Most York gas furnaces have a control board inside the furnace blower compartment that will flash a diagnostic code that shows the operating status of the furnace.

Diagnostic Codes:
Slow Green	Normal Operation - no call for heat
Slow Amber	Normal Operation - with a call for heat
Steady On Red	Control failure	Replace control board (S9200U is a suitable replacement)
Rapid Red	Twinning error, incorrect 24V phasing	Try switching polarity at transformer.
Rapid Amber	Low flame sense current	Clean flame sensor
4 Amber	Y present without G	Low voltage wiring problem
1 Red	Flame present with gas off	Stuck gas valve Shorted flame sensor Faulty control
2 Red	Pressure switch stuck closed	Bad pressure switch Inducer fan relay stuck on
3 Red	Pressure switch stuck open	Bad pressure switch Bad inducer fan Bad inducer fan relay Blocked or restricted vent Condensate drainage issue
4 Red	Limit switch open / open fuse	Bad limit Low airflow Over-Fired
5 Red	Rollout switch open / open fuse	Bad switch Cracked heat exchanger Bad inducer motor Over-Fired
6 Red	Pressure switch opened 4 times	Bad pressure switch Bad inducer fan Blocked or restricted vent Condensate drain issue
7 Red	Ignition lockout due to 3 retries	Bad igniter Dirty flame sensor No gas flow
8 Red	Flame signal lost 5 times	Dirty flame sensor Combustion air problem
9 Red	Grounding or line polarity fault	Try reversing transformer power
10 Red	Flame sensed with no call for heat	Stuck gas valve Shorted flame sensor Faulty control
11 Red	Extended limit switch open	Bad limit Blower failure
12 Red	Open igniter circuit	Bad igniter Loose wiring to igniter
13 Red	2nd Stage Pressure switch stuck open	Bad pressure switch Bad inducer fan Blocked or restricted vent Condensate drain issue

Fault Code Retrieval: With no thermostat call push and release the LAST ERROR button. The LED will flash up to 5 stored fault codes, beginning with the most recent. If there are no codes in memory, the LED will flash two green flashes.

Clear the Fault Codes: Push and hold the LAST ERROR button for at least five seconds. The LED will flash three green flashes when the memory has been cleared.