SLP99V

Device off the iComfort bus. Native, expected, first-class on SLP99.

1. 1 Data pair, not 24 V

   On Lennox SLP99V indoor/outdoor comm is a data pair (AB, S1/S2, U, ComfortBridge, ComfortLink — whatever THIS plate uses). It is not a 24 V Y. Check polarity, continuity, and that nobody landed it on R/C. i+/i− polarity, one communicating control, indoor and outdoor on the bus. Data, not 24 VAC.

   Expect: Pair continuous, not shorted to chassis, polarity per the diagram.

2. 2 Power both ends, then the outdoor status

   Indoor transformer and outdoor control both alive. Read the outdoor LED / 7-seg. Water in the outdoor control is the usual dead-end after a storm. Do not convert to 24V. Isolating devices one at a time finds the bad node.

   Expect: Both controls powered. Outdoor showing a comm or address state, not a blank board.

3. 3 Address / bias, then the control

   Communicating plates need a matching indoor address and sometimes a terminating bias. Swap is last: known-good pair + power both ends + correct address.

   Expect: Indoor sees the outdoor. No comm code after a power cycle.

Serial blower dropped off the IFC.

1. 1 Reseat the serial plug

   On Lennox SLP99V this is a communicating / serial ECM, not a tap motor. Kill power, reseat the 14-pin (or OEM serial plug) at the motor and at the IFC. Pins straight, no corrosion. Harness, motor power, then motor. This is a communicating VS — same family as EL296V E201, native here.

   Expect: Plug fully seated both ends. Correct motor for this cabinet.

2. 2 Power and the data pair at the motor

   Line voltage at the motor, then the serial/data pair continuity. A half-seated plug or a pinched harness throws comm on a good motor.

   Expect: Line voltage present. Data pair not shorted to chassis.

3. 3 Motor vs IFC serial driver

   Known-good motor still throws this code = Lennox SLP99V IFC serial driver. Known-good IFC still throws it = motor module. Do not put a PSC or 9-speed tap motor on a serial IFC.

   Expect: One clean CFM request after a known-good pair.

Modulating prove failed at that rate. Stat may say low/high PS even though the hardware is a band, not two EL296 discs.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the Lennox SLP99V active code. Write E223 / E225. Is the inducer spinning on this call? Silent inducer is not a pressure switch. Vent, trap, inducer, hose. 96% PVC physics.

   Expect: 120 VAC at the inducer on a heat call. Wheel turning.

   If fail: 120 VAC missing → IFC / door switch / incoming power. Voltage + dead motor → inducer.

   If pass: Inducer running + this code → trap, hose, switch, vent (step 2).

2. 2 Trap, collector, and the hose

   A drowned 96% trap will not make collector pressure. Pull the trap, flush it, confirm the collector box and inducer outlet are not full. Hose cracked, off the port, or full of water fakes an open switch. Do not treat it as a two-stage W1/W2 swap.

   Expect: Trap flowing, hose dry and downhill, ports not swapped on a two-switch board.

3. 3 Switch ohms, then vent load

   Ohm the pressure switch: NO, must close when that inducer speed is making the collector setpoint. If the switch is good, walk the PVC — bird screen, ice, long 2" run, wind, shared chase.

   Expect: Switch closes on known draft. Vent unrestricted.

   If fail: Replace the open switch only after the vent and trap are proven.

   If pass: Switch closes on draft and the code stays → IFC input.

Weak µA or max retries with no flame current.

1. 1 Measure µA while it is running

   Do not clean first and guess. Series-µA the rod on Lennox SLP99V during a call. Falling µA is oxide, ground, or a dying flame. A hard drop is often the pressure switch or limit opening, not the rod. Watch a try. Rod, HSI, gas. Same SureLight ignition as EL296V.

   Expect: Flame: µA per SLP99 card — same F-menu idea if the board exposes it.

2. 2 Rod, ground, then gas

   Clean the rod with a non-scratch pad — not sandpaper. Chassis ground to burner box and IFC (painted screw and missing star washer are classic). Then manifold on the stage that is dropping and confirm LP conversion leftovers.

   Expect: Manifold on the rating plate. Solid burner-box ground.

3. 3 Prove it is not a switch opening

   Watch the live Lennox SLP99V display when flame drops. A PS or limit code that appears at the drop means the rod was a victim. Inducer, hose, vent wind, or a limit on a wrong heat tap.

   Expect: µA stable and the live code stays a flame code — then rod/valve/IFC. Otherwise chase the new code.

Limit / rollout string. Stat names it in English.

1. 1 Treat this as flame outside the exchanger

   Do not reset the Lennox SLP99V rollout and walk away. Write the code, kill the call, and look for flame at the burners, crossover, and vestibule before you do anything else.

   Expect: No flame outside the exchanger. Burners seated. Crossover lit.

   If pass: Still inspect secondary / flue / condensate before you reset.

2. 2 Secondary, condensate, and flue

   Blocked secondary, drowned collector, or a plugged flue pushes flame out the front. Pull the inducer/collector look, confirm the trap is flowing, and check PVC/vent. Heat CFM follows %. Filter, coil, rise. Rollout is still manual reset.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on Lennox SLP99V is a CO / fire call — cracked primary, misaligned burners, or a failed secondary. Confirm CO in the space before you leave.

   Expect: One clean cycle after the cause is fixed. No second reset as a repair.

Ignitor open or trigger fault.

1. 1 Ohms at the igniter, then at the IFC

   On Lennox SLP99V measure cold ohms at the igniter plug, then at the IFC connector. An open at the plug is the element or local harness. Open only at the IFC is the harness rub-through on the collector. Ohms and 120 VAC during trial. Confirm on the SLP99 door card.

   Expect: Nitride cold ohms per THIS door sticker — do not use an 80% Norton chart.

2. 2 120 VAC during warm-up

   Call for heat and measure 120 VAC at the igniter during the warm-up window. 0 V + good ohms = IFC igniter relay. Voltage + no glow = cracked nitride.

   Expect: 120 VAC only during warm-up. Visible glow before the valve opens.

3. 3 Do not keep cycling a cracked element

   A cracked nitride can ohm ‘good’ cold and open hot. If ohms and voltage are good and there is no glow, replace the igniter. Confirm polarity/ground (this plate’s polarity code) before you condemn a new igniter.

   Expect: One clean light after a known-good igniter and a proven ground.