Package Unit: YSC

Re: Heating - Code 4

Code 4 = High Limit Switch Open

► The YSC rooftop units can have either a Belt-Drive or Variable Speed Blower.

CHECKOUT:

1. Check Blower Belt (Belt-Drive Models)

• Remove the blower door and check the blower belt.
  • Replace Belt if broken or damaged.
  • Tighten Belt if loose.

2. Check Limit Circuit

• At the Ignition Control Board (ICB):
  • Check for 24VAC from Pin 9 (109A Red) to Gnd.
    • No Power ⇒ Suspect Bad Ignition Control Board (ICB)
  • Check for 24VAC from Pin 8 (108A BR) to Gnd.
    • Power + Code 4 = Bad Ignition Control Board (ICB)
    • No Power = Open Limit Circuit

3. Check Primary Limit

• The Primary Limit is located below the burners.
• Check for 24VAC between each side of the Primary Limit and GND.
  • Power on Both Sides = Limit Switch Closed → Step 4. Check Aux Limit
  • Power on One Side Only = Open Limit
    • Open Limit + Cool Chamber = Bad Limit
    • Open Limit + Hot Chamber ⇒ Allow limit to cool the re-check.

4. Check Aux Limit 

• On Direct-Drive Models, the Aux Limit is mounted on the side of the blower housing.
• You have to take the top of the unit off to access the blower housing.
• Check for 24VAC between each side of the Aux Limit and GND.
  • ​​​​​​​Power on Both Sides = Limit Switch Closed → Recheck Limit Circuit (Step 2).
  • Power on One Side Only = Open Limit
    • ​​​​​​​Open Limit + Cool Chamber = Bad Limit
    • Open Limit + Hot Chamber ⇒ Allow limit to cool the re-check.

5. Check Blower Operation

• Set the thermostat to call for full fan.
• Ensure the the blower motor starts.
• On Direct-Drive models, the fan speed is controlled by a Turn-Dial in the motor control cabinet (above the burner compartment).
  • Push the knob in and turn Clockwise to increase speed, Counter-Clockwise to decrease speed.