Carrier Gas Furnaces: 395CAV

Re: Flashing Code 33 / Fault Code 33

Flash Sequence = 3 Short Flashes + 3 Long Flashes (- - - , — — —)

Code 33 = Open Limit or Roll-Out

The Primary Limit is wired in series with other limits, including up to two Roll-out Switches or a Fusible-Link roll-out switch.

Therefore, a Fault Code 33 can be caused by one of the following:

• Primary Limit Switch Open.
• Rollout Switch Open

► If the limit circuit stays open for more than 3 minutes, the code changes to Code 13.

CHECKOUT:

1. Check Furnace Operation

• Start the furnace with a call for heat.
  • If there is power on "W" at start-up, the furnace will run the blower for 90 seconds and flash a Code 12.
  • Code 33 Immediately or Before Burners → Step 6. Check Limit Circuit
• Blower motor should start within 30 - 60 seconds after ignition.
  • Blower Never Starts → Step 2. Check Blower Motor
  • Motor starts and runs normally → Step 3. Check Temperature Rise

2. Check Blower Motor

• Restart an ignition sequence and allow burners to ignite.
• Check for 115VAC between "Neutral" and "Heat" on the control board.
  • No Power = Bad Control Board
  • Power + No Blower = Bad Motor or Capacitor 

3. Check Temperature Rise

• Check the temperature rise as the furnace operates.
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 40 - 70 deg.)
  • Temperature rise below maximum + Limit Trips = Bad Limit
  • Temperature rise at or above maximum ⇒ Remove the air filter and recheck the temperature rise.
    • Temperature rise drops into normal range = Clogged Air Filter
    • Temperature rise still too high → Step 4. Check Gas Pressure

4. Check Gas Pressure

• Monitor the manifold gas pressure when gas valve opens.
  • Natural Gas = 2.5" w.c. minimum to 3.5" w.c. maximum
  • LP Gas = 9" w.c. minimum to 11" w.c. maximum
    • Pressure below maximum → Step 5. Check Duct Static Pressure
    • Pressure above maximum ⇒  Adjust Gas Pressure & 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

6. Check Limit Circuit

• Check for 24VAC between "24V Com" and the RED wire on Pin #1 of the 9-pin plug.
• Check for 24VAC between "24V Com" and the RED wire on Pin #7 of the 9-pin plug.
  • Power on Pin #1 + Power on Pin #7 + Code 33 = Bad Control Board
  • Power on Pin #1 + No power on Pin #7 = Open Circuit --> Check Each Switch
    • Primary Limit
    • Check for 24VAC between "24V Com" and both sides of the Primary Limit Switch.
    • Power on Both Sides = Limit OK
    • Power on One Side Only = Open Switch
      • Heat Exchanger Cool = Bad Limit
      • Heat Exchanger Hot ⇒ Allow time to cool and re-check.
    • Rollout Switch
    • Check for 24VAC between "24V Com" and both sides of the Rollout Switch(es).
    • Power on Both Sides = Limit OK
    • Power on One Side Only = Open Switch ⇒ Reset switch and Check Heat Exchanger!