Rheem furnace 4 blinks: Lockout after five failed ignition attempts
The furnace tried to light five times, failed to prove flame each time, and locked out. It will retry in an hour by itself, or immediately if the thermostat is opened for ten seconds.
- Pressure switch failure — switch remains closed — Rheem standard gas furnaces
Applies to: Rheem, Ruud and Weather King *RC 92+ full size upflow and *RL 90+ full size downflow condensing furnaces, per installation instructions 708503a. On Rheem's standard gas furnaces four blinks means a pressure switch stuck closed instead.
What it means
The control must prove flame through the flame sensor five seconds after the gas valve opens. If it does not, the gas valve closes immediately and the inducer keeps running, and a new trial begins. Rheem's instructions specify that on the fifth try the control locks out and blinks four times repetitively. Two details on this control are worth knowing. The reset is explicit: the thermostat has to be opened for at least ten seconds to clear the lockout, otherwise the furnace attempts another ignition sequence in one hour on its own. And the warm-up time is adaptive — it shortens by three seconds on each successful ignition until a failure occurs, then lengthens by three seconds, within limits of six and fifty-four seconds. A furnace that has been slowly drifting toward failure will therefore already be running a longer igniter warm-up before it ever locks out.
Most common causes
- A flame sensor coated in oxide, so flame is present but not detected.
- No gas reaching the burners — a closed manual valve or an interrupted supply.
- An igniter that glows but is out of position, or one that has failed.
- A poor chassis ground or reversed line voltage polarity, since flame rectification depends on both.
- A gas valve not opening when energised.
- Burners partially blocked so the flame does not reach the sensor.
How to diagnose it
- Confirm the model number begins RC or RL. On a standard Rheem furnace four blinks is a pressure switch stuck closed, an unrelated fault.
- Confirm the gas supply is on, including any manual valve at the furnace and the meter.
- Note whether the burners light at all before shutting down, which separates an ignition failure from a flame sensing failure.
- Note whether the furnace has been taking longer to light over recent weeks, which fits the adaptive warm-up behaviour described above.
How to fix it
- Opening the thermostat for at least ten seconds resets the lockout, which is safe and is Rheem's own documented method.
- If it locks out again, stop resetting it. Repeated ignition attempts put unburnt gas into the heat exchanger each time.
- Cleaning the flame sensor is the most common repair and is technician work in the burner compartment.
- Ground and polarity should be confirmed before any part is replaced, since both produce this fault with everything else intact.
Do it yourself or call someone
Two things on this page are homeowner work, and only two. Confirming the gas supply — the manual valve at the furnace, the meter or tank — is the first, and Rheem's documented reset is the second: open the thermostat for at least ten seconds, once, after fixing anything obvious. Because this control makes five ignition trials per sequence, a single reset already commits the furnace to up to five more gas-valve openings over a burner that just failed five times; doing that repeatedly to chase a warm house pushes unburnt gas into the heat exchanger over and over, which is exactly the scenario the lockout exists to prevent. If the second lockout arrives, the useful homeowner contribution becomes history rather than action — mention whether the furnace has been taking noticeably longer to light in recent weeks, since this control's adaptive igniter warm-up stretches before a failure and that detail corroborates a sensor or igniter drifting rather than a sudden supply fault. Flame sensor cleaning, igniter position and resistance checks, ground and polarity verification, and anything touching the gas valve are burner-compartment work for a licensed technician.
Notes
Five trials is more generous than the three most manufacturers allow, so a furnace reaching this lockout has failed more times than a Goodman or Carrier would have tolerated. The retry allowance varies by brand more than the fault does: ICP's SmartValve furnaces lock out after four trials, Goodman's one-flash lockout follows the common three-trial pattern, and Trane's two-flash code names the event directly: retries exceeded. Carrier stores the same state as code 14, and Lennox wraps it in its Watchguard logic as alert 270. Different counts, different trial budgets, one underlying event: gas offered, flame never proven.
Related
- 4 blinks — Pressure switch failure — switch remains closed
- Which blink code list applies — Rheem publishes two blink code lists that contradict each other
All codes for this manufacturer
Rheem, Ruud and Weather King furnace blink codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 4 flashes means on every brand puts them side by side.
What the fix costs
The most common repair behind this lockout is also the cheapest: flame sensor cleaning at $80–200, which the furnace repair cost guide describes as a ten-minute job. If the igniter has failed, the igniter replacement cost guide puts a standard-visit install at $150–350 — the part is $20–60 for a universal silicon nitride igniter or $40–120 for brand OEM — and an after-hours emergency install at $250–500, the cold-night premium this hourly-retry lockout tends to invite. A gas valve that will not open is $300–900 and never a DIY part. Ground and polarity problems, which produce this code with every part healthy, usually cost only the $75–150 diagnostic to find. More codes in this family: igniter fault codes.
What this looks like
From the outside, this usually shows up as the furnace turns over but never lights — that walkthrough covers what to check in order, and when it stops being a homeowner job.
This can also show up as the furnace not turning on at all — that page is a 5-minute checklist before calling anyone.