LGT

This board

Lennox current rooftop unit controller (not Landmark Prodigy)

• Alarms and economizer live on the unit display in this cabinet.
• If you still see a Prodigy label, you are on an LGH — switch charts.

Sequence

1. Call. Occupied / G starts the indoor fan. Y1 is cool. W is gas heat. They are different boards in the same box.
2. Fan. Indoor blower must prove (belt, ECM, or current switch) before some controllers allow heat or cool.
3. Econo. If OA is below the economizer setpoint, the damper takes the cool call. Compressor stays off. That is not a dead contactor.
4. Cool. Y1 → unit board → compressor contactor through HPS (and LPS when equipped). OD fan on the same call or on head pressure.
5. Heat. W → IGC / ignition module: inducer → pressure switch → HSI → valve → flame rod → indoor blower on delay.

Faults on this IFC

• Heat

  Heat fail

  lockout

  Heat section did not prove.

  • Ignition path in the heat section. Unit display names the fail.
• Cool

  Cool fail

  lockout

  Mechanical cool did not prove.

  • Economizer first, then contactor / HPS.
• Fan

  Blower fail

  lockout

  Indoor fan did not prove.

  • Direct-drive ECM vs belt — this generation is often ECM.
• HPS

  High pressure

  lockout

  Discharge / liquid opened the high-pressure switch.

  • OD coil packed. Fan running and correct rotation on 3-phase.
  • Overcharge or non-condensables. Recirc over a low roof screen.
  • Indoor airflow so bad the refrigerant has nowhere to go — still an HPS trip.
• LPS

  Low pressure / freeze

  lockout

  Suction dropped out the low-pressure switch.

  • Charge / leak. Filters and indoor coil first — a dirty rooftop ices like a starved piston.
  • TXV / orifice. Economizer dumping 40 °F OA on the coil can look like a freeze-stat trip.
  • Do not add gas because suction is low.
• Contactor

  No start / hum

  lockout

  Y1 at the board, compressor not running.

  • 24 VAC across the coil. Then dual cap (if this chassis has one) or VFD / two-stage unload.
  • 3-phase: phase loss, reverse rotation. A scroll run backwards is loud and pumps no gas.
  • Internal overload. Let it cool; find why it overheated.
• Fan prove

  Indoor fan proving

  lockout

  Controller did not see blower current / pressure.

  • Belt off, ECM alarm, or current switch around the wrong lead.
  • VFD in limp mode on some Trane/VFD cabinets will not make the prove switch.
  • Do not jumper fan-prove and walk away — heat will fire into a dead blower.
• Smoke

  Smoke / firestat

  hazard

  Supply or return smoke detector, or a firestat, has the unit locked out.

  • Reset only after you know why it tripped. Welding and dirty returns nuisance-trip these.
  • The unit board sees this as a shutdown, not a refrigerant fault.
• Phase

  Phase loss / rotation

  lockout

  Three-phase monitor opened.

  • Measure all three legs at the disconnect and at the contactor.
  • Condenser fan and compressor rotation. Swap two legs only after you confirm this unit is 3-phase on the plate.
• Econo

  Economizer FDD

  warn

  Start here when the hood is the complaint. Title 24 FDD classes: sensor, not-economizing, false-economizing, damper not modulating, excess OA.

  • Read the module in THIS hood (JADE W7220 / W7212 / Reliatel / Prodigy / factory FDD). Type the live LCD string — examples: MA SENS ERR, OA SENS ERR, CO2 SENS ERR, ACT ERROR.
  • Blade vs command first. Then OA / RA / MAT vs a real thermometer. Then Y1 vs compressor.
  • Do not add refrigerant because mixed air is cold — that is often the damper.
• MA ERR

  MA SENS ERR (JADE LCD example)

  warn

  JADE two-line LCD: MA / SENS ERR. Mixed-air sensor failed. This one alarms immediately on power-up — others wait the 60-minute setup window.

  • Probe mixed air after the damper vs MAT on the module.
  • Ohm the MA sensor. Open/short or a sensor in the OA stream is this code.
  • MAT lying high keeps mechanical cool off. MAT lying low drives the damper shut or trips FREEZING.
• OA ERR

  OA SENS ERR (JADE LCD example)

  warn

  JADE LCD: OA / SENS ERR (or OA SYLK ERR). Outside-air thermistor failed or lost SYLK. Board thinks the roof is 20 °F or 120 °F.

  • Probe OA at the hood vs the module. Several degrees off is a sensor, not a damper.
  • Ohm / SYLK the OA sensor. Cut 2-wire in the hood is weekly.
  • Sun-soaked hood vs a shaded probe — measure the air the sensor sees.
• RA ERR

  RA SENS ERR (JADE LCD example)

  warn

  JADE LCD: RA / SENS ERR. Return-air thermistor failed. Differential / enthalpy changeover compares OA to RA.

  • Probe RA in the return vs the module.
  • A missing RA sensor on a differential setup looks like false economizing.
• CO2 ERR

  CO2 SENS ERR (JADE LCD example)

  warn

  JADE LCD: CO2 / SENS ERR. DCV sensor failed or the 0–10 V input is floating. Damper holds open; compressor never runs.

  • Read CO2 in an empty space. 400–800 ppm typical empty retail. 2000 ppm empty is a failed sensor.
  • 0–10 V at the DCV input vs the sensor. If DCV is unused, set the input OFF on JADE.
• ACT ERR

  ACT ERROR (JADE LCD example)

  warn

  JADE LCD: ACT ERROR / ACT RANGE. Actuator did not follow command. Feedback pot off the shaft is the weekly.

  • Command min → 100%. 2–10 V (or 24 V floating) at the actuator vs blade travel.
  • Linkage on the shaft. Seized blades. A 24 V actuator on a 2–10 V output.
• FREEZE

  FREEZING (JADE LCD example)

  warn

  JADE LCD: FREEZING. Mixed air below the low-limit. Damper drives shut; mechanical cool may lock.

  • MAT vs a real probe. A failed MAT lying low is this, not a frozen DX coil.
  • Damper stuck open dumping 30 °F OA on the mixed-air sensor — walk ACT ERROR first.
  • True freeze: indoor airflow and charge after the damper is explained.
• Not econ

  Not economizing when it should

  warn

  OA is in the free-cooling window and the damper stayed shut. Title 24 FDD class.

  • Confirm OA really is below the high-limit / enthalpy changeover on THIS module (not a 55 °F guess).
  • Y1 present? Some modules will not economize without a cool call.
  • Min-position only, jammed blades, or a MAT low-limit holding the damper closed.
  • Mechanical cool running in 50 °F OA is this fault until the damper moves.
• False econ

  Economizing when it should not

  warn

  Damper open with OA above changeover. Space hot, high OA bills, or wet coil.

  • OA vs changeover setpoint. Sensor lying low (see OA T) is the usual.
  • Min position cranked to 40% on a small store. DCV failed high.
  • Power-fail default on some actuators is spring-open — they dump OA with 24 V lost.
• Excess OA

  Excess outdoor air

  warn

  Damper open past the commanded minimum with no economizer or DCV request.

  • Commanded position vs blade. Feedback saying 80% when the board asked 15% is actuator/linkage.
  • Barometric relief / power exhaust stuck open looks like excess OA at the space.
  • Unoccupied damper should go to the unoccupied minimum, often 0. A 24 V occupancy point stuck high holds min all night.
• No mod

  Damper not modulating

  warn

  Damper is two-position (0 or 100) instead of tracking 2–10 V. Title 24 FDD class.

  • 2–10 V at the actuator while OA is in the window. 0 V or 24 V only = module output or a two-position actuator installed by mistake.
  • JADE set for 2-position vs modulating. Direct-coupled actuator vs a 24 V floating that someone landed on analog.
  • Stuck at 50% is a failed actuator, not ‘modulating.’
• VFD

  VFD / indoor-fan drive

  lockout

  The indoor fan is a drive, not a contactor + cap. Type the keypad string: oC, Uv, ov, GF, oH, PF.

  • Photograph the drive keypad before you cycle power. Yaskawa/ABB show oC / Uv / ov; EcoBlue / Reliatel may say fan fail instead.
  • Enable present? Speed reference present? Motor spinning and proving?
  • Do not throw a dual run cap at a VFD motor.
• Enable

  VFD enable missing

  warn

  Drive has line voltage but will not run — the unit board never enabled it.

  • 24 VAC (or the digital enable this drive uses) at the enable terminals while G / occupied / heat / cool is on.
  • 0 V enable with a live G = unit board output or a smoke/firestat shutdown still open.
  • Some EcoBlue / Reliatel cabinets enable on a board relay, not on G from the stat.
• 0-10V

  VFD speed reference

  warn

  Drive is enabled but stays at min Hz or 0 because analog / serial speed is missing.

  • 0–10 VDC at the speed-reference pair under a high-fan call. Typical occupied cool is not 10 V — check the unit-board fan table.
  • 0 V with enable = broken analog wire, board analog dead, or the drive is set for a different reference (serial vs 0–10).
  • 10 V at all times = board commanding 100%. High static / oC will follow.
• oC

  oC / OC — overcurrent

  lockout

  Keypad example: oC (Yaskawa) or OC. Output current trip. Locked rotor, dirty filters, or FLA set too low.

  • Spin the wheel by hand (power off). Debris, seized bearing, belt wrapped.
  • Total external static vs this blower table. A rooftop with 1.4" TES and a 0.8" motor is an oC waiting to happen.
  • FLA / motor-data parameters vs the motor nameplate. Do not raise FLA to hide a dirty coil.
  • Ramp-up too fast on a high-inertia wheel — lengthen accel before you change a drive.
• Uv

  Uv / LU — undervoltage

  lockout

  Keypad example: Uv (Yaskawa) or LU. DC bus dropped. Incoming power or a missing phase.

  • Three legs at the drive line terminals, under load. A single-phased 3-phase drive nuisances Uv / oC.
  • Do not ignore Uv on a 480 V roof with a 208 V drive, or the reverse.
• ov

  ov / OU — overvoltage

  lockout

  Keypad example: ov (Yaskawa) or OU. DC bus high. Usually regen on decel, not a new drive.

  • ov on shutdown = decel too fast; lengthen decel.
  • Incoming voltage high or a utility swell. Measure line first.
• GF

  GF — ground fault

  hazard

  Keypad example: GF. Drive sees current to ground on the motor leads.

  • Megger the motor with the drive disconnected. A wet rooftop motor after a rain is this code.
  • Cable from drive to motor — insulation rubbed through the curb or the fan deck.
  • Do not reset-and-run a GF. That is a grounded winding or a damaged VFD output.
• oH

  oH — heatsink overtemp

  warn

  Keypad example: oH. Drive thermal trip.

  • Drive fan spinning. Heat sink packed with roof lint.
  • Cabinet door left off in the sun is not always the drive — still clean the sink.
• PF

  PF — input phase loss

  lockout

  Keypad example: PF. Missing or imbalanced incoming phase.

  • L1 L2 L3 at the drive and at the disconnect. Tight lugs.
  • A rooftop fuse that opened one leg looks like a dead motor until you read PF.
• Limp

  VFD limp / min Hz — fan prove

  warn

  Drive is running too slow to close the prove switch, so the unit board reports fan fail. Reliatel example: Fan Fail with the drive still at min Hz.

  • Min-frequency / limp parameter vs the prove-switch setpoint (often ~0.07" on some Reliatel cabinets).
  • A dirty filter that drops the motor into current-limit limp will never make prove. Clean first.
  • Do not jumper the prove switch. Heat will fire into a near-dead blower.
• Drain

  Condensate / float

  warn

  Float switch or tripped condensate opened 24V.

  • Roof drain pan, trap, and the overflow to the roof — not a furnace collector.
  • A clogged pan on a curb soaks the deck. Fix the drain before the board.

Workflows

Test points

Y1 at controller

~24 VAC on a mechanical-cool call. 0 V if the economizer is satisfying.

Contactor coil

~24 VAC when the board commands the compressor. 0 V with Y1 present = board or lockout.

HPS / LPS

Closed in the run circuit when pressures are legal. Open is a switch or a real pressure problem.

3-phase

All three legs, correct rotation. Nameplate voltage ±10%.

TES / static

External static vs the blower table for this tonnage — rooftops run dirty.

OA / MAT

Module reading vs a real thermometer at the hood and in the mixed-air stream.

Actuator

2–10 V (or 24 V floating) tracks commanded damper. Blade moves with voltage.

VFD enable

~24 VAC (or this drive’s digital enable) while G / occupied / a call is on.

VFD 0–10 V

Analog speed present under a high-fan call. 0 V with enable is a board/wire, not a motor.

IGC W

~24 VAC on a heat call.

Flame rod

µA DC per the IGC sticker in this heat section. Rising while running.

Manifold

Rating plate wins. Roof gas piping often starves high-fire.

Gotchas

• Packaged rooftop — one nameplate. Do not pick a split furnace + condenser pair for this call.
• Economizer can run the building without the compressor. Look at the damper before you condemn a scroll.
• 3-phase rotation and a roof full of lint are the first two measurements, not a charge chart.
• Type the live string in Diagnose. Examples: IGC 2-flash / 8-flash, JADE MA SENS ERR / ACT ERROR, Reliatel Fan Fail, VFD oC / Uv / GF. The sticker or LCD in THIS cabinet is the book.

Related plates

• lennox-lgh — Landmark Prodigy predecessor. Still most of the roofs.