The short answer
On a Lennox air conditioner or heat pump in Northern Virginia, a 400-series code comes from the outdoor unit and shows as an alert on your S30 or S40 thermostat. The ones that matter most: 400 means the compressor’s internal overload tripped, 404 means the compressor rotor is locked, and 411 and 413 mean a pressure switch opened five times in one call and shut the outdoor unit down. Those need a tech. 401 and 403 are warnings, worth a visit before they turn into one of the others.
The Lennox outdoor alerts
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| Alert | What Lennox says | Priority | What it usually is |
|---|---|---|---|
| 180 | Outdoor air temperature sensor problem | Service Soon | The sensor or its wire. It still runs. |
| 400 | Compressor internal overload tripped | Service Soon | The compressor got too hot. A dirty outdoor coil, a weak capacitor, low charge or a dead fan. |
| 401 | Long run cycle, over 18 hours on one call, or low system pressure | Information | Often low refrigerant, which means a leak. On a heat pump Lennox ignores it under 65°F outside. |
| 403 | Short-cycling, the compressor ran less than 3 minutes | Service Soon | The thermostat, a pressure switch or the charge. |
| 404 | Compressor rotor locked | Service Urgent | Lennox lists a shorted run capacitor, seized bearings or too much liquid refrigerant. |
| 410, 411 | Low pressure switch open, then locked out after 5 opens in one call | Information, then Service Urgent | Low charge, a dirty filter or indoor coil, a frozen coil. |
| 412, 413 | High pressure switch open, then locked out after 5 opens in one cooling call | Information, then Service Urgent | A dirty outdoor coil, an outdoor fan that stopped, too much charge. |
| 414 | High discharge line temperature, higher than 279°F | Service Soon | Low charge or a restriction. Clears once it cools under 225°F. |
| 416 | Outdoor coil sensor faulty | Service Soon | Heat pumps. Without it, defrost stops, and the coil ices up. |
| 417 | Discharge sensor faulty | Service Soon, urgent after 10 minutes | The sensor or its wire. |
| 426 | Excessive inverter alarms, 10 faults in 60 minutes | Service Urgent | Inverter models. Lennox says disconnect power to the outdoor control and restart. |
Service Urgent shows on your thermostat. Service Soon and Information alerts often go only to the dealer, so a 401 can sit there for weeks without you seeing it (how S40 alerts work).
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
What you can check yourself
The filter. A packed filter starves the indoor coil, pressures drop, and you get 410, 411 or a frozen coil.
The outdoor unit. Clear leaves, grass and snow away from all sides. In summer, with the power off at the disconnect, rinse the coil gently with a garden hose from the inside out if you can reach, or from the outside if you can’t. A dirty outdoor coil is behind a lot of 400 and 413 alerts.
The outdoor fan. If the compressor hums and the fan on top isn’t spinning, turn the system off at the thermostat. That’s a 400 or 413 waiting to happen (AC fan not spinning).
Ice in winter. Frost on a heat pump is normal. A solid block isn’t (heat pump covered in ice), and a 416 means defrost may have stopped. While it’s iced, the backup heat carries the house (aux heat vs emergency heat).
One power reset. For 411, 413 and 426, Lennox clears the lockout by cycling power. You can turn the outdoor disconnect off for a minute and back on, once. If it locks out again, the reason is still there.
What a tech does
Gauges go on, and we read the pressures, superheat and subcooling to see whether the charge is right. The capacitor and contactor get tested with a meter, because those two are behind a lot of 400 and 404 alerts and both are usually on the van. A locked rotor sometimes needs a hard start kit, and sometimes it means the compressor is done.
What the usual fixes cost
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| The fix | Our price |
|---|---|
| Run capacitor replaced | $240 to $370 |
| Contactor replaced | $290 to $425 |
| Refrigerant topped up | $190 to $740 |
| Defrost board replaced (heat pumps) | $450 to $1,150 |
| Control board replaced | $550 to $1,180 |
| Condenser fan motor replaced | $1,100 to $1,500 |
The diagnostic is a flat $99. If it needs refrigerant every year, it’s leaking, and we’ll tell you where. A leak repair or a compressor gets priced in writing, both numbers, repair and replace, side by side. More on what repairs run in AC repair cost and heat pump repair cost.
If the repair is a big one on an older system, get a second look before you spend it. Our free second opinion costs you nothing, and the estimator shows what a new system costs with $1,500 off already in the price. Thinking about brands? Lennox vs Trane lays out the difference.
Every Lennox code in one place: Lennox error codes.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
Call or book a Lennox repair
Call (703) 997-9757. We answer day or night. Or book online and pick a time. The diagnostic is a flat $99, and you see the repair price in writing before we touch anything. Heat pump running fine and you want to keep it that way? Book the $67 heating check from goswiftpro.com/cookiesoffer (book by November 30) and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276. More on Lennox repair in Northern Virginia.
Questions people ask
What does code 404 mean on a Lennox?
Compressor rotor locked. Lennox lists a shorted run capacitor, seized bearings or too much liquid refrigerant as causes. It’s a Service Urgent alert and needs a technician.
What does code 411 mean on a Lennox heat pump?
The low pressure switch opened 5 times in one call and the outdoor unit locked out. Low refrigerant, a dirty filter or coil, or a frozen coil are the usual reasons.
How do I reset a Lennox outdoor unit lockout?
Lennox clears 411, 413 and 426 by cycling the power. Turn the outdoor disconnect off for a minute and back on, once. If it locks out again, call.
What does code 401 mean on a Lennox?
The compressor ran more than 18 hours on one call, or system pressure is low. Often that’s low refrigerant from a leak. It’s an information alert that may only reach the dealer.
