Trane / American Standard Error 79: Low Pressure Cut-Out (LPCO) Lockout (Meaning, Causes, Fixes)

What the code means

Trane's System Alert Guide lists three versions of the alert. ERR.079.00, LPCO Soft Lockout, means the low pressure cut-out opened and the unit takes a 5-minute soft lockout; the guide gives the switch as opening at 50 PSI and closing at 70 PSI in cooling, and opening at 35 PSI and closing at 55 PSI in heating. ERR.079.01 means the switch has not reset within that 5-minute period, and the unit regains operation once the switch closes. ERR.079.02, LPCO Hard Lockout, means the switch tripped multiple times within a short time. Trane's alert code table for its communicating controls (CL2.079.xx) gives the same picture with slightly different numbers: the short lock-out lists heating at 25 PSI open and 45 PSI close (cooling is the same 50/70), and on the third short lock-out within the same call the control issues a hard lock-out (CL2.079.02, Critical) that is reset by cycling power. The setpoints vary by equipment, so a technician should use the Service Facts for your unit. Trane also notes that if power to the outdoor unit is interrupted, Err 79 can show on the comfort control simply because the low pressure switch can't be found on the communication bus. Related alert: 80 is the opposite problem, the high pressure switch tripping.

Likely causes

• System low on refrigerant charge, usually from a leak (Trane lists this first)
• A refrigerant restriction, such as a clogged filter drier or a kinked line
• Indoor airflow restriction in cooling mode, such as a dirty filter, dirty indoor coil or blower problem
• Outdoor airflow restriction in heating mode, such as a coil blocked by snow, ice, leaves or debris
• Wiring to the low pressure switch open or shorted
• Power to the outdoor unit interrupted, so the control can't see the low pressure switch on the communication bus

Safe DIY checks

1. Write down the exact alert (79.00, 79.01 or 79.02) and whether it happened in cooling or heating; the likely cause differs by mode.
2. Check the air filter and replace it if it's dirty. In cooling, low indoor airflow is one of the causes Trane lists.
3. Make sure supply registers and return grilles are open and not blocked.
4. In heating, look at the outdoor unit. Clear snow, ice, leaves or anything else blocking the coil, and keep about 2 feet of space around it.
5. Check that the outdoor unit's disconnect and breaker are on. Trane notes a power loss to the outdoor unit can show as Err 79.
6. For a soft lockout (79.00), wait at least 5 minutes; the unit retries on its own. For a hard lockout (79.02), Trane's reset is to cycle power: turn the system off at the thermostat, switch off power to the indoor and outdoor units for a minute, then turn it back on.
7. If Err 79 comes back after a filter change and a reset, stop resetting it. Running a compressor on low charge can damage it, and the remaining checks need gauges.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 106.00 Trane HVAC System 106.00
• 126.00 Trane HVAC System 126.00
• 156 Trane HVAC System 156
• 166.00 Trane HVAC System 166.00
• 91 Trane HVAC System 91

All Trane codes →

Sources

1. Trane System Alert Guide (Aug 2020): ERR.079.00 LPCO Soft Lockout (5 minute; heating opens 35 PSI closes 55 PSI, cooling opens 50 PSI closes 70 PSI), ERR.079.01 LPCO not reset during soft lockout, ERR.079.02 LPCO Hard Lockout; causes low charge, refrigerant restriction, indoor (cooling) or outdoor (heating) airflow restriction, low pressure switch wiring
2. Trane alert codes table (CL2 codes): CL2.079.00 Normal short lock-out (heating 25/45 PSI, cooling 50/70 PSI), third short lock-out in the same call issues hard lock-out and ERR79; CL2.079.02 Critical hard lock-out, cycle power to reset; outdoor power interruption can show ERR79