Rheem Furnace 5 Flashes Error Code - Flame Sensor Fault Fix

Error Code: Rheem Furnace 5 Flashes

What it means: Five LED flashes on a Rheem, Ruud, or WeatherKing furnace indicate a flame sense fault. The control board attempted ignition, the burner lit, but the flame sensor failed to send a valid signal back to the board confirming stable flame. After two or three ignition attempts with no confirmed flame signal, the board locks out and blinks 5 times in a repeating sequence.

This is one of the most common furnace faults in the field. The flame sensor itself is usually the culprit — it’s a $15–$25 part that corrodes over time — but don’t replace it blindly. A cracked heat exchanger, gas pressure issue, or failed control board can produce the same lockout.

Common Causes

• Dirty or oxidized flame sensor rod — The most common cause by far. A thin layer of oxidation on the sensor’s stainless rod insulates it enough to drop the microamp signal below the board’s 1–2 µA detection threshold.
• Cracked or broken flame sensor — Physical cracks in the ceramic insulator cause shorts to ground, killing the signal.
• Weak gas pressure — Low inlet or manifold gas pressure produces a small, unstable flame that doesn’t fully engulf the sensor rod, causing intermittent detection.
• Incorrect flame sensor position — If the sensor rod is not sitting in the burner flame (bent, misaligned, or wrong replacement part), it will never read correctly.
• Failed control board — If a new sensor doesn’t resolve the fault, the sensing circuit on the board itself may be defective.

Step-by-Step Fix {#step-by-step-fix}

1. Cut power to the furnace. Turn the thermostat to OFF, then flip the furnace power switch or circuit breaker. Wait 60 seconds for capacitors to discharge before opening the access panel.

2. Locate the flame sensor. On most Rheem 80% and 90%+ units (Classic, Classic Plus, Prestige series), the flame sensor is a single rod mounted in a ceramic insulator at the left side of the burner assembly, directly in the path of the flame. It connects to the control board via a single orange or white wire with a push-on terminal.

3. Disconnect the sensor wire and remove the sensor. Use a 1/4” hex driver to remove the single mounting screw. Pull the sensor out carefully — the ceramic insulator is brittle.

4. Inspect the sensor rod. Look for a dull white, gray, or greenish coating on the stainless steel rod. That’s oxidation. Also check the ceramic for cracks.

5. Clean the rod with fine steel wool or emery cloth (#400 or finer). Polish the rod until it’s bright and shiny. Do NOT use sandpaper with a grit coarser than 400 — you’ll scratch the rod and accelerate re-oxidation. Do NOT use spray cleaners or lubricants on the rod.

6. Reinstall the sensor and restore power. Reconnect the wire, reinstall the screw snugly (do not overtighten — you can crack the ceramic), and close the access panel. Restore power and cycle the thermostat through a heat call. Watch for normal ignition: inducer on → hot surface igniter glows → gas valve opens → burner lights → flame sensor confirms → blower starts.

7. If the fault returns within one heating season: Replace the sensor outright. The Rheem OEM replacement is part number 42-24195-01 (~$20). Verify the part number against your model’s wiring diagram — some units use a different sensor geometry.

8. Check gas pressure if a new sensor doesn’t resolve the fault. Connect a manometer to the manifold port. Rheem furnaces typically require 3.5” W.C. manifold pressure for natural gas. Low pressure = call the gas company or a licensed tech.

Parts That May Need Replacement {#parts-that-may-need-replacement}

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 Professional

If you’ve cleaned or replaced the flame sensor and the 5-flash lockout persists, you’re dealing with either a gas pressure problem, a cracked heat exchanger, or a failed control board — none of which are safe or simple DIY repairs. A cracked heat exchanger is a carbon monoxide hazard and requires immediate shutdown of the unit. A licensed HVAC technician can measure microamp output at the sensor terminal (should read 2–6 µA on a healthy Rheem system), verify gas pressure at manifold, and perform a combustion analysis. Don’t run the furnace in lockout-reset cycles while troubleshooting — repeated ignition attempts without resolution can damage the igniter.

Pro tip: Before ordering a replacement flame sensor, measure the microamp signal with a multimeter set to DC microamps in series with the sensor wire. A reading below 1.5 µA confirms a bad sensor. A reading of 0 µA often means a broken wire or cracked ceramic. A reading above 1.5 µA but still faulting usually points to the control board’s sense circuit — save yourself a wasted parts order.

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