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Lennox

OEM Lennox At140A1042/U Transformer 120/240V 24V 40VA — X9899

SKU: LNX-X9899 Brand: Lennox
$29.95 $39.95

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Texas CMBL #19328542014
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Govt POs Accepted

The Lennox X9899 is a genuine OEM AT140A1042/U step-down transformer designed to convert a 120/240 VAC primary input to a 24 VAC secondary output at 40 VA, providing stable low-voltage power to your Lennox system's control board and thermostat circuits. Symptoms of a failing transformer include a blank thermostat display, loss of heating or cooling, or blown control-circuit fuses. This OEM replacement transformer ensures compatibility and reliable performance with applicable Lennox furnaces and air handlers.

Shipping Information

  • Orders placed before 3PM CST ship same business day
  • Standard delivery: 2-5 business days
  • Express shipping available at checkout
  • Free shipping on orders over $75
  • Tracking number emailed upon shipment

Return Policy

  • 30-day returns on unused parts in original packaging
  • Electrical parts (capacitors, contactors, control boards) are final sale once installed
  • Email info@nationalhvacparts.com to initiate a return
  • Refunds processed within 5-7 business days

Frequently Asked Questions

Does this Transformer fit my Lennox OEM Lennox At140A1042/U Transformer 120/240V 24V 40VA — X9899?
Yes. This is the factory-specified OEM transformer for the Lennox OEM Lennox At140A1042/U Transformer 120/240V 24V 40VA — X9899. It is a direct drop-in replacement that fits without modification. Always verify the model number on your equipment nameplate before ordering.
How do I replace the transformer on my Lennox OEM Lennox At140A1042/U Transformer 120/240V 24V 40VA — X9899?
Difficulty: Medium (30–45 min)
Step 1: Turn off breaker. Step 2: Photo wiring connections. Step 3: Disconnect wires from old transformer. Step 4: Remove mounting hardware. Step 5: Install new transformer, reconnect wires matching voltage ratings. Step 6: Verify 24V output with multimeter.
My thermostat has no power. What failed?
Thermostat with no display = failed 24V transformer (or blown control board fuse). Check the 3A/5A fuse on the control board first ($1–$5 fix). If fuse is good but no 24V output from transformer, replace transformer.
Is this transformer compatible with Bryant or Payne too?
Many Lennox parts are cross-compatible with Bryant and Payne since all three brands are manufactured by Carrier Global Corporation. The same physical part is often used across brand labels with different model numbers. To verify: check your Bryant or Payne equipment service manual for cross-compatible part numbers, or email info@nationalhvacparts.com with your model number.
Should I use OEM or aftermarket for my Lennox transformer?
OEM recommended for reliability and warranty compatibility. OEM parts match exact factory specifications for proper fit, performance, and safety. While aftermarket alternatives cost less upfront, the risk of improper fit and reduced lifespan usually makes OEM the better value long-term.
How fast does National HVAC Parts ship?
Orders placed before 3:00 PM CST ship the same business day. Standard delivery is 2–5 business days anywhere in the continental United States. Express shipping available at checkout. We ship from Houston, TX to all 50 states. 30-day return policy on all parts.
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Mike Rivera
HVAC Parts Specialist — Online