Goodman Gas Furnace: GMP
Re: Furnace Cycles Off and On
The gas furnace occasionally kicks off and then re-fires.
| Qty |
Possible Causes: |
|
Pressure Switch Fault |
|
Primary Limit Switch Open |
| 1 |
Low Flame Signal |
|
Bad Control Board |
|
Thermostat Issue |
CHECKOUT:
1. Check Diagnostic Indication
- Try to catch the furnace cycling-off & check the diagnostic indication on the control board.
- Furnace Cycling Off and No Diagnostic = "W" Dropping Out, Low Flame Signal or Bad Control Board
- Place a jumper between "R" and "W" and re-check.
- Furnace Runs Normally = Thermostat Issue
- Furnace Still Cycles & No Fault Indication = Low Flame Signal or Bad Control Board → Step 4. Check Flame Sensor
2. Check Pressure Switch
- Monitor for 24VAC between the two "Yellow" wires attached to the pressure switch when the furnace shuts off.
- Check the rated pressure of the switch.
- Place a pressure gauge in-line with the pressure switch hose.
- Monitor the pressure as the furnace operates.
- Pressure at or near the rating and the switch opens = Restricted Vent or Inducer
- Pressure well above the rating and the switch opens = Bad Pressure Switch
3. Check Limit Switch
- Monitor for 24VAC between the two "Red" wires attached to the limit switch when the furnace shuts off.
- Check the rated temperature rise on the furnace nameplate.
- Monitor the temperature rise as the furnace operates.
- Temperature rise at or above rating and limit opens = Low Airflow or Over-Fired
- Temperature below rating and limit opens = Bad Limit
4. Check Flame Sensor
- DO NOT CLEAN THE FLAME SENSOR BEFORE TESTING.
- On the multi-meter:
- Switch the test leads to "COM" and "mA".
- Set the meter on DC 200ù.
- Disconnect the wire from the flame sensor.
- Place one meter lead on the flame sensor terminal and the other on the flame sensor wire.
- Check the reading when the burners ignite.
- Minimum acceptable reading = 1 MicroAmp
- Low Reading ⇒ Clean Flame Sensor & Re-check
- Normal Reading (2 to 6 mA) but Shuts Off = Bad Ignition Control
- MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!