Rheem and Ruud furnaces use a small LED on the control board that blinks in a repeating pattern to report a fault. Count the blinks, watch for the pause, count again to confirm.
Rheem and Ruud really are the same equipment. Both are Rheem Manufacturing brands (Ruud sold through HVAC contractors, Rheem through retail and dealer channels), and their own professional tech-support sites, my.rheem.com and my.ruud.com, publish the identical flash-code reference word for word. If your furnace is a Rheem or Ruud standard (single-stage, non-modulating) gas furnace, the codes below apply to both brands equally.
The catch: that official chart only defines 5 codes, and it explicitly excludes modulating, LXE, and two-stage furnaces. If you’ve seen a longer list elsewhere online with 6 through 9 blinks, that’s not confirmed by Rheem’s or Ruud’s own documentation. More on that below.
How to read the flash code
- Open the lower door panel (slides off or unclips on most Rheem/Ruud units).
- Find the diagnostic LED on the main control board, visible through a small sight glass or opening in the blower compartment.
- Watch the blink pattern: a count of flashes, a pause of a few seconds, then the same count repeats.
- Count twice to confirm before acting on it.
Steady light, no flashing: furnace running normally or in standby. No light at all: no power reaching the control board. Check the breaker, the door interlock switch, and the fuse on the board itself before assuming the board has failed.
Rheem/Ruud standard furnace flash codes: the verified chart
This is Rheem’s and Ruud’s own “Standard Gas Furnace Flash Code” table, unchanged from their tech-support documentation.
| Flashes | Fault | What it means | DIY or call? |
|---|---|---|---|
| 1 | Soft lockout (1 hour) | The board attempted and failed to complete ignition, and has locked out heat calls for one hour before it will retry automatically. | DIY: diagnose the cause below |
| 2 | Pressure switch failure (remains open) | The switch that confirms the inducer is drafting properly isn’t closing when it should. | DIY check |
| 3 | Limit switch open | A high-limit or rollout safety switch has opened, usually from overheating or restricted airflow. | DIY first |
| 4 | Pressure switch failure (remains closed) | The pressure switch is reading closed before the inducer has even started. | DIY check |
| 5 | Twinning circuit failure | Only applies to boards with a twinning feature (two furnaces paired to one system). Rheem’s own field notes add that this code can also appear on non-twinning boards if an electronic (non-mechanical) thermostat is installed. | DIY check |
Confirm against your own door sticker. Boards vary by production year and by exact model (Rheem lists several different Integrated Furnace Control board part numbers across its furnace lineup), so the label inside your specific unit is the final authority if it conflicts with anything here.
Code-by-code: what to check first
1 flash: soft lockout
The furnace tried to light and couldn’t, then stopped trying for an hour as a safety measure. It will attempt ignition again on its own once the hour is up. Use the wait to check the likely causes: a closed gas valve, a dirty flame sensor, or a failed igniter (see the ignition troubleshooting guides linked below). If you don’t want to wait out the hour, cutting power at the furnace switch for 30 seconds and restoring it forces an earlier retry, but only do that once you’ve addressed a likely cause, or it’ll just lock out again.
2 flashes: pressure switch failure (remains open)
The inducer is running, but the switch that should close once draft pressure is established isn’t closing.
Check:
- Failed or seized inducer motor. Listen for it running when the furnace calls for heat.
- A blocked flue or blocked condensate drain (on 90%+ AFUE units) restricting draft. Clear or white PVC tubing running from the furnace to a drain is the condensate line; a full trap points to a downstream blockage.
- A disconnected, kinked, or cracked pressure hose between the switch and the inducer housing.
- On 80% AFUE furnaces with no condensate drain, check the flue pipe itself for blockage (bird nests, ice, debris at the rain cap).
3 flashes: limit switch open
A high-limit or rollout safety switch tripped, most often from overheating.
Most common cause: a dirty air filter. Replace it first. Restricted airflow across the heat exchanger is the leading cause of nuisance limit trips. After a filter change, cut power for 30 seconds and restore; the switch should reset on its own once the heat exchanger cools.
If the filter is clean and the code returns:
- Confirm all supply registers in the house are open.
- Check the return air intake isn’t blocked by furniture, rugs, or debris.
- With power off, spin the blower wheel by hand; heavy dust buildup cuts airflow significantly.
- If airflow checks out and the code keeps returning, that pattern is worth a professional look rather than repeated resets. A recurring limit trip can point to a failing blower motor or, on older units, a heat exchanger issue.
4 flashes: pressure switch failure (remains closed)
The pressure switch is reading closed before the inducer has even started, which the board reads as a stuck contact.
Check:
- Disconnect the rubber pressure hose from the switch and blow through it. Water coming out means condensate is backing up into the hose; clear the condensate drain first.
- Inspect the hose itself for a kink or collapsed section that could hold false pressure.
- With the furnace off, test the switch for continuity across its two terminals. There should be none at rest; continuity means the switch is stuck and needs replacing (roughly $15–25 at an HVAC supply house).
5 flashes: twinning circuit failure
If your system doesn’t use twinned furnaces, this code is most often triggered by an electronic (non-mechanical) thermostat interacting with the twinning terminals on the board, per Rheem’s own field notes, not a hard failure. Check your thermostat wiring against the board’s twinning jumper/terminal configuration in the installation manual before assuming a board fault.
What about 6, 7, 8, and 9 flashes?
You’ll find longer lists on other sites claiming 6 flashes means a polarity fault, 7 a gas valve circuit fault, 8 an ignitor failure, and 9 a rollout switch trip. We can’t verify those against Rheem’s or Ruud’s own published documentation. Their official “Standard Gas Furnace Flash Code” reference stops at 5, and different sites that list a 9-code version don’t agree with each other on what 2 through 4 mean, which is a sign of copied, unverified content rather than a shared manufacturer source.
Two things are likely going on if your board shows more than 5 flashes:
- Your furnace is modulating, LXE, or two-stage. Rheem’s high-efficiency modulating models (RGFE, RGFG, and similar) don’t use simple LED blink counts at all; they display alphanumeric fault codes on a digital readout instead.
- Your board is a different revision or a commercial-grade unit with its own troubleshooting guide printed on the door sticker.
Either way, the door sticker inside your specific furnace, or the installation manual for your exact model number, is the correct source, not a generalized number-to-fault mapping. If your furnace does show rollout, gas valve, or ignition symptoms (flames outside the burner box, no gas reaching the burners, or repeated lockouts), those are real fault categories on Rheem/Ruud equipment regardless of which blink count your specific board uses for them. Treat them with the same urgency described in the safety box below.
Safety box: if you smell gas, see flames outside the burner box (rollout), or the furnace repeatedly locks out after you’ve fixed the obvious cause, stop troubleshooting. Close the manual gas shutoff valve near the furnace and call a licensed HVAC technician. A rollout event or a gas valve that won’t fully close can point to a cracked heat exchanger or a gas leak. Both are beyond a DIY fix, and a rollout switch should never be reset without a technician finding the cause first.
FAQ
Are Rheem and Ruud furnace flash codes really identical? Yes, for standard (non-modulating) furnaces. Rheem’s and Ruud’s own tech-support sites publish the exact same “Standard Gas Furnace Flash Code” table on both my.rheem.com and my.ruud.com: same 5 codes, same wording. Both brands come from Rheem Manufacturing and use the same control boards.
How do I reset a Rheem or Ruud furnace after a lockout? Turn the furnace’s power switch off for 30 seconds, then restore it. That clears a soft lockout and lets the board attempt ignition again. If the underlying cause (dirty flame sensor, no gas, failed igniter) isn’t fixed, it will lock out again.
My furnace has two LEDs, not one. How do I read it? Some Rheem/Ruud boards pair a status LED with a separate flame-sensing LED (on when flame is detected, off when it isn’t). Check your door sticker — it documents your board’s specific LED layout, since this varies by control board revision.
Why do other sites list 9 different flash codes instead of 5? Most aggregate multiple furnace lines and board generations into one list without citing a source, and they disagree with each other on what codes 2 through 4 mean. Rheem’s and Ruud’s own published chart only defines 5 codes for standard furnaces; treat anything beyond that as unverified unless your specific door sticker confirms it.
Related guides
- Rheem HVAC: Brand Overview, Reliability, and Repair Help: product lines, Prestige vs. Classic tiers, serial number age lookup, and Rheem vs. Ruud explained
- Furnace Not Heating: Diagnostic Order: if no fault code is showing and the furnace simply isn’t heating
- Furnace Flame Sensor: Symptoms and Cleaning: for ignition lockouts traced to a dirty flame sensor
- Furnace Hot-Surface Igniter: Symptoms and Replacement: for a lockout with no igniter glow through the sight glass
Last reviewed: August 17, 2026.
