Error Code Flame Lost During Heating

York TM9T100C16MP11 Error Code Flame Lost During Heating: Flame Lost During Heating

TL;DR
Your York furnace keeps losing its flame during operation, cycling off and restarting until it locks out. The flame sensor is likely dirty and needs cleaning to restore a strong flame signal.
Disclaimer
This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code Flame Lost During Heating Mean?

The 8-red-flash code on the York S1-33103010000 board means the furnace successfully ignited but lost the flame 5 times (4 recycles plus the final failure) during the heating call, triggering a 1-hour soft lockout. Unlike code 7 (failure to ignite at all), code 8 tells you the furnace IS lighting — it just cannot keep the flame going.

This intermittent flame loss pattern is the classic signature of a flame sensor that is borderline. The sensor detects enough current to initially prove flame, but as the burner cycles or as slight fluctuations occur, the weak signal momentarily drops below the threshold and the board shuts off gas as a safety measure. The furnace then restarts, proves flame briefly, loses it again, and eventually hits the recycle limit.

A dirty or oxidized flame sensor is the most common cause. Cleaning it restores the microamp reading to a healthy range. Less common causes include low or fluctuating gas pressure, a loose flame sensor wire connection, or wind/downdraft affecting the flame.

How to Fix It: Clean the Flame Sensor on Your York Furnace

âš  Safety First
Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

Steps

  1. Turn off power AND shut off gas supply Turn off the furnace power switch. Turn the gas shutoff valve to OFF (perpendicular to the pipe). Wait 5 minutes. If you smell gas at any point, leave the area immediately and call your gas company.
  2. Access the burner compartment Remove the upper and lower access panels on your York furnace to reach the burner assembly.
  3. Remove and clean the flame sensor Locate the flame sensor — a thin metal rod with one wire, mounted with a single 1/4-inch screw on the burner assembly. Disconnect the wire, remove the screw, and slide the sensor out. Clean the metal rod with fine-grit emery cloth or a Scotch-Brite pad until shiny. Do NOT use steel wool. Wipe clean.
  4. Reinstall the flame sensor Reinsert the sensor, tighten the mounting screw, and reconnect the wire connector firmly. Ensure the rod will sit in the burner flame path.
  5. Restore and test Replace access panels. Turn gas ON (parallel to pipe). Restore power. Call for heat and let the furnace run through a complete heating cycle to verify it does not cut out.
How to Verify
Let the furnace run for at least 10-15 minutes through a full heating cycle. The LED should show slow amber flash during the call for heat and should NOT switch to red flashes. If the furnace runs a complete cycle without dropping flame, the fix was successful. If 8 red flashes return, there may be a gas pressure issue or the flame sensor needs replacement.

When to Call a Professional

Contact a licensed HVAC technician if:

Sources

  1. Johnson Controls S1-33103010000 Installation Manual (ManualsLib)
  2. GHAC Knowledgebase
  3. Gray Furnaceman (grayfurnaceman.com)
  4. Hunker (hunker.com)