THM / DGH FC2 Fault Code – Combustion Airflow & Flue Fault

(Quick Check & Diagnostics)

✅ 1‑Minute FC2 Quick Check (Do This First)

Use this checklist before deep diagnostics. It resolves most FC2 faults on the first visit.

If FC2 is displayed:

  1. Check the flue terminal (outside)

    • Is anything blocking the terminal (debris, bird nests, plastic, insulation)?

  2. Check pressure switch hoses (inside heater)

    • Are all small rubber hoses connected to the combustion fan and pressure switch?
    • Are any hoses split, brittle, kinked, or left off after service?

  3. Listen to the combustion fan

    • Does it start promptly and sound strong?
    • Any scraping, slow start, or unusual noise?

  4. Reset and retry

    • Reset the fault at the SCC
    • If FC2 returns immediately → airflow or flue issue still present

Remember: FC2 occurs before gas lighting. It is never a gas supply problem.

If FC2 is not resolved, continue with the detailed steps below.

Applies to

  • Braemar THM Ducted Gas Heaters
  • Braemar DGH systems using THM heaters
  • Units post‑serial T12071732
  • Systems using Spectrolink Comfort Control (SCC)

SCC (Spectrolink Comfort Control)

SCC stands for Spectrolink Comfort Control.

It is the Braemar wall controller used on THM and DGH systems to:

  • Display active fault codes (e.g. FC2)
  • Store and access fault history (last 10 faults)
  • Enter Service Mode for diagnostics
  • Reset Lockout 1 and Lockout 2
  • Monitor heater operation and system settings

The SCC is the primary diagnostic interface for service technicians.


Click here to view YouTube video Spectrolink SCC accessing and clearing fault code history



Fault Description – FC2

FC2 indicates the 25/50% (45 Pa) combustion air pressure switch failed to close during the ignition sequence.

The heater did not confirm sufficient combustion airflow after the combustion fan started, so ignition was inhibited.


What the Heater Is Expecting

During a normal start sequence:

  1. Combustion fan starts
  2. Suction pressure increases
  3. 25/50% (45 Pa) pressure switch closes
  4. Ignition sequence proceeds

If step 3 does not occur, FC2 is generated and the heater enters Lockout 1.

Lockout Type

  • Lockout 1
  • Resettable via SCC or power reset

If the cause is not rectified, FC2 will return immediately after reset.

Common Causes (Most Likely First)

  1. Restricted combustion airflow
  2. Flue blockage or incorrect flue installation
  3. Pressure switch suction hose disconnected, split, or blocked
  4. Weak or failing combustion fan
  5. Faulty 25/50% (45 Pa) pressure switch
  6. Wiring issue to pressure switch or MCB

FC2 Explained

How a ducted gas heater works

For safe operation, the heater must confirm:

  1. Air is moving through the heater
  2. Exhaust gases can escape via the flue
  3. Gas only lights after airflow is proven

Before lighting gas, the heater performs an airflow safety check.

What the combustion fan does

  • Pulls in air for combustion
  • Pushes exhaust gases out through the flue
  • Creates suction inside the combustion chamber

What the pressure switch does

The 45 Pa pressure switch confirms that the combustion fan is moving air.

Think of it as a proof‑of‑airflow safety switch.

✅ Airflow proven → heater lights
❌ No airflow → heater shuts down with FC2

Why FC2 occurs

FC2 means:

“The heater started the combustion fan but did not see enough airflow to safely light the gas.”

Important safety note

The heater is operating correctly when FC2 occurs.
It is preventing:

  • Overheating
  • Flame rollout
  • Exhaust gases entering the dwelling

Bypassing safety devices is not permitted.

Key takeaways

FC2 is almost always an airflow or flue problem — not a gas problem.

Start with:

  1. Flue condition
  2. Combustion fan operation
  3. Pressure hose integrity

Common Mistakes We See

1. Flue cowl is obstructed

  • Partially blocks exhaust airflow
  • Common after roof or heater servicing

Fix: Remove any cowl obstructions.

2. Pressure switch hose left off

  • Hose not reconnected after servicing
  • Hose on wrong port

Fix: Refit and secure all hoses.

3. Split or brittle pressure tubing

  • Hardened or cracked hoses
  • Kinked or pinched tubing

Fix: Replace tubing and avoid tight bends.

4. Combustion fan runs but is weak

  • Slow start, noisy, or seized bearings

Fix: Replace fan if performance is poor.

5. Partial flue blockage

  • Bird nests, debris, condensate buildup

Fix: Inspect the entire flue path, not just the terminal.

6. Assuming it’s a gas fault

  • Gas valve or regulator replaced unnecessarily

Fix: Remember — FC2 occurs before gas ignition.

Step‑by‑Step Diagnostic Procedure

Step 1 – Confirm the Fault and Check History (SCC)

  • Confirm FC2 is active on the SCC
  • Enter Service Mode (SCC OFF → open cover → hold ZONE 3 for 5 seconds)
  • Review stored fault history
  • Do not clear history until diagnostics are complete

Step 2 – Visual Inspection

  • Flue clear and correctly fitted
  • Flue cowl position correct
  • Heater panels fitted
  • No damaged tubing or wiring

Step 3 – Pressure Switch Tubing

  • Hoses connected, intact, and clear
  • Fan pressure tapping not blocked

Step 4 – Combustion Fan

  • Starts promptly
  • Runs smoothly and strongly

Step 5 – Flue System

  • Check for obstruction, collapse, or condensate

Step 6 – Pressure Switch Test

  • Open circuit with no suction
  • Closed circuit with suction
Pressure switch check: Power OFF and wires removed from spade terminals → meter across switch → no suction = open, gentle suction on hose = closed → if it doesn’t change, replace switch; if it does, check airflow/flue.

Step 7 – Electrical Checks

  • Wiring to MCB secure
  • No damaged terminals


Additional Checks for THM heaters Pre-Serial Number T12071732 

If the fault code experienced is FC02 and the combustion/flue fan does not start when the thermostat is calling for heat, along with the usual pressure switch checks, we recommend you check the combustion fan relay terminals of the Fenwal EFS – disconnect power supply to heater, remove wires from IND and L1 terminals from Fenwal and check for continuity between IND and L1. 

The normal condition with power disconnected from the heater is open circuit.


If you have a closed circuit between IND and L1 (continuity detected) replace with one of the following kits (instructions are included) -   

• Kit 642161 to suit THM5’s   

• Kit 642178 to suit THM6’s   

IMPORTANT: Kits are suitable for units Pre Serial Number T12071732


Click below link to view solution article:

THM Ducted Gas Heater FC2: Seeley International Pty Ltd 

After Repair Checks

  • Reset lockout
  • Run multiple start cycles
  • Confirm FC2 does not reoccur