Trane Furnace Won’t Ignite — No Heat Diagnosis and Repair Guide

When you call for heat and nothing happens — the furnace starts but never lights, or the burners ignite briefly and then shut off — you have an ignition failure. On Trane furnaces, this typically triggers a 2-flash fault code after the control board has attempted and failed ignition three times.

This guide walks through every reason a Trane furnace won't ignite, what to check, and how to distinguish between problems you can resolve yourself and those that need a technician. For Trane furnace repair in St. Louis, call Thomas Hoffmann Air Conditioning & Heating at (314) 471-7625.

How Trane Furnace Ignition Works

Modern Trane gas furnaces use a hot surface ignitor — a silicon carbide or silicon nitride element that heats to over 1800°F to light the gas. When you call for heat, the sequence goes like this:

• The inducer motor starts and runs for a pre-purge period (30–60 seconds)
• The pressure switch closes, confirming the inducer is working
• The control board energizes the hot surface ignitor
• After the ignitor reaches temperature (15–30 seconds), the gas valve opens
• Gas ignites, the flame sensor detects the flame, and the blower starts after a brief delay

If any step in this sequence fails, the furnace will not ignite. The control board allows 3 attempts before locking out and displaying a 2-flash fault code.

Causes of Trane Furnace Ignition Failure

Failed Hot Surface Ignitor

This is by far the most common cause of ignition failure in Trane furnaces. The hot surface ignitor is a fragile ceramic element that glows orange-hot to ignite the gas. Over time — typically 5–7 years — the element cracks or burns out.

How to tell: When the furnace starts a heating cycle, watch through the sight glass on the lower access panel. You should see the ignitor glow orange before gas flows. If you see the inducer motor start but no glow, the ignitor has likely failed.

Repair: Ignitor replacement is one of the most common furnace repairs. The part typically costs $30–80, and replacement is straightforward for a technician. Never touch the ignitor surface with bare hands — skin oils can cause it to fail prematurely.

Dirty or Failed Flame Sensor

The furnace may actually ignite — the burners light — but then shut off within 3–5 seconds. This points to the flame sensor rather than the ignitor. The flame sensor confirms the burner has lit, and if it cannot detect a flame (due to oxide buildup on the sensing rod), it commands the gas valve closed as a safety measure.

A Trane furnace that lights and then shuts off within seconds is almost always a flame sensor issue. See our detailed Trane flame sensor guide for cleaning instructions.

No Gas Supply

Before assuming a mechanical failure, confirm that the furnace has a gas supply:

• Check that the gas shutoff valve on the supply line to the furnace is open — the handle should be parallel to the pipe, not perpendicular
• Check that other gas appliances in your home (water heater, gas range) are working. If none are, the issue may be with your gas supply from the utility.
• If you smell gas strongly near the furnace without it running, do not attempt to operate the furnace — leave the house and call your gas utility

Failed Gas Valve

The gas valve controls the flow of gas to the burners. If the valve's solenoid coil has failed or the valve has stuck closed, gas will not flow even with a working ignitor. This requires professional diagnosis — a technician will test voltage at the valve during an ignition attempt and check valve resistance.

Pressure Switch Not Closing

The furnace will not attempt ignition if the pressure switch has not closed. A blocked vent, failed inducer motor, or faulty pressure switch can prevent the ignition sequence from progressing past the pre-purge stage. This shows as a 3-flash fault code on Trane furnaces. See our Trane pressure switch guide.

Control Board Failure

If the control board is not sending voltage to the ignitor at the right time in the ignition sequence, the furnace will fail to light regardless of the condition of other components. Control board diagnosis requires testing voltage output at multiple points during a call for heat — a job for a technician with the right test equipment.

No Power to the Furnace

Before any other diagnosis, confirm the furnace has power:

• Check the power switch on or near the furnace — it looks like a light switch and is easy to accidentally turn off
• Check the breaker for the furnace in the electrical panel
• Check for a blown fuse on the furnace control board — most Trane furnaces have a 3-amp automotive-style fuse on the board that protects the control circuit. A blown fuse is a very common and inexpensive cause of a “dead” furnace.

Trane Furnace Ignition Failure: Step-by-Step Diagnosis

• Step 1: Check the power switch and circuit breaker
• Step 2: Check the 3-amp fuse on the control board
• Step 3: Check the gas shutoff valve and confirm other gas appliances work
• Step 4: Read the fault code from the LED on the control board
• Step 5: Watch through the sight glass — does the ignitor glow? Do the burners light and then shut off?
• Step 6: If the ignitor does not glow: likely ignitor failure
• Step 7: If burners light then shut off in seconds: likely flame sensor
• Step 8: If the inducer runs but ignition never starts: likely pressure switch or gas issue

Trane Furnace No Heat Repair in St. Louis

Thomas Hoffmann Air Conditioning & Heating stocks ignitors, flame sensors, gas valves, and control boards for common Trane furnace models. If your Trane furnace won't ignite or is locking out on a 2-flash code, call us at (314) 471-7625. We serve the St. Louis metro including Chesterfield, Ballwin, Kirkwood, Webster Groves, Des Peres, and surrounding areas.

Related: Trane furnace fault codes complete guide | Trane furnace repair St. Louis | Trane flame sensor cleaning