Ameristar Package Unit: M4PG4

Re: Flashing Code 4

Code 4 = Main Limit switch or Aux Limit (not a Roll-Out Limit) has opened its normally closed contacts.

• The control will operate the supply air blower and inducer while the open limit condition exists.

► The manual-reset Aux Limit is located in the blower compartment.

► The automatic-reset Main Limit Switch is located in the main burner compartment.

CHECKOUT:

1. Check Limits

• At the 9-pin plug of the control board:
  • Check for 24VAC between the Purple wire at Terminal 8 and "C".
  • Check for 24VAC between the Red wire at Terminal 9 and "C".
    • Power on Both Terminals = Limit Circuit OK → Step 2. Check Temperature Rise
    • Power on One Terminal Only = Open Limit Circuit
      • Check for 24VAC between each Red wire of the Main limit
        • Power on One Terminal Only = Open Limit → Step 2. Check Temperature Rise
      • Check for 24VAC between each Red wire of the Aux limit
        • Power on One Terminal Only = Open Limit ⇒ Reset by pressing the center button on the limit → Step 5. Check Blower Motor & Blower Speed

2. Check Temperature Rise

• 1st Stage Heat
• Un-hook the thermostat wire from W2 at the furnace.
• Start the furnace with a call for heat.
• Check the temperature rise across the furnace.
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. 
  • Temperature Rise At or Above Maximum → Step 3. Check Air Filter
  • Temperature Rise Below Maximum but Limit Trips ⇒ Suspect Bad Limit
• 2nd Stage Heat
• Reconnect the thermostat wire to W2 at the furnace.
• Place a temporary jumper between W1 & W2.
• Start the furnace with a call for heat.
• Check the temperature rise across the furnace.
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 40 - 70 deg.)
  • Temperature Rise At or Above Maximum → Step 3. Check Air Filter
  • Temperature Rise Below Maximum but Limit Trips ⇒ Suspect Bad Limit

3. Check Air Filter

• Remove air filter and recheck temperature rise.
  • Temperature rise drops into normal range = Clogged or Restrictive Air Filter
  • Temperature rise still too high → Step 4. Check Gas Pressure

4. Check Gas Pressure

• De-energize the call for heat and allow the furnace to cool.
• Check the manifold gas pressure.
• 1st Stage Heat
  • Natural Gas = 1.5 to 2" w.c.
  • LP Gas = 6" to 8" w.c. 
• 2nd Stage Heat
  • Natural Gas = 2.5 to 3.5 " w.c.
  • LP Gas = 9" to 11" w.c. 
    • Pressure Above Maximum ⇒ Adjust Gas Pressure & Re-check Temperature Rise
    • Pressure Below Maximum + Still Over-Heating → Step 5. Check Blower Speed

5. Check Blower Motor & Blower Speed

• Check the Blower motor speed wire attached to the "Low" and "Single/Hi" outputs of the control board.
  • Red = Low
  • Yellow = Med Low
  • Gray = Med
  • Blue = Med Hi
  • Black = Hi
• If possible, increase the Heat blower speed and Re-Check Temperature Rise

6. Check Duct Static Pressure

• Check the return static pressure (in the blower door if possible).
• Check the supply static pressure (between the furnace and evaporator coil if possible).
  • High Duct Static = Above 0.5" w.c.
    • High Return Static + Low Supply Static = Restricted Return Ductwork
    • Low Return Static + High Supply Static = Clogged Coil or Restricted Supply Ductwork
      • Compare static on both sides of evaporator coil.
        • High Static Entering Coil + Low Static Leaving Coil = Clogged Coil