Trane TUD1B080A9H31B Furnace Error Code 8 Flashes: Low Flame Sense Signal

What Does Code 8 Flashes Mean?

The flame sensor is a thin metal rod that sits in the burner flame and passes a tiny microamp current to prove the flame is present. The 50A65 IFC compares that current to a minimum threshold; an 8-flash code means a flame is detected but the signal is too weak to be fully trusted. The furnace may still light but can run briefly and shut down, or cycle on and off as the marginal signal comes and goes.

The most common cause is carbon and oxidation building up on the sensor rod, which insulates it and cuts the current. Less often the sensor is cracked, or the flame-sensing ground path is poor — the same grounding weakness that can trigger the 6-flash power-reversed/grounding code. Left unaddressed, a weak signal tends to worsen until the board loses flame entirely and escalates to a 2-flash system lockout.

Cleaning the sensor rod is one of the most common, lowest-risk furnace maintenance tasks and typically restores the signal. If the code returns soon after a proper cleaning, the rod may be cracked or its porcelain insulator damaged, and the sensor should be replaced by a technician.

What You'll Notice

• The control LED flashes eight times, pauses, and repeats.
• The furnace lights, runs for a short time, then shuts down.
• The furnace short-cycles — igniting and dropping out repeatedly.
• Heating becomes less reliable over days or weeks as the signal degrades.
• The problem may eventually turn into a 2-flash lockout if the flame signal is lost completely.

Common Causes

How This Is Diagnosed

Because a fouled rod is by far the most common cause, cleaning it is both the first step and the fix. A technician confirms it by measuring the flame-sense current in microamps with the burners lit and comparing it to the board's minimum. If cleaning does not restore the reading, they check the sensor for cracks and verify the flame-sensing ground path before replacing the sensor. This keeps the furnace out of the escalating lockout that a lost flame signal would cause.

How to Fix It: Clean the Flame Sensor

What You'll Need

• Scotch-Brite pad
• 1/4-inch nut driver or hex-head screwdriver
• Clean dry cloth
• Flashlight

Steps

1. Turn off power at the breaker and shut off the gas supply valve Flip the furnace circuit breaker to OFF and turn the gas supply valve to OFF (perpendicular to the pipe) before opening anything. If you smell gas at any point, leave your home immediately and call your gas company from outside.
2. Open the burner-compartment access panel Remove the furnace access panel that exposes the burners. It typically lifts out or is held by a few screws. Set it aside where the door-interlock switch will not be tripped accidentally.
3. Locate and remove the flame sensor The flame sensor is a thin metal rod, often bent at an angle, mounted near the burners on the opposite end from the igniter, with a single wire and one mounting screw. Note its orientation, disconnect the wire, remove the screw, and carefully withdraw the sensor.
4. Clean the sensor rod Gently clean the flame sensor rod until the metal is dull-bright. A non-abrasive Scotch-Brite pad is preferred because it leaves no abrasive residue on the rod; if you use fine steel wool or emery cloth instead, wipe the rod thoroughly afterward, since residue left on the rod can insulate it. Do not clean or scratch the white porcelain base, and take care not to bend the rod. Wipe the rod with a clean dry cloth when finished.
5. Reinstall the flame sensor Slide the sensor back into its original position so the rod will sit in the burner flame, secure the mounting screw, and firmly reconnect the wire.
6. Restore gas and power, then test Replace the access panel, turn the gas valve back to ON (parallel to the pipe), and flip the breaker to ON. Set the thermostat to call for heat and watch the furnace run its ignition sequence.

When to Call a Professional

Contact a licensed HVAC technician if:

• The 8-flash code returns soon after a careful cleaning of the flame sensor.
• The furnace lights and then shuts off within a minute, or short-cycles.
• The sensor rod is visibly cracked or corroded rather than just dirty.
• The code appears together with a 6-flash grounding fault or a 2-flash lockout.

✓ Verified against manufacturer service manual — March 2026