Gas Furnace Technician Repair Chart

York / Coleman Gas Furnace Sequence of Operation Flow Chart

Thermostat calls for heat ↓

Inducer motor starts (establishes draft) ↓

Pressure switch proves draft (safety check) ↓

Ignition sequence begins (hot surface ignitor or spark) ↓

Gas valve opens ↓

Burner ignition ↓

Flame sensor proves flame ↓

Blower motor starts after delay ↓

Furnace runs until thermostat is satisfied ↓

Gas valve closes & burners shut off ↓

Blower runs post‑purge (clears residual heat/gases) ↓

Blower shuts off → Cycle complete

If at any point the furnace quits during operation the first thing to check is the control board to see if there are any LED lights blinking on it --> See Flash Code Chart

Coleman / York Furnace Flash Code Chart

Flash Code	Result (Meaning)	Fix (Recommended Action)
Slow Green Flash	Normal operation	No action needed
Slow Amber Flash	Normal operation with call for heat	No action needed
Rapid Red Flash	Twinning error, incorrect 24V phasing	Check twinning wiring
Rapid Amber Flash	Flame sense current < 1.5 µA	Clean flame sensor, check gas flow, verify >1.5 µA at test pad
4 Amber Flashes	Y signal from thermostat without G signal	Correct thermostat wiring
1 Red Flash	Flame sensed with no call for heat	Check gas valve for leaks or slow closing
2 Red Flashes	Pressure switch stuck closed	Inspect/replace pressure switch, check wiring
3 Red Flashes	Pressure switch failed to close after inducer energized	Check inducer, vent pipe, hose, or pressure switch
4 Red Flashes	Limit switch open	Check filter, duct sizing, blower speed, firing rate, blower motor; also check fuse on board
5 Red Flashes	Rollout switch open	Reset rollout, check combustion air, inducer, heat exchanger, burners; cycle power after reset
6 Red Flashes	Pressure switch opened 4 times during heat call	Inspect pressure switch, venting, inducer; furnace locks out for 1 hr then retries
7 Red Flashes	Flame not established after 3 tries	Check gas pressure, gas valve, flame sensor, ignitor, burners
8 Red Flashes	Flame lost 5 times during cycle	Inspect gas pressure, flame sensor, gas valve
9 Red Flashes	Reversed line voltage polarity or grounding problem	Verify polarity, grounding, flame probe not shorted
10 Red Flashes	Flame sensed with no call for heat	Inspect gas valve and wiring
11 Red Flashes	Limit switch open >5 minutes (hard lockout)	Likely failed blower motor or wheel; cycle 24 VAC power after correction
12 Red Flashes	Open igniter circuit	Check for loose/disconnected wire or cracked igniter
Steady On Red	Control failure	Replace control board
60-Minute Auto Reset	Watchdog resets lockout after 60 minutes	Temporary protection; correct underlying issue
Flame Sense Levels	Normal ~3.7 µA DC; warning <1.5 µA; lockout <0.1 µA	Verify flame sensor current, clean/replace sensor if low

By Darin DeVries