TM9T - Flashing Code 4

York Gas Furnace: TM9T

Re: Flashing Code 4

Code 4 = Main limit switch has opened its normally closed contacts.

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

► If the limit switch has not closed within five minutes, the control will assume that the blower is not functioning, will start a hard lockout and will begin to flash the 11 Red Flashes error code. Power will have to be cycled off and on to reset the control after the problem has been corrected. See “Flashing Code 11”

► If the main limit switch opens five times within a single call for heat, the control will also indicate 4 Red Flashes and will enter a one-hour soft lockout.

CHECKOUT:

1. 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. (typically 40 - 70 deg.)
  • Temperature Rise At or Above Maximum → Step 2. 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 2. Check Air Filter
  • Temperature Rise Below Maximum but Limit Trips ⇒ Suspect Bad Limit

2. Check Air Filter

• Remove air filter and recheck temperature rise.
  • Temperature rise drops into normal range = Clogged Air Filter
  • Temperature rise still too high → Step 3. Check Gas Pressure

3. 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 4. Check Blower Speed

4. Check Blower Speed

• Check the blower motor speed wire attached to the "Lo Heat" and "Hi Heat" 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

5. 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