Trane TUD1B080A9H31B Error Code 5 Flashes: Flame Sensed When No Flame Should Be Present
What Does Code 5 Flashes Mean?
The 50A65 IFC watches the flame sensor not only during firing but also when the gas valve is supposed to be closed. A 5-flash code means it is seeing a flame signal at a time when there should be none, so it locks out to prevent unsafe operation. This is essentially the mirror image of the 8-flash low flame sense signal code: instead of too little signal while running, there is signal present when the burners should be off.
The usual causes are electrical rather than an actual fire. A short in the flame-sensor wiring or a control-board fault can make the sensing circuit read a phantom flame. In rare cases a gas valve that is stuck open can allow real, unwanted combustion, which is why the board treats this as safety-critical.
Because every possible cause here — sensor wiring, control board, or gas valve — is inside the furnace and involves either line-voltage circuitry or the gas system, there is no homeowner fix. If you smell gas at any time, leave your home immediately and call your gas utility.
What You'll Notice
- The control LED flashes five times, pauses, and repeats.
- The furnace will not start a normal heating cycle and stays locked out.
- The inducer or blower may run to purge even though no heat is being delivered.
- In rare stuck-valve cases you may notice a gas smell — treat that as an emergency.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Short in flame sensor wiring | Most common | ✗ Call a pro → |
| Faulty control board | Common | ✗ Call a pro → |
| Stuck-open gas valve | Uncommon | ✗ Call a pro → |
How This Is Diagnosed
A technician first confirms the gas valve is actually closed and there is no residual flame, then isolates the sensing circuit. They inspect the flame-sensor lead for a short to the cabinet or a pinched, melted section of insulation and check the sensor and its routing. If the wiring is sound, they test the control board's flame-sense input, and finally verify the gas valve is closing fully and not leaking by. Because this involves the gas valve and line-voltage sensing, it is technician-only work.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The furnace shows a 5-flash code and will not clear with a single power cycle.
- You smell gas near the furnace (leave the home first, then call your gas company and a technician).
- The furnace runs the inducer or blower but refuses to enter a normal heat cycle.
- The code appeared after other work was done on the furnace wiring or gas valve.
Frequently Asked Questions
Is a 5-flash code on my Trane furnace dangerous?
It can be, which is why the board locks the furnace out. Most cases are a wiring short or board fault, but a stuck-open gas valve is possible, so it needs prompt professional service.
Can I reset the 5-flash code myself?
You can cycle power once, but if the code returns do not keep running the furnace. Every cause involves the sensing circuit or gas valve and requires a technician.
What should I do if I smell gas with this code?
Leave your home immediately, do not touch switches, and call your gas utility from outside. Then arrange for a technician to inspect the furnace.
✓ Verified against manufacturer service manual — March 2026