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.

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

1. Confirm the model number begins RC or RL. On a standard Rheem furnace four blinks is a pressure switch stuck closed, an unrelated fault.
2. Confirm the gas supply is on, including any manual valve at the furnace and the meter.
3. Note whether the burners light at all before shutting down, which separates an ignition failure from a flame sensing failure.
4. 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

1. Opening the thermostat for at least ten seconds resets the lockout, which is safe and is Rheem's own documented method.
2. If it locks out again, stop resetting it. Repeated ignition attempts put unburnt gas into the heat exchanger each time.
3. Cleaning the flame sensor is the most common repair and is technician work in the burner compartment.
4. Ground and polarity should be confirmed before any part is replaced, since both produce this fault with everything else intact.

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.

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.

Sources

• Rheem Gas Furnaces Installation Instructions 708503a — *RC 92+ Full Size Upflow and *RL 90+ Full Size Downflow Condensing Furnaces, Sequence of Operation