A blinking LED on a York furnace control board is a diagnostic code, not a warning light. Count the flashes, pause for the three-second gap, count again to confirm, and the furnace is telling you exactly what failed — if you match the count against the correct chart for your model. York YP9C modulating, TM9Y two-stage, TG9S single-stage, and the LX-series furnaces each use similar but non-identical code tables, and each fault category points to a short list of Source 1 OEM parts that are almost always the cause.
The quick answer: on a York furnace, 3 flashes is a pressure switch fault, 4 flashes is an open high-limit, 6 flashes is ignition lockout (ignitor or flame sensor), 7 flashes is a gas valve circuit fault, and 9 flashes is reversed line-voltage polarity. Confirm your model number against the Johnson Controls chart, then call HVACMORE at 855-575-4822 with the model, serial, and flash count to confirm the correct Source 1 OEM part.
Why the York flash code chart depends on your model number
A flash code is not a universal standard. The pattern on a York control board is defined by Johnson Controls per product family, and the fault category each count maps to differs between the YP9C modulating platform, the TM9Y two-stage platform, the TG9S single-stage platform, and the older LX-series boards. Three flashes is a pressure switch fault on all four, but which Source 1 pressure switch (single-stage, two-stage, or modulating) you order depends on the model.
The same logic applies to the parts you buy to clear the code. A 6-flash ignition lockout on a TM9Y furnace points to a specific Source 1 hot surface ignitor SKU; the same lockout on a YP9C points to a different ignitor with a different resistance range and connector. Order the generic “York ignitor” without the model and there is a 50 percent chance it will not fit the ceramic bracket or will burn out on first ignition.
If you have not pulled your York model number yet, here is where to find and decode it before you call parts. Everything below assumes you have the full M/N and S/N in hand.
How to count a York furnace flash code the right way
The LED is on the control board, visible through the sight glass on the lower access panel or directly on the board when the panel is off. Shut the thermostat off, wait for the furnace to finish its post-purge, and watch one full cycle before you count.
- The LED flashes a count — three, four, six, however many. Each flash is roughly a quarter-second on, a quarter-second off.
- Then a longer pause, about three seconds, with the LED off.
- Then the count repeats.
Watch at least two full cycles before locking in a number. On very old York LX-series boards the gap can be shorter and the counts can run together if you blink at the wrong moment.
Three patterns are not flash codes and should not be counted:
- Solid-on LED — normal standby, no fault. The furnace is powered and the control sees no problem. If the furnace won't run with a solid LED, the issue is upstream (thermostat, 24V transformer, or wiring).
- Rapid continuous flash with no pause — on many York boards this is “normal operation” or “call-for-heat received.” Not a fault.
- LED off entirely — no power to the board. Check the 120V service switch, the fuse on the board itself, and the door interlock switch before condemning the board.
York flash code chart: 1 through 9 flashes
The counts below cover the YP9C, TM9Y and TG9S residential platforms, which account for the vast majority of 2010–2026 York gas furnace installs. For LX-series or older 80-series equipment, confirm against the installation manual stamped inside the blower panel.
1 flash — internal control fault
The board has detected a fault with itself. Not a sensor, not a motor — the control board. Cycle 120V power at the service switch; if the code repeats on power-up, the Source 1 OEM control board is the replacement part.
Before condemning a board, verify the 24V transformer output (should read 24–28V AC between R and C) and the ground connection on the furnace chassis. A loose ground or a brownout can mimic a board fault.
2 flashes — external lockout / pressure or limit circuit
On most York boards, 2 flashes means the control has entered external lockout from a repeated limit or pressure switch open. The fault is not the board; it is one of the safety switches. Inspect for a dirty filter, blocked return, closed registers, or vent blockage first. If those are clear, the Source 1 OEM part is usually a pressure switch or high-limit switch — confirmed by which switch is open at the time of lockout.
3 flashes — pressure switch fault
The pressure switch is stuck open or the inducer is not creating enough negative draft to close it. On a 95–98% AFUE condensing furnace, 3 flashes is the most common service call code. Walk through this order before ordering a part:
- Check the vent pipe and intake for a bird nest, ice buildup, or a disconnected joint.
- Pull the pressure switch hose and look for condensate blockage or a cracked hose. Blow it clear.
- Verify the inducer motor is spinning up to full speed on call for heat. A failing inducer bearing drops airflow below the switch threshold.
- Finally, if hoses are clear and the inducer runs correctly, the Source 1 pressure switch itself is the OEM part to order.
On two-stage TM9Y furnaces, the pressure switch is a dual-pressure assembly (one switch for low fire, one for high). Match the Source 1 SKU to the furnace model before ordering — a single-stage switch will not fit the two-stage harness.
4 flashes — open high-limit switch
The high-limit sensor has opened because the heat exchanger overheated. In order of likelihood: dirty filter (replace and reset), missed annual tune-up with blocked blower wheel, failed blower motor or capacitor, or an actual failed limit switch.
If the airflow checks pass and the limit still opens on run, the Source 1 OEM part is the high-limit switch. Many York furnaces use two limits (primary and auxiliary); only replace the one that is open at lockout, not both.
If the blower motor is suspect, verify capacitor microfarad rating with a meter before ordering. A weak capacitor is a $20 fix; an ECM blower motor is $300–$600.
5 flashes — flame sensed with no call for heat
The flame sensor is reading current when the gas valve is supposed to be closed. The most common cause is a shorted or carbon-coated flame sensor rod reading “phantom flame.” Pull the sensor, buff it lightly with fine Scotch-Brite (never sandpaper — it leaves grit on the ceramic), reinstall, and recycle power.
If 5 flashes returns, the Source 1 OEM flame sensor rod is $25–$45 and takes five minutes to swap. If the fault persists with a new sensor, the gas valve has an internal leak and the Source 1 gas valve is the replacement part.
6 flashes — ignition lockout
The control attempted ignition the maximum number of retries (usually three) and never proved a flame signal. The board has shut off the gas valve and locked out for one hour. 6 flashes is the single most common York furnace service call in winter.
The failed Source 1 OEM part is almost always one of two components:
- Hot surface ignitor (HSI) — a cracked or burned-out ignitor glows weakly or not at all. A new OEM ignitor measures 40–80 ohms depending on model. An open circuit (OL on the meter) means replace.
- Flame sensor — a dirty or coated sensor can't detect the flame even though the burners are lit. Clean first, replace if cleaning doesn't restore microamp signal above 2.0 µA.
Less commonly, the gas valve has failed closed, the gas supply pressure has dropped (frozen regulator in a cold snap), or the burner orifices are blocked by rust from the heat exchanger. Verify incoming gas pressure before ordering a Source 1 gas valve.
7 flashes — gas valve circuit fault
The board is sending the signal to open the gas valve but not seeing the expected current draw back. On modulating YP9C furnaces this points to the modulating valve coil; on single-stage TG9S furnaces it is the simpler on-off valve.
Before ordering a Source 1 gas valve, verify the wiring harness from the board to the valve is intact and the ground connection is clean. A corroded ground pin can mimic valve failure.
8 flashes — low flame signal
The furnace is running but the flame rectification current is below the minimum threshold (usually 1.0 µA). The burners stay lit but the control throws a warning and may drop into reduced-fire mode on modulating units. Clean the flame sensor first; replace if the microamp reading does not recover.
On condensing furnaces, 8 flashes paired with 3 flashes (intermittent) often means the burner is running rich because the condensate drain is partially blocked and pulling the flame off the sensor. Clear the drain before replacing parts.
9 flashes — reversed line-voltage polarity
Hot and neutral are swapped at the 120V furnace power connection. The flame rectification circuit uses chassis ground as a reference, and reversed polarity breaks that reference. 9 flashes is a wiring fault, not a failed part. Shut off the breaker and have an electrician or HVAC tech swap the leads at the service switch or junction box. Do not order a replacement board, flame sensor, or gas valve for a 9-flash code — none of them are at fault.
From flash code to Source 1 OEM part: the diagnostic tree
The right order of operations prevents the two most expensive mistakes in furnace repair: condemning a $400 control board when the real fault is a $25 flame sensor, and ordering an aftermarket ignitor that burns out in six weeks because the resistance was wrong for your model.
Step 1: Count the flash accurately
Two full cycles minimum. Write the number down. If the count varies between cycles (3 flashes, then 4 flashes, then 3 again), the board is throwing multiple intermittent faults and the issue is usually airflow, drainage, or a loose connection — not a single failed part.
Step 2: Pull the full model and serial off the data plate
Inside the upper access panel, right-hand interior wall. Photograph the entire plate. The model determines which chart applies and which Source 1 part number fits. Do not skip this step — see how to find and decode a York model number if you need a refresher.
Step 3: Check the free fixes before ordering any part
- Replace the air filter. A dirty filter triggers 4-flash high-limit codes constantly in winter.
- Clear the pressure switch hose and inspect the vent. Clears most 3-flash codes without a parts order.
- Clean the flame sensor. Resolves most 5-flash and 8-flash codes, and roughly half of 6-flash ignition lockouts.
- Verify 120V polarity at the furnace. If 9 flashes, stop and call an electrician.
- Clear the condensate drain on a condensing furnace. Blocked drain can mimic pressure switch and limit codes.
If the code persists after the free fixes, move to parts.
Step 4: Match the fault to the Source 1 OEM part
Each flash category points to a short list of components. The Source 1 (S1-) part number is stamped on the component itself in most cases — pull the suspect part, read the number, and cross-reference against your model.
- 3 flashes → pressure switch; inducer motor assembly as secondary
- 4 flashes → high-limit switch; blower motor and capacitor as secondary
- 5 flashes → flame sensor; gas valve as secondary
- 6 flashes → hot surface ignitor and flame sensor (verify both)
- 7 flashes → gas valve; wiring harness as secondary
- 8 flashes → flame sensor; condensate drain and burner inspection
- 1 or persistent multi-fault → control board, after upstream checks
Step 5: Call HVACMORE with the model, serial, and flash count
The parts team at 855-575-4822 cross-references your model and serial against the Source 1 catalog, confirms the exact SKU, and ships the correct part. Reading the three numbers (model, serial, flash count) over the phone takes two minutes and prevents a wrong-part return.
Worked example: TM9Y080B12MP11 throwing 6 flashes
A two-stage condensing furnace, 80,000 BTU input, cabinet revision B, has locked out with 6 flashes repeating every three seconds. Walking the tree:
- Count confirmed: 6 flashes, two cycles observed.
- Model confirmed: TM9Y080B12MP11 — York LX two-stage condensing, 95–96% AFUE.
- Free fixes attempted: flame sensor cleaned with Scotch-Brite; furnace cycled; code returns within three ignition attempts.
- Fault isolated: ignitor measured at infinite resistance (open circuit) with a multimeter — cracked ceramic.
- Source 1 part identified: OEM hot surface ignitor matched to TM9Y platform. Call HVACMORE with the model and serial to confirm the current Source 1 SKU (York supersedes ignitor numbers periodically).
- Install verified: new ignitor measures 50 ohms, glows uniformly orange within 25 seconds of call for heat, flame proof achieved on first attempt, lockout cleared.
Total time: 35 minutes. Total cost: Source 1 ignitor plus shipping. No wasted parts, no wrong SKU, no condemned board.
Where aftermarket is OK and where you need the Source 1 OEM part
Not every flash-code part needs to be OEM. The rule is simple: anything with firmware, anything paired to a specific control board, and anything under warranty needs the York Source 1 part. Everything else is a cost-and-fitment decision.
Aftermarket is usually fine
- Flame sensor — a quality universal flame rod of the correct bracket pattern works
- Pressure switch — universal with matching MBAR / inches-WC rating fits most single-stage
- High-limit switch — universal snap-disc rated to the correct temperature
- Hot surface ignitor — universals labeled for the specific York bracket pattern (41-408, 271A, etc.) work; verify resistance range matches your model
- Capacitors — commodity, match microfarads and voltage
You need the Source 1 York OEM part
- Control boards — firmware is model-specific; a universal will not run modulating or two-stage logic correctly
- Modulating gas valves — paired to the YP9C control; universals do not exist
- ECM / variable-speed blower motors — motor module is programmed to the furnace board
- Dual-pressure switch assemblies on two-stage TM9Y furnaces — harness is proprietary
- Any part on an in-warranty furnace — aftermarket voids the parts warranty for the remainder of the 10-year coverage period
For the full breakdown of OEM-vs-aftermarket decisions on York equipment, see the York model number and OEM parts guide.
Common mistakes reading York flash codes
Counting the pre-purge blink as a code. On power-up, many York boards flash once as a self-test. That is not a fault code. Wait for the thermostat to call for heat, let the furnace lock out, then count.
Reading the wrong LED. Some York boards have two LEDs — a green status and an amber or red fault. Count the fault LED, not the status LED. The installation manual inside the blower panel labels which is which.
Ordering a “York ignitor” without the model. York uses at least four different ignitor bracket patterns across its residential furnaces. Order generic and there is a 50 percent chance the ceramic bracket does not fit the burner box opening.
Replacing the control board on a 1-flash code without checking the 24V supply. A loose transformer secondary or a corroded ground post can trigger a 1-flash internal fault code. Verify the 24V output at R-to-C with a meter before condemning a board.
Ignoring the serial number when ordering a superseded part. Johnson Controls supersedes Source 1 part numbers regularly. A board stamped with a 2019 S1 number may now ship as a 2026 superseded SKU — which fits, but ships under a new catalog number. Call HVACMORE to confirm the current SKU for your serial.
Treating a 9-flash code as a parts failure. 9 flashes is a wiring issue. No Source 1 part will fix reversed line-voltage polarity — the fix is correcting the hot/neutral wiring at the service switch or junction box.
Frequently asked questions
What does 3 flashes mean on a York furnace? Pressure switch fault — the switch is stuck open or the inducer is not producing enough draft. Check the vent, hose, and inducer before ordering the Source 1 pressure switch.
What does 4 flashes mean on a York furnace? Open high-limit — the furnace has overheated. Most common cause is a dirty air filter restricting airflow. The Source 1 part is the high-limit switch, if the airflow checks pass.
What does 6 flashes mean on a York furnace? Ignition lockout — the control attempted ignition three times without proving flame. The failed Source 1 OEM part is almost always the hot surface ignitor or the flame sensor.
What does 7 flashes mean on a York furnace? Gas valve circuit fault — the board is not seeing feedback from the gas valve. Verify wiring and ground before ordering a Source 1 gas valve.
What does 9 flashes mean on a York furnace? Reversed line-voltage polarity — a wiring fault, not a failed part. Have an electrician correct hot/neutral at the service switch.
Why is my York furnace LED solid on with no flashes? That is normal standby. The control is powered and has not detected a fault. If the furnace will not run, the issue is upstream — thermostat, 24V transformer, or wiring to the R terminal.
Does the flash code chart apply to Coleman, Luxaire, Champion, and Fraser-Johnston furnaces? In most cases yes. Johnson Controls builds all five brands on the same control platforms, and the flash codes and Source 1 OEM parts are usually identical within the same product family.
Do I need the York model number to decode a flash code? Yes. The count-to-fault mapping is similar across York furnaces, but the Source 1 OEM part number for each fault depends on the model family (YP9C, TM9Y, TG9S, LX).
Can I clear a York flash code without replacing the part? Soft lockouts clear after one hour or when 120V power is cycled at the service switch. But the fault itself returns on the next call for heat unless the underlying cause is fixed.
Does HVACMORE sell York Source 1 flash code replacement parts? Yes — ignitors, flame sensors, pressure switches, high-limit switches, inducer motors, gas valves, and control boards. Call 855-575-4822 with your model, serial, and flash count.
Buying from HVACMORE
HVACMORE stocks York and Johnson Controls Source 1 OEM parts for every component a flash code can point to — hot surface ignitors, flame sensors, pressure switches, high-limit switches, inducer motor assemblies, gas valves, and control boards. Every part is cross-referenced against your model and serial before it ships, so you don't open the box and find the wrong SKU.
What ordering flash-code parts from HVACMORE gets you:
- Fitment verified against your York model and serial before the order ships
- Free shipping on eligible orders over $500
- Price match if you find the same Source 1 part number cheaper elsewhere
- Original 10-year York parts warranty preserved when the unit is in-warranty and the replacement is OEM
- 60-Day Money Back Guarantee
- 30-Day Hassle-Free Returns with no restocking fees
Not sure which part the code points to? Text or email a photo of the furnace data plate, a photo of the failed component, and the flash count to the HVACMORE parts team, or call 855-575-4822 — US-based, Monday to Friday 8am–9pm ET, weekends included. Confirming the Source 1 SKU over the phone takes two minutes and prevents the two most expensive mistakes in furnace repair: a wrong-part return, and condemning a healthy board when a $25 flame sensor was the real culprit.
The bottom line
A York furnace flash code is a diagnosis in plain sight — count the flashes, pause for the gap, confirm with a second cycle, and the control has already told you where the fault lives. The missing piece is the model number: without it, the code points to a category; with it, the code points to a specific Source 1 OEM part. Clean the flame sensor and change the filter before you order anything. Call HVACMORE with the model, serial, and flash count in hand. OEM where it counts — boards, modulating valves, ECM motors, anything under warranty — aftermarket where the physics are commodity. That rule holds for every flash code from 1 through 9.
Sources
- Johnson Controls — York residential furnace installation and service manuals (model-specific flash code charts and Source 1 OEM parts)
- U.S. Department of Energy — Furnaces and Boilers (AFUE efficiency tiers referenced in model families)
- U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center (why pressure switch and limit faults should not be bypassed)
- Related reading on HVACMORE — York Model Number Lookup & OEM Parts, Repair vs Replace, HVAC Maintenance Guide, Furnace Buying Guide 2026
Flash code tables, Source 1 part supersessions, and warranty terms change over time. Always verify the current flash code chart and part fitment with Johnson Controls, an authorized York dealer, or HVACMORE parts before ordering. Never bypass a safety switch (pressure, limit, or rollout) to clear a code — the switch is there to prevent carbon monoxide release or heat exchanger damage.
Ready to upgrade your system?
Free ZIP-based pricing on top-rated HVAC systems
Get instant local pricing and expert help choosing the right AC, furnace, or heat pump for your home.
Shop HVAC Systems
