White-Rodgers 21V51U Flash Codes: Red, Amber and Green

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

White-Rodgers 21V51U Flash Codes: Red, Amber and Green

Short answer

On the White-Rodgers 21V51U two stage furnace board, one green flash is standby, amber 1 and 2 are normal first and second stage heat, and red counts are faults: 3 is the first stage pressure switch, 4 the limit, 5 the rollout or fuse, 7 no flame after the retries. White-Rodgers warns its codes may differ from the chart on the furnace door.

The White-Rodgers 21V51U is a replacement control kit for two stage gas furnaces with a variable speed blower. Its control board, the 50V51, has one light that shows green, amber or red, and each colour counts out a different kind of message.

Everything on this page comes from White-Rodgers’ own installation instructions for the 21V51U-843 kit, published by Copeland, which now owns the brand. White-Rodgers adds one warning that matters: these codes may be different from the chart on the furnace door, because this board replaced the one that chart was written for.

Before you start

What to carry: a torch and a pen. Ten minutes.

Gas or electric? Gas, natural gas or propane, with a hot surface igniter.

Check it is this board. The kit’s control module is the 50V51-843, so look for that number on the board. For single stage White-Rodgers boards see the 50M56U codes.

One red flash at power up is normal. White-Rodgers says the light flashes red once when power comes on.

Where you stop. Red 1: flame when there should be none. White-Rodgers says to verify the gas valve is shutting down properly and the flame goes out promptly at the end of the cycle. Turn the gas off and call a technician.

Red 5: the rollout switch or the fuse. A rollout switch opens when flame comes out of the burner area. Find out why before anyone resets it: what a rollout switch is telling you.

Solid red. White-Rodgers says the fault is inside the control. Reset it once, and replace the module if it stays.

What the colours mean

Green and amber are the board saying all is well. Red is a count of the fault.

Check 1Free, two minutes

Read the light

Green 1: standby. Waiting for the thermostat, or a call for cooling. Normal.

Amber 1 and 2: heating. Normal first stage heat, and normal second stage heat.

Amber 3: W2 with no W1. The thermostat is calling second stage without first. White-Rodgers says to check the DIP switches are set for a two stage thermostat, and the first stage wire.

Amber 4: Y with no G. Cooling still runs, but White-Rodgers says the thermostat should energize both Y and G. Check the G connection.

Rapid amber: low flame current. White-Rodgers names carbon on the flame sensor, poor grounding, or a sensor out of position: how to clean a flame sensor safely.

The red codes

Red flashes	What White-Rodgers says
1	Flame sensed when no flame should be present
2	Pressure switch stuck closed, or inducer error
3	First stage pressure switch stuck open, or inducer error
3 double	Second stage pressure switch stuck open, or inducer error
4	Open limit switch
5	Open rollout switch, or open fuse
6	First stage pressure switch cycled five times in one call for heat
7	Lockout, retries exceeded: no flame sensed
8	Lockout, recycles exceeded: flame lit and then lost
9	Grounding or reversed polarity
10	Gas valve contacts energized with no call for heat
11	Limit open, possibly the blower failing
12	Igniter contacts on the module not working
Solid	Internal fault in the module

The checks that are yours

Check 2

Work the owner checks

Red 3, 3 double and 6: the pressure switch. White-Rodgers says to check the switch and its tubing, and that the inducer is pulling enough vacuum. Look at the vent outside for blockage: the places a pressure switch fault hides.

Red 4 and 11: the limit. For 11, White-Rodgers names a possible blower failure or restricted airflow through the furnace or the ducts. Start with the filter: why a furnace overheats and cuts out.

Red 7 and 8: no flame. White-Rodgers says a failure to sense flame is often carbon on the flame sensor, a disconnected or shorted sensor lead, or a poorly grounded furnace. It cleans the sensor with emery cloth. Check the gas is on first.

Red 9: grounding or polarity. Wiring, which is an electrician’s job.

Reading the history and resetting

White-Rodgers' Last Error button plays back up to five codes, newest first. Two green flashes means none are stored.

Check 3Free, five minutes

Read the history, then reset

Last Error. Push and release the LAST ERROR button for between a fifth of a second and five seconds. The light shows solid green while you hold it.

What plays back. Up to five stored codes, newest first, two seconds apart. Two green flashes means there are no codes in memory. White-Rodgers says solid light codes are not played back.

Reset at the power. Remove the 24 volt power to the control for 20 seconds or longer.

Reset at the thermostat. Remove the call for heat for between one and 20 seconds. White-Rodgers says this does not reset the control if flame was sensed with the gas valve off.

Or wait. After one hour of lockout, the control resets itself and runs a 15 second purge.

Questions people ask

What does 3 red flashes mean on a White-Rodgers 21V51U?

The first stage pressure switch is stuck open, or the inducer is not pulling enough vacuum. Check the vent, the tubing and the inducer. A double 3 is the second stage switch.

Is a flashing amber light a fault?

Amber 1 and 2 are normal first and second stage heating. Amber 3 and 4 are thermostat wiring messages, and a rapid amber flash is low flame current.

Why do the codes not match the chart on my furnace?

White-Rodgers says its codes may differ from the furnace label or manual. The 21V51U replaced the original board, so use its own list.

How do I see the last faults?

Press and release LAST ERROR for under five seconds. Up to five codes play back, newest first. Two green flashes means none are stored.

How do I reset it?

Cut the 24 volt power for 20 seconds, or remove the call for heat for a few seconds, or wait an hour for the automatic reset.

What does solid red mean?

A fault inside the control. White-Rodgers says to reset it, and replace the module if the condition stays.

Sources: White-Rodgers (Copeland) installation instructions 37-7115002 (replaces 37-7115D) for the 21V51U-843 integrated two stage, variable speed hot surface ignition control kit