Trane HVAC Error Code 40: How to Fix It

Hard 1–3 hours (diagnostics); additional time if refrigerant work or part replacement is required Medium Severity Verified June 2026
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Error Code
40
Brand
Trane
Product Type
hvac
Severity
Medium
DIY Difficulty
Hard
Estimated Fix Time
1–3 hours (diagnostics); additional time if refrigerant work or part replacement is required
Trane HVAC error code 40 indicates a problem with the Electronic Expansion Valve (EXV) — it is either stuck open or stuck closed and cannot properly regulate refrigerant flow into the system. When the EXV fails to modulate correctly, your system loses the ability to control superheat, which can lead to poor cooling or heating performance, compressor damage, or a complete system shutdown. Addressing this error promptly is important to prevent costly secondary damage to the compressor and other refrigerant-circuit components.
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Tools You'll Need

How to Fix Error Code 40

  1. Power Down the System Safely

    Never work on HVAC electrical components or refrigerant lines without cutting power first. High-voltage capacitors can hold a lethal charge even after shutdown.

  2. Document and Clear the Fault Code

    Clearing codes without diagnosing the root cause only masks the problem temporarily. Do not leave the system running if code 40 returns immediately.

  3. Inspect the EXV Wiring Harness and Connector

    Do not tug aggressively on wires. If insulation is melted or wires are exposed, do not restore power — call a licensed HVAC technician immediately.

  4. Test EXV Coil Resistance with a Multimeter

    Only perform resistance tests with power completely off. Never apply voltage directly to the valve terminals during this step.

  5. Check the Control Board Output Voltage to the EXV

    Exercise extreme caution when measuring live voltages. Keep one hand away from the unit and use insulated probes. Do not touch refrigerant lines during this step.

  6. Inspect for Physical EXV Obstructions or Ice Buildup

    Do not use heat guns or open flames to melt ice on refrigerant components. Allow natural thawing only.

  7. Check System Refrigerant Charge

    Handling refrigerants without EPA 608 certification is illegal in the United States. Do not attempt to add or remove refrigerant yourself — contact a licensed HVAC technician.

  8. Replace the EXV Coil or Full EXV Assembly if Faulty

    Opening the refrigerant circuit to replace the valve body releases refrigerants into the atmosphere if not properly recovered — this is illegal and environmentally harmful. Only a certified technician should perform valve body replacement.

  9. Restore Power and Verify Normal Operation

Parts You May Need

EXV coil (Electronic Expansion Valve coil)
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Electronic Expansion Valve (EXV) assembly
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HVAC control board
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EXV wiring harness
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EXV coil (Electronic Expansion Valve coil)
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Electronic Expansion Valve (EXV) assembly
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HVAC control board
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EXV wiring harness
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EXV coil (Electronic Expansion Valve coil)
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Electronic Expansion Valve (EXV) assembly
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HVAC control board
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EXV wiring harness
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When to Call a Professional

Call a licensed HVAC technician if: the EXV valve body (not just the coil) needs replacement since this requires refrigerant recovery and recharging; your refrigerant charge is incorrect and must be adjusted; you measure no output voltage from the control board and suspect a failed control board requiring replacement; the system repeatedly returns code 40 after all wiring and coil checks; you are not comfortable working around live electrical components or refrigerant lines; or your system is still under a Trane warranty (DIY repairs may void coverage). Refrigerant handling legally requires EPA Section 608 certification in the United States.

Frequently Asked Questions

What does Trane error code 40 mean?
Trane error code 40 means the Electronic Expansion Valve (EXV) is stuck open or stuck closed. The EXV controls how much refrigerant flows through the system, and when it cannot move properly, the system cannot regulate temperature or pressure correctly, leading to poor performance or a shutdown.
Can I fix Trane error code 40 myself?
Some parts of the diagnosis — such as inspecting wiring, testing coil resistance with a multimeter, and replacing just the EXV coil — can be done by a handy homeowner. However, anything involving the refrigerant circuit (valve body replacement, refrigerant charge checks) must be done by an EPA-certified HVAC technician by law.
How much does it cost to fix Trane error code 40?
Replacing an EXV coil typically costs $50–$200 for the part plus 1–2 hours of labor. If the full EXV valve body needs replacement (requiring refrigerant recovery and recharge), total repair costs can range from $400 to $900 or more depending on your region and refrigerant type.
Will Trane error code 40 damage my compressor?
Yes, if left unaddressed, a stuck-open EXV can flood the compressor with liquid refrigerant (called liquid slugging), causing serious internal damage. A stuck-closed EXV starves the system of refrigerant, which can cause the compressor to overheat. Fixing code 40 promptly is important to protect the compressor.
How do I reset Trane error code 40?
You can reset the fault by cycling the system power off at the breaker for at least 30 seconds and then restoring it, or by using the diagnostic board reset function. However, the code will return if the underlying EXV problem is not repaired. A reset alone is not a fix — always diagnose and resolve the root cause first.

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