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LAST FAULT MODE

To retrieve fault codes, push and release the "LAST ERROR"

button for more than 1/5 second and less than 5 seconds.

(Control will indicate this period by solid GREEN for 1/5 to

5 seconds). The LED will flash up to five stored fault codes,

beginning with the most recent. If there are no fault codes in

memory, the LED will flash two green flashes. The control will

flash the most recent error first and the oldest error last (last in

first out). There shall be 2 seconds between codes. Solid LED

error codes will not be displayed.

DIAGNOSTIC FEATURES

The 50M51 control continuously monitors its own operation and

theoperationof the system. If a failureoccurs, the redLEDon the

control will flash a failure code.

If the failure is internal to the

control, the light will stay on. In this case, the entire control

should be replaced, as the control is not field-repairable.

If the sensed failure is in the system (external to control), the

LEDwill flash in the following flash-pause sequences to indicate

failure status (each flash will last approximately 0.25 seconds,

and each pause will last approximately 2 seconds.)

During a second-stage error condition, the red LED when in

lockout will flash groups of double pulses. The red LEDwill flash

on for approximately 1/15 second then off for 1/15 second then

on for 1/15 second, then off for 3/10 second. The pause between

groups of flashes is approximately 2 seconds.

DIAGNOSTIC TABLE

Green

LED

Flash

Amber

LED

Flash

Red LED

Flash

Error/Condition

Comments/Troubleshooting

1 Flame sensed when no flame should

be present

Verify the gas valve is operating and shutting down properly. Flame in burner

assemble should extinguish promptly at the end of the cycle. Check orifices and

gas pressure.

2 Pressure switch stuck closed/ inducer error

Pressure switch stuck closed. Check switch function, verify inducer is turning off.

3 1st-stage pressure switch stuck open/inducer

error

Check pressure switch function and tubing. Verify inducer is turning on the pulling

sufficient vacuum to engage switch.

4 Open limit switch

Verify continuity through rollout switch circuit.

5 Open rollout/open fuse detect

Verify continuity through rollout switch circuit, check fuse.

6 1st-stage pressure switch cycle lockout

if the first stage pressure switch cycles 5 times (open, closed) during one call

for heat from the thermostat the control will lockout. Check pressure switch for

fluttering, inconsistent closure or poor vacuum pressure.

7 External lockout (retries)

Failure to sense flame is often caused by carbon deposits on the flame sensor, a

disconnected or shorted flame sensor lead or a poorly grounded furnace. Carbon

deposits can be cleaned with emery cloth. Verify sensor is not contacting the

burner and is located in a good position to sense flame. Check sensor lead for

shorting and verify furnace is grounded properly.

8 External lockout (ignition recycles exceeded

where flame is established and then lost)

Check items for exceeded retries listed above and verify valve is not dropping out

allowing flame to be established and then lost.

9 Grounding or Reversed polarity

Verify the control and furnace are properly grounded. Check and reverse polarity

(primary) if incorrect.

10 Module gas valve contacts energized with no

call for heat

Verify valve is not receiving voltage from a short. If a valve wiring is correct and

condition persists, replace module.

11 Limit switch open – possible blower failure

overheating limit

Possible blower failure, restricted air flow through appliance or duct work. Verify

continuity through limit switch circuit and correct overheating cause.

12 Module Ignitor contact failure

Fault code indicates the module ignitor contacts are not functioning properly.

Replace module.

Solid Module - internal fault condition

Module contacts for gas valve not operating or processor fault. Reset control. if

condition persists replace module.

Rapid Twinning error

Check wire connections. If condition persists, replace module.

3 double 2nd-stage Pressure Switch Stuck Open/Inducer

Error

Check pressure switch function and tubing. Verify inducer is turning on and

pulling sufficient vacuum to engage switch.

1

Normal Operation with call for first stage heat

Normal operation - first stage

2

Normal Operation with call for second stage heat Normal operation - first stage

3

W2 present with no W1

Second stage call for heat on thermostat circuit with no call for first stage. Verify

dip switches are set for two stage thermostat and check thermostat first stage

circuit. Configured for a multi-stage thermostat the Module will not initiate heating

unless first stage call from thermostat is received.

4

Y present with no G call

Module will allow cooling to operate with only a "Y signal from the thermostat but

will also trigger this code. Verify thermostat is energizing both "Y" and "G" on call

for cool. Check "G" terminal connections.

Rapid

Low flame sense current

Low flame sense current is often caused by carbon deposits on the flame sensor,

a poorly grounded furnace or a mis-aligned flame sense probe. Carbon deposits

can be cleaned with emery cloth. Check for improve furnace and module ground.

Verify sensor is located in or very near flame as specified by the appliance

manufacturer.

1

Standby or Call for Cool

Normal operation. Waiting for call from thermostat or receiving thermostat

call for cool.

INTEGRATED

FURNACE CONTROLS

21M51U-843

DIAGNOSTIC TABLE

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180

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