Error Code Low Flame Signal

York TM9T100C16MP11 Error Code Low Flame Signal: Low Flame Signal

TL;DR
Your York furnace's LED is flashing amber rapidly, indicating the flame sensor's microamp reading is below 1.5 — weak but still functional. Clean the sensor now to prevent a future ignition lockout.
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 Low Flame Signal Mean?

A rapid amber flash on the York S1-33103010000 board is an early warning that the flame sense current has dropped below 1.5 microamps. The furnace is still operating, but the flame signal is weak and unreliable. This is the York board's way of telling you to service the flame sensor before the problem escalates.

A healthy flame sensor on a York furnace typically reads between 2 and 6 microamps. When the rapid amber flash appears, the reading has degraded to a point where any slight fluctuation could cause a flame loss event. If left unaddressed, this condition will progressively worsen until the furnace begins losing flame mid-cycle (code 8 red flashes) or failing to prove flame during ignition (code 7 red flashes).

The rapid amber flash is essentially a maintenance reminder. In most cases, a simple flame sensor cleaning restores the signal to a healthy range and the amber warning disappears immediately.

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 access panels on your York furnace to expose the burner assembly and flame sensor.
  3. Remove and clean the flame sensor The flame sensor is a thin metal rod with one wire, held by a single 1/4-inch screw. Disconnect the wire, remove the screw, and pull the sensor out. Clean the rod with fine-grit emery cloth or a Scotch-Brite pad until shiny. Do NOT use steel wool. Wipe clean with a dry cloth.
  4. Reinstall and restore Remount the sensor, reconnect the wire, replace access panels. Turn gas ON (parallel to pipe), restore power, and call for heat.
How to Verify
During the next heating cycle, check the LED through the viewport. The rapid amber flash should be gone. You should see a slow amber flash (normal call for heat) while the furnace is running. If the rapid amber flash persists after cleaning, the flame sensor may need replacement, or there may be a gas pressure or sensor positioning issue.

When to Call a Professional

Contact a licensed HVAC technician if:

Sources

  1. Johnson Controls S1-33103010000 Installation Manual (ManualsLib)
  2. GHAC Knowledgebase