Lennox AC Error Code 413 – Meaning, Causes & Fixes
Error code 413 on a Lennox air conditioner or heat pump indicates a low-pressure lockout — the refrigerant circuit's suction-side pressure dropped below the minimum safe operating threshold, and the system shut down to protect the compressor from damage. This is among the most serious fault codes an air conditioning system can display, and it should not be reset repeatedly and ignored. Low refrigerant pressure almost always indicates either a refrigerant leak somewhere in the system, or a significant airflow restriction that is starving the evaporator coil of heat to absorb. Code 413 requires professional diagnosis in most cases because refrigerant handling is federally regulated and requires EPA Section 608 certification. However, there are a few homeowner-accessible checks that can rule out simple causes before scheduling a service call.
What it means
Low-pressure lockout — refrigerant pressure dropped below the safe minimum and the system shut down to protect the compressor.
Severity
HighCan you fix it yourself?
Partially — check airflow and filters. Refrigerant issues require a licensed technician.
Call a technician when
Yes — if the error returns after resetting, a refrigerant leak or pressure switch fault requires professional repair.
What Does 413 Mean?
Code 413 is a low-pressure fault on Lennox iComfort, iComfort S30, and communicating system configurations. The system's low-pressure switch monitors the suction-side refrigerant pressure — the pressure of the refrigerant vapor returning to the compressor after passing through the evaporator coil.
For R-410A systems (the most common refrigerant in modern Lennox equipment), normal suction-side pressure during cooling operation typically runs 100–130 PSI depending on ambient temperature and system load. When pressure falls below the low-pressure cutout threshold — approximately 40–50 PSI on most residential units — the low-pressure switch opens its circuit. This immediately de-energizes the compressor contactor, shutting down the compressor.
Code 413 is logged on the iComfort thermostat or communicating control board. The system will attempt a restart after a delay, but if pressure remains low, it will enter a hard lockout after repeated failed attempts.
The reason this protection matters: when refrigerant pressure is too low, the refrigerant inside the compressor doesn't carry adequate lubricating oil to the moving parts. Metal-to-metal contact begins inside the compressor within minutes of operation under low-charge conditions. Compressor replacement is one of the most expensive HVAC repairs, often costing $1,500–$3,000 including labor. Code 413's lockout exists specifically to prevent this.
What Causes This Error?
- Refrigerant leak — the most common cause of persistent code 413; leaks occur at brazed joints, flare fittings, Schrader valve cores, or the coil itself
- Severely clogged air filter reducing evaporator coil airflow — without adequate airflow, the coil absorbs too little heat, refrigerant doesn't fully evaporate, and suction pressure drops
- Frozen evaporator coil — ice buildup blocks airflow entirely and causes rapid pressure drop; often caused by a dirty filter, dirty coil, or low charge
- Faulty low-pressure switch tripping at an incorrect threshold — less common, but possible on aging equipment
- Expansion valve (TXV or EXV) stuck closed or restricted — reduces refrigerant flow to the evaporator and causes suction pressure to fall
- Refrigerant undercharge from a previous service where the system was not fully charged to specification
How to Fix 413
- 1
Turn the system off at the thermostat and locate the air filter. Even a moderately clogged filter can reduce evaporator airflow enough to contribute to a pressure drop event. Replace it with a new filter and inspect the indoor unit's return air path for any obvious blockages.
- 2
Inspect the indoor unit's evaporator coil for ice. If the refrigerant lines running from the outdoor unit to the indoor air handler are frosted, or if you can see ice through the access panel on the evaporator coil housing, the coil is frozen. Set the system to fan-only mode (no cooling) to thaw the coil — this takes 1–4 hours depending on the amount of ice. Do not chip or scrape ice off the coil.
- 3
With a fresh filter and a thawed coil, reset the system: set the thermostat to Off, switch the outdoor unit's circuit breaker off for 30 seconds, restore power, and wait 5 minutes before calling for cooling. This allows the low-pressure switch to reset and the system to re-initialize.
- 4
If code 413 returns within the first cooling cycle after these steps, stop resetting the system. Continued compressor starts under low refrigerant pressure will cause damage. Inspect the refrigerant lines at the outdoor unit for oily, greasy residue near fittings or the service valve caps — refrigerant oil escapes with the refrigerant at leak points and leaves a visible residue.
- 5
Confirm that the service valve caps on the outdoor unit are tight. These brass caps (on the service ports) should be finger-tight plus a quarter turn. Missing or loose caps allow refrigerant to escape slowly through the Schrader valves underneath.
When to Call a Professional
A Lennox-authorized HVAC technician must perform a refrigerant pressure test using manifold gauges to confirm whether the system is undercharged. If pressure is below specification, the technician will perform a leak search using electronic leak detection equipment or UV dye. Once the leak is located and repaired, the system must be evacuated (all remaining refrigerant and moisture removed) and recharged to the manufacturer's exact weight specification. Simply adding refrigerant without finding the leak is a non-compliant, temporary fix — and under EPA Section 608, knowingly venting refrigerant is a federal violation. Budget $300–$600 for a full leak search and recharge; compressor replacement if damage has occurred can run $1,500–$3,500.
Can You Still Use the Device?
No. Do not attempt to run the system while code 413 is active, especially not by repeatedly resetting it. The compressor is the single most expensive component in the system. Each restart attempt under low-charge conditions risks permanent damage. If you need cooling urgently, use portable air conditioners or fans while you arrange professional service.
See Also
Official Resources
Related Error Codes
Disclaimer: This article is for informational purposes only and does not constitute professional repair advice. CodeExplain is not affiliated with Lennox or any other brand mentioned. Always consult a qualified technician for complex repairs.