Heat call troubleshooting walk-through (two-stage communicating IFC)

What is going on

Order of checks from the factory diagnostic chart when a heat call does not end in high-fire heat.

Likely causes

• No W1/W2 or communicated call
• Dipswitch or test-mode setting
• Inducer not starting or pressure switches not closing
• No spark or no flame proof
• Blower not starting on heat speed

Check in this order

1. Communicating systems pull E1/E2 off the thermostat and jumper W1-R and W2-R to force high heat. Legacy: confirm 24 VAC on W1 and W2 at the IFC.
2. Displays should light and show a steady H (h = low heat only). Flashing H means check the dipswitches.
3. Inducer should pre-purge about 30 seconds at high speed. If it runs 60 seconds then stops for five minutes with 45, 46 or 57 showing, work the pressure switch fault.
4. Watch for spark; the gas valve is powered about 7 seconds per trial, four trials before a one-hour lockout. 11 or 13 = flame not proven, 12 = weak flame (furnace still runs).
5. The blower should come on at high heat speed roughly 15 to 20 s after the burners light; if 22 or 33 shows, check airflow, limits, rollouts and overfiring.
6. Remove the W2 call and confirm the inducer and blower switch to low; then end the call and confirm post-purge and the blower off delay.
7. To reset a lockout cut main power 30 seconds, or remove the heat call for more than 2 but less than 20 seconds (soft lockout).

More furnace guides

• Blower codes 66, 61 and 60 - what to look for
• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Communicating system not found / no communication
• ECM blower does not start on heat
• ECM blower will not run - power and communication checks (code 68)