Lennox XC21 air conditioner and XP21 heat pump (Dave Lennox Signature, iComfort-enabled): quick sheet

Covers: Lennox XC21 (air conditioner, service literature Corp. 1007-L2) and XP21 (heat pump, Corp. 1031-L7), 2, 3, 4 and 5 ton, R-410A, expansion valve indoor coils only. Both use an A175 main control (outdoor control part number 101798-xx) that works in communicating (iComfort) or conventional 24 V mode, plus a separate fan motor control. The XP21 literature describes a two-stage scroll compressor; the 2018 XC21 literature describes the compressor as one-stage while the same control section covers two-stage units, so confirm staging on the nameplate. For ML14XC1/EL16XC1 single-stage units see the separate sheet.

Single sourceBrands: Lennox4 components

In conventional mode the A175 shows diagnostics on two LED pairs: DS11 (green) and DS14 (red) for system status, fault and lockout codes, and DS15 (yellow) and DS13 (red) for compressor fault and lockout codes. Newer controls also have a seven-segment display that shows unit type, capacity and fan profile on power-up. With an iComfort thermostat the same events appear as numbered alert codes (400-series).

Safety first

Pressure, charge and compressor replacement work are for an EPA 608 certified technician.
Single-pole contactors leave one leg live: lock out the disconnect.

How it works

Only the latest active fault or lockout shows; with none active, the status LEDs show normal operation.

Unit capacity and fan profile must be configured on a new or replacement control (via the FIELD TEST pins E33 or the push button / seven-segment display, depending on revision). Three bars on the display during power-up mean the control was not configured.

XP21 defrost: termination temperature is set by a jumper (factory 50 F); with no jumper the default is 90 F; the coil sensor RT21 measures it.

Compressor fault LEDs are monitoring only; they do not provide safety protection.

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

Lennox residential literature (installation and service documents)

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Air-source heat pump outdoor unit beside a house in the snow by PeterEastern, CC BY-SA 4.0

Diagnostic guides for this appliance

Guide · Air Conditioner

PTAC not cooling (fan runs, air not cold)

Most PTAC no-cool calls are airflow (filter, coils), a wrong control setting or a failed compressor start component. Rule those out before suspecting the sealed system.

6 steps ~60 minmoderate

Guide · Air Conditioner

PTAC not heating (electric heat or heat pump)

PTACs heat with electric strip heat, a heat pump, or both. Heat pump models switch to strip heat in cold weather. Check settings, then the heater element and its limits, then the reversing valve.

5 steps ~60 minmoderate

Guide · Air Conditioner

PTAC error codes: how to read and clear them

Most PTAC brands store fault codes that can be shown with a button combination on the control (for example, on GE Zoneline (2019 technical service guide) you hold FAN and the red AUX button together while the unit is OFF, and AUX clears stored faults; other models use other sequences, so use the model's tech sheet). Read the active code, fix the cause, then clear it.

4 steps ~20 mineasy

Other air conditioners manuals

Manual · Air Conditioner

Carrier Infinity Greenspeed 24VNA6 air conditioner and 25VNA4 heat pump (variable speed): quick sheet

Carrier Infinity Greenspeed Intelligence variable-speed outdoor units: 24VNA6 air conditioner (up to 26 SEER) and 25VNA4 heat pump (up to 24 SEER, up to 13 HSPF), 2-5 ton. Nomenclature from the service manual: 24 = AC, 25 = HP; V = variable speed; N = Infinity series; A = Puron (R-410A); 6 / 4 = SEER tier; two digits = nominal capacity (e.g. 36 = 36,000 BTU/h); voltage digit 3 = 208/230-1; compressor ID 4 = variable-speed scroll, 5 = variable-speed rotary. Serial format: week + year + plant letter + sequence (e.g. 0118E00001). Other 24VNA families (24VNA0, 24VNA9) use different software and code lists.

5 components4 common failuresCarrier

Manual · Air Conditioner

Trane XV20i / XV18 variable-speed outdoor units (4TTV0 AC, 4TWV0 heat pump, ComfortLink II): quick sheet

Trane variable-speed TruComfort / ComfortLink II communicating outdoor units: 4TTV0 air conditioners (XV20i family, e.g. 4TTV0024A1000C, 4TTV0036, 4TTV0048, 4TTV0060, 4TTV0061) and 4TWV0 heat pumps (Service Facts 4T-V0-SF). They use an Integrated Variable Speed Control (IVSC) board and a Communicating Display Assembly (CDA) in the outdoor unit. Distributor listings note several 4TTV0 revisions; confirm the full model on the nameplate before ordering parts. Single-stage XR14/XR16 units are on a separate sheet.

4 components3 common failuresTrane

Manual · Air Conditioner

Goodman / Amana ComfortNet two-stage condensers and heat pumps (GSXC, DSXC, ASXC, GSZC, ASZC 16-18 SEER): quick sheet

Goodman (G), Goodman Deluxe (D) and Amana (A) communicating-ready two-stage split units: SXC = air conditioner, SZC = heat pump; 16 or 18 = SEER class; then capacity (024-060) and voltage digit (1 = 208/230 V single phase), e.g. GSXC180481, DSXC160361, ASZC180601. Revisions moved through Copeland UltraTech 2-stage scroll compressors (later UltraTech 3) and 7 mm coils. They work with a ComfortNet communicating thermostat over a 4-wire connection or with conventional 2-stage 24 V thermostats. Single-stage GSX/GSZ units are on a separate sheet.

4 components4 common failuresGoodman, Amana, Daikin