Trane TUH1B080A9H31A Error Code 8 Flashes: Low Flame Sense Signal
What Does Code 8 Flashes Mean?
When the White-Rodgers 50A65 control board in your Trane furnace shows 8 flashes, it is reporting a low flame sense signal. Unlike the 2-flash code where the furnace cannot ignite at all, with 8 flashes the furnace IS lighting and running — but the flame sensor's electrical signal is below the optimal threshold. The board is warning you that the flame signal is marginal.
A healthy flame sensor typically reads between 2 and 6 microamps of DC current. The 8-flash code means the reading has dropped to a borderline level — often between 0.5 and 1.5 microamps. The furnace may still operate, but it is unreliable. You may notice the furnace running for a while, then cutting off mid-cycle, then restarting. This intermittent behavior is the hallmark of a weak flame sense signal.
The most common cause is the same oxidation buildup on the flame sensor rod that causes code 2. The difference is that code 8 catches the problem earlier, before total ignition failure. If you ignore this warning, the flame signal will likely continue to degrade until the furnace can no longer prove flame at all, resulting in a code 2 lockout.
How to Fix It: Clean the Flame Sensor on Your Trane Furnace
What You'll Need
- 1/4-inch hex-head driver or nut driver
- Fine-grit emery cloth or Scotch-Brite pad
- Clean dry cloth or paper towel
Steps
- Turn off power AND shut off gas supply Switch the furnace power switch to OFF (usually a standard light switch on or near the furnace). Then locate the gas shutoff valve on the gas supply pipe leading to the furnace and turn it to the OFF position (perpendicular to the pipe). Wait 5 minutes before proceeding. If you smell gas at any point, leave the area immediately and call your gas company.
- Open the furnace access panel Remove the lower access panel on the front of your Trane furnace. Most panels slide up and off, or are held by a couple of screws.
- Locate the flame sensor The flame sensor is a thin metal rod (about 2-3 inches long) mounted with a single 1/4-inch hex screw on the burner assembly. On Trane furnaces with the White-Rodgers 50A65 board, it is typically on the right side of the burner assembly when facing the furnace, opposite the igniter. It has a single wire with a ceramic insulator.
- Remove the flame sensor Disconnect the single wire connector by pulling it straight off the sensor terminal. Remove the 1/4-inch hex screw and slide the sensor out of its mounting bracket.
- Clean the sensor rod Gently rub the metal rod with fine-grit emery cloth or a Scotch-Brite pad until the oxidation is removed and you see clean, shiny metal. Do NOT use steel wool — steel fibers can embed in the rod and worsen the problem. Do NOT sand the ceramic insulator portion. Wipe clean with a dry cloth.
- Inspect the ceramic insulator While you have the sensor out, examine the white ceramic insulator where the rod passes through the mounting bracket. If the ceramic is cracked, the sensor needs to be replaced entirely — cleaning will not help, as the current will leak through the crack.
- Reinstall and test Reinstall the flame sensor, reconnect the wire, replace the access panel, turn gas back ON (valve parallel to pipe), and restore power. Call for heat from the thermostat and observe the startup sequence.
When to Call a Professional
Contact a licensed HVAC technician if:
- The 8-flash code returns after cleaning the flame sensor
- The ceramic insulator on the flame sensor is cracked
- The furnace starts but shuts down within 5-10 seconds of flame ignition
- You notice a weak, yellow, or flickering burner flame instead of steady blue