame sensor rod |
| 8 flashes | Rollout switch open | Inspect heat exchanger; manual reset |
| 9 flashes | Reversed line voltage polarity | Check wiring at disconnect |
| 57 | Abnormal furnace behavior | Board fault; check all wiring |
| Slow single | Normal standby | No fault |
| Continuous on | Normal operation (heating) | — |
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.
Most Common Codes
1 Flash: System Lockout
The furnace attempted to light multiple times and locked out for safety. The most common causes are: (1) flame sensor coated with oxidation — clean with steel wool; (2) cracked igniter — resistance over 200 ohms or OL means it’s bad; (3) no gas pressure — check main shutoff and gas meter.
2 Flashes: Low-Pressure Switch Open
On Rheem Classic and Classic Plus 90%+ furnaces, this fault often traces to the condensate drain trap. A blocked trap holds water that increases back-pressure and prevents the pressure switch from closing. Pour water through the trap to verify it flows freely, or disassemble and clean it.
4 Flashes: High Limit Open
The heat exchanger temperature rose above the safety limit. Replace the air filter — Rheem recommends replacing 1” filters every 30–60 days on furnaces running continuously. Also check all supply and return registers are open and unobstructed. If the limit trips repeatedly with clean filters, the heat exchanger or blower may need professional inspection.
5 Flashes: Flame Roll-Out
Flame rolled out of the heat exchanger into the burner compartment, tripping the roll-out switch. This switch has a manual reset button located on the switch body. Before resetting, identify the cause — blocked flue, cracked heat exchanger, or excess gas pressure.
7 Flashes: Low Flame Signal
The burner is lit but the flame sensor signal is marginal. Remove the flame sensor (single screw), clean the 1–2 inch metal rod with 400-grit sandpaper, and reinstall. Replace the sensor if cleaning doesn’t improve the microamp reading above 1.5 µA.
8 Flashes: Rollout Switch Open
Similar to code 5, but specifically identifies the manual-reset rollout switch as the tripped device. Do not bypass or repeatedly reset this switch without finding root cause.
When to Call a Pro
Roll-out faults (5 or 8 flashes) and 9-flash polarity faults require attention beyond basic homeowner troubleshooting. If the furnace locks out more than twice in one heating season for the same code, schedule a diagnostic visit.