Trane furnace pressure switch
Match the pressure setpoint printed on the existing switch before replacement.
$30-$90
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Error Code: Trane 2 Flashes
What it means: Trane code 2 indicates the pressure switch is stuck closed. The control board detected that the pressure switch contacts were closed before the inducer motor started, or they failed to open when the inducer stopped.
This is different from code 3 (three flashes), which means the pressure switch stayed open after the inducer motor started. Code 2 is a closed-when-should-be-open fault. Code 3 is an open-when-should-be-closed fault. Understanding this distinction tells you where to start looking.
Trane Flash Code System
Trane furnaces use a diagnostic LED on the control board to display error codes. On older models, a single green or amber LED blinks the error code in repeating sequences. On newer models with a two-digit display, you will see a numeric code instead.
| Flash Count | Meaning |
|---|---|
| 1 flash | Ignition failure (no flame sensed) |
| 2 flashes | Pressure switch stuck closed |
| 3 flashes | Pressure switch stuck open |
| 4 flashes | Open limit switch |
| 5 flashes | Flame sensed without call for heat |
| 6 flashes | Circuit board or wiring fault |
| 7 flashes | Gas valve circuit fault |
| 8 flashes | Blower motor fault |
American Standard furnaces use the same control boards and the same flash code mapping. If you have an American Standard showing two flashes, follow this guide.
How the Pressure Switch Circuit Works
The pressure switch is a safety device that proves the inducer motor is pulling proper draft through the heat exchanger. It is normally open when the furnace is off. When the thermostat calls for heat, the inducer motor starts. The negative pressure created inside the heat exchanger pulls the switch closed. The control board sees the closed circuit and allows the gas valve to open.
Code 2 triggers when the control board sees the pressure switch closed before the inducer starts, or when the switch fails to open after the inducer stops. A stuck-closed pressure switch means the control board cannot verify the inducer is actually running. It blocks ignition as a safety measure.
Why This Happens
The pressure switch is stuck closed for one of these reasons:
Failed pressure switch. The internal diaphragm has failed or the switch contacts are welded shut. This is most common on older furnaces where the switch has cycled thousands of times.
Water or debris in the switch. A clogged condensate drain line can push water up the pressure switch hose and into the switch body. Water inside the switch can bridge the contacts and keep them closed. Debris from cracked tubing can do the same.
Short-circuit in the wiring. The pressure switch wires may be touching each other or touching ground, creating a closed circuit that bypasses the switch entirely.
Pressure switch installed backward or wrong part. If someone replaced the pressure switch with the wrong part or installed it backward, the switch may show closed when it should be open.
Step-by-Step Diagnosis
1. Check the Condensate Drain
On 90%+ Trane furnaces, water from the condensing process drains through a tube to the floor drain. If this drain is blocked, water backs up into the inducer housing and can travel up the pressure switch hose. Open the drain tee and look for standing water. Clear the drain with a wet/dry vacuum. If water comes out of the pressure switch hose when you disconnect it from the switch, the switch is water-damaged and needs replacement.
2. Inspect the Pressure Switch Tubing
Trace the small silicone tube from the pressure switch to the inducer housing. Look for cracks, splits, or loose connections. A kinked tube can also cause pressure problems. Replace any cracked tubing with new silicone tubing of the correct diameter.
3. Test the Pressure Switch Wiring
Disconnect the two wires from the pressure switch terminals. With the furnace off, test for continuity between the two wires. There should be no continuity (open circuit). If you get continuity with the wires disconnected from the switch, the wiring harness has a short. Trace the wiring and repair the damaged section.
4. Test the Pressure Switch Itself
Disconnect the switch from the tubing. With the furnace off, test the switch terminals with a multimeter set to continuity mode. A properly functioning normally-open pressure switch should show no continuity. If it shows continuity (closed circuit) with no vacuum applied, the switch has failed and needs replacement.
5. Test With a Manometer (Required for Accurate Diagnosis)
Connect a manometer to the pressure switch tap on the inducer housing. Start the furnace and watch the pressure reading during the inducer pre-purge. The inducer should pull negative pressure that meets or exceeds the pressure switch setpoint. Trane pressure switch setpoints typically range from -0.40 to -1.25 inches WC depending on the model and vent configuration. If the inducer pulls enough vacuum but the switch does not close, the switch is failed. If the inducer does not pull enough vacuum, you have an inducer or vent problem.
6. Check the Inducer Motor
The inducer motor must run at full speed to create enough draft. Check voltage at the inducer motor during operation. For PSC motors, you should see 120VAC. For X13/ECM motors, the voltage varies based on the control signal. Listen for unusual sounds: grinding bearings, rattling from a loose wheel, or a hum with no spin (failed capacitor). Clean the inducer wheel if it is coated with debris.
7. Check the Vent Termination
Go outside and look at the flue vent. Snow, ice, a bird nest, or debris can block the vent. A blocked vent prevents the inducer from pulling draft. On a high-efficiency furnace with two PVC pipes (intake and exhaust), check both terminations.
Parts Reference
| Part | Typical Cost | Notes |
|---|---|---|
| Pressure switch (Trane OEM) | $30-$70 | Match setpoint to existing switch |
| Silicone tubing (1/4” ID) | $5-$15 | Replace if cracked or brittle |
| Inducer motor capacitor | $10-$30 | Match µF rating on existing cap |
| Inducer motor assembly | $150-$400 | Trane OEM recommended |
| Condensate drain trap kit | $15-$35 | Common Trane part |
Resetting After Repair
Turn the thermostat to OFF. Flip the furnace breaker to OFF and wait 30 seconds. Flip the breaker back on. Set the thermostat to call for heat. Listen for the inducer motor to start, then listen for the gas valve click. If the furnace fires and runs through one complete cycle without returning to two flashes, the repair is successful. If the code returns, recheck the switch and inducer.
When To Call a Professional
If you have cleared the condensate drain, inspected the tubing, confirmed the switch wiring is not shorted, verified the switch itself is not stuck closed, and the code still appears, call a technician. Manometer testing requires the right equipment and experience reading the pressure measurements. A technician can also test the inducer motor draft output and measure combustion gas temperatures to rule out heat exchanger issues.
If you disconnected and reconnected the pressure switch tubing and the furnace is now making unusual noises or the flame looks distorted, stop and call a technician. A blocked flue can cause carbon monoxide to enter the living space.