Lennox Error Code 434 (E434): Outdoor Control Lost Communication With the Inverter (Meaning, Causes, Fixes)

What the code means

Lennox's inverter outdoor units (for example the SL25KCV, SL28XCV, XC20 and EL/SL variable-capacity models) have two boards that talk to each other: the Lennox outdoor control and the compressor inverter. Alert 434 means the outdoor control hasn't heard from the inverter for more than three minutes. Lennox's Unit Information for the SL25KCV says the outdoor unit then stops all compressor demand and keeps trying to re-establish communication, clearing on its own if it succeeds. Lennox's alert code guide describes how: it turns the outdoor contactor off for two minutes, then tries to talk to the inverter for three minutes, up to three times. If communication isn't restored, the unit locks out (Lennox says after 60 minutes, or after three failures in one thermostat call on older models) and the thermostat shows a Service Urgent or critical alert. The inverter's own LEDs show code 53: the red LED flashes five times and the green LED flashes three times. Normal is red on steady and green off. Lennox also notes the indoor blower won't run, even in test mode, while 434 is active, until the system is reset.

Likely causes

• The outdoor disconnect is off or the outdoor breaker tripped while the indoor unit (which supplies the 24 VAC control power) is still on, or the reverse (Lennox lists this first)
• A generator powering the indoor unit but not the outdoor unit
• An interruption of the main 230 V power to the inverter
• Loose or disconnected electrical power connections in the outdoor unit
• Power being switched off at one unit during service or maintenance, which Lennox says can trigger 434 on its own

Safe DIY checks

1. Check the thermostat's alert screen and note whether 434 is the only alert. If other codes appear with it, write them all down for the technician.
2. Make sure the outdoor disconnect next to the unit is on and the outdoor unit's breaker in the panel isn't tripped. Reset a tripped breaker once; if it trips again, stop and call a technician.
3. If you're running on a generator, note that Lennox lists a generator that powers the indoor unit but not the outdoor unit as a cause. The code is expected until the outdoor unit has power again.
4. Reset the system the way Lennox describes: turn the outdoor unit's power off and back on, then turn the indoor unit's power off (the furnace or air handler switch or breaker, which feeds 24 VAC to the outdoor control) and back on. Cycling the indoor power de-energizes the outdoor control and inverter through the contactor.
5. Give it a few minutes. The alert clears by itself once the outdoor control and inverter are talking again.
6. If you can safely see the outdoor control's display, E434 tells you it's this fault. Don't open the inverter section yourself; Lennox's service literature has technicians wait for the capacitors to discharge before working inside.
7. If 434 comes back after a reset with power confirmed on both units, it needs a technician to check the 230 V supply at the inverter, tighten power connections, and follow the unit's service manual.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 292 Lennox HVAC System 292
• 31 Lennox HVAC System 31
• 413 Lennox HVAC System 413
• 106.00 Trane HVAC System 106.00
• 126.00 Trane HVAC System 126.00

All Lennox codes →

Sources

1. Lennox Unit Information SL25KCV (Corp. 100102, Jan 2025), Table 10: E434 / inverter code 53, causes (outdoor power off with indoor on, loose connections, main power interruption, generator) and corrective actions
2. Lennox Residential Communicating Systems Alert Code Guide (V2, 100017): 434 OD Inverter Comm Error to Main Control, contactor cycling, reset steps, inverter LED code 53
3. Lennox iComfort S30 installation and setup manual, alert codes 434-438 (ManualsLib p.45)
4. Lennox XC20 service literature, Table 6: E434 lost communication with inverter, lockout after three occurrences in one thermostat call