350MAV - Flashing Code 33/Fault Code 33

Carrier Gas Furnaces: 350MAV

Re: Flashing Code 33 / Fault Code 33

Diagnostic Light Sequence: 3-Short Flashes, 3-Long Flashes (- - - , — — —)

On Carrier Gas Furnaces, the Primary Limit is wired in series with other limits, including up to two Rollout Switches or a Fusible-Link rollout 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.
  • It is common for a Carrier furnace to run the blower for 90 seconds and flash a Code 12 on start-up.
  • Code 33 Immediately = Tripped Rollout or Bad Limit Switch → Step 7. Check Limit Circuit
• Blower motor should start within 30 - 60 seconds after ignition.
  • No Blower = Bad Motor / Capacitor or Bad Board → Step 2. Check Blower Motor
  • Motor starts and runs normally → Step 3. Check Temperature Rise

2. Check Blower Motor

• Check for 115VAC between "Neutral" and "Heat" on the control board.
  • No Power = Bad Control Board
  • Power but No Blower = Bad Motor / Capacitor

3. Check Temperature Rise

• Check and record the return air temperature near the furnace (at the filter if possible) with the blower on.
• Check the supply air temperature (in a straight duct run near the furnace) as the furnace operates.
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 40 - 70 deg.)
  • Temperature Rise Near or Above Maximum → Step 4. Check Air Filter
  • Temperature Rise Below Maximum + Limit Trips = Bad Limit

4. Check Air Filter

• Remove the air filter and recheck the temperature rise.
  • Temperature Rise Drops Into Normal Range = Clogged or Under-Sized Air Filter
  • Temperature Rise Still Too High → Step 5. Check Gas Pressure

5. Check Gas Pressure

• Check the Manifold Gas Pressure.
• • 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 6. Check Duct Static Pressure
    • Pressure Above Maximum ⇒ Adjust Gas Pressure & 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

7. Check Limit Circuit

• Check for 24VAC between "24V Com" and the Red wire on Pin #6 of the 9-pin plug.
• Check for 24VAC between "24V Com" and the Red wire on Pin #8 of the 9-pin plug.
  • Power on Both + Code 33 = Bad Control Board
  • Power on One Side Only = 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!