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Lennox

OEM Lennox AT40A1121/U Transformer 120 Vac Primary 24 Vac 40 Va — 59N90

SKU: LNX-59N90 Brand: Lennox
$29.95 $39.95

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Texas CMBL #19328542014
SAM.gov Registered
Govt POs Accepted

The Lennox 59N90, model AT40A1121/U, is a genuine OEM 40 VA control transformer that converts 120 VAC primary voltage to 24 VAC secondary voltage to power the low-voltage thermostat and control circuitry in Lennox HVAC systems. If your system shows symptoms such as no thermostat display, failure to energize heating or cooling, or repeated blown low-voltage fuses, the transformer may need replacement. This OEM Lennox part is designed for precise fit and reliable operation in compatible Lennox equipment.

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 AT40A1121/U Transformer 120 Vac Primary 24 Vac 40 Va — 59N90?
Yes. This is the factory-specified OEM transformer for the Lennox OEM Lennox AT40A1121/U Transformer 120 Vac Primary 24 Vac 40 Va — 59N90. 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 AT40A1121/U Transformer 120 Vac Primary 24 Vac 40 Va — 59N90?
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