N9MSE

Memory empty or power was cycled.

1. 1 Confirm this is not a fault

   On the ICP N9MSE this readout (11) is run status, not a lockout. Write what the display actually shows and whether W/Y/G is present at the IFC.

   Expect: Idle / Ht / CF / stage+CFM words change when you apply and remove the call.

   If fail: If the word never changes on a real call, the call is not reaching this IFC.

   If pass: The board is alive. Diagnose the comfort complaint, not this code.

2. 2 Prove the call at the board

   Measure R to W / Y / G at the ICP N9MSE IFC, not at the thermostat. Communicating plates need the data pair and a live indoor address — 24 V on W is not the test.

   Expect: Not a fault. Recreate the call and watch the live code.

   If fail: Fix the stat, harness, or data pair before you change parts.

3. 3 Watch one full sequence

   Leave power on. Watch Carrier-platform two-digit IFC through one cycle and write the first real fault that replaces this status word.

   Expect: W: Inducer on → the single PS must close (31 if it will not) → HSI → valve → flame (34 if no prove) → blower on the heat tap.

Powered up on a W call — blower runs ~90 s.

1. 1 Confirm this is not a fault

   On the ICP N9MSE this readout (12) is run status, not a lockout. Write what the display actually shows and whether W/Y/G is present at the IFC.

   Expect: Idle / Ht / CF / stage+CFM words change when you apply and remove the call.

   If fail: If the word never changes on a real call, the call is not reaching this IFC.

   If pass: The board is alive. Diagnose the comfort complaint, not this code.

2. 2 Prove the call at the board

   Measure R to W / Y / G at the ICP N9MSE IFC, not at the thermostat. Communicating plates need the data pair and a live indoor address — 24 V on W is not the test.

   Expect: Normal after a brownout. Repeating 12 with a limit complaint → airflow, not a bad 12.

   If fail: Fix the stat, harness, or data pair before you change parts.

3. 3 Watch one full sequence

   Leave power on. Watch Carrier-platform two-digit IFC through one cycle and write the first real fault that replaces this status word.

   Expect: W: Inducer on → the single PS must close (31 if it will not) → HSI → valve → flame (34 if no prove) → blower on the heat tap.

Limit/rollout stayed open. Auto-reset ~3 h. Hardened 33.

1. 1 Treat this as flame outside the exchanger

   Do not reset the ICP N9MSE 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. Treat as a 33 that won. Filter, heat tap, coil, return, rollout reset. Do not sit on 3-hour resets.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on ICP N9MSE 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.

Hardened 34. Auto-reset ~3 h.

1. 1 Watch one full try — do not reset

   Write the ICP N9MSE code. Watch inducer, HSI glow, valve click, flame, µA. The step that dies is the repair. Resetting only burns another try into a harder lockout. HSI, gas, flame sensor, ground. Same prove path as 59SC5.

   Expect: W → No W2 path → Limits

2. 2 Gas and the valve

   Cock open, inlet pressure, then manifold on the stage that is failing vs the rating plate. 24 VAC at the valve during trial? Coil ohms? A valve that never opens looks like a ‘bad igniter’ if you did not watch the sequence.

   Expect: Inlet and manifold on the plate. 24 VAC only during trial.

3. 3 Flame prove, then the lockout timer

   Glow + valve + no prove → rod, ground, or IFC sense. Prove then drop → µA / PS / limit. Soft lockouts on this family often auto-reset on a long clock — do not sit on resets.

   Expect: Stable µA after light-off. No new PS/limit code.

Flame sense while the gas valve is de-energized.

1. 1 Shut the gas cock first

   On ICP N9MSE the rod sees flame with the valve de-energized. Shut the cock. If flame stays, the valve is leaking — replace it. Do not jump or energize it as a test. Leaking valve. Shut the cock.

   Expect: Flame dies when the cock is shut.

   If fail: Leaking valve. Replace. Do not leave the unit.

2. 2 Rod and IFC flame circuit

   If flame dies with the cock shut, the rod is shorted to ground or the IFC flame circuit is lying. Unplug the rod, inspect ceramic, and check the harness for a rub-through on the collector.

   Expect: Rod isolated from ground. IFC does not report flame with the rod unplugged.

3. 3 Confirm valve solenoids are actually off

   Measure 24 VAC at both solenoids after the call drops. A welded IFC relay or a shorted W will keep a solenoid wet and look like ‘unexpected flame.’ Replace the IFC only after the valve and rod are proven.

   Expect: 0 VAC at both solenoids after the call. No flame.

24 V fuse on the IFC.

1. 1 Find the 24 V short before you replace the fuse

   On ICP N9MSE an open IFC fuse is a short on W, Y, G, HUM, EAC, or a data pair landed on 24 V. Unplug the thermostat / data harness first. Tstat short, HUM/EAC, valve harness. Replace the fuse only after the short is gone.

   Expect: Fuse out. 24 V transformer still good. Harness isolated.

2. 2 Replace the fuse with the harness off

   New fuse, harness still off. If it holds, the short is in the field wiring or accessory. Plug one circuit back at a time (W, then Y, then HUM/EAC).

   Expect: Fuse holds with harness off. Dies when the shorted lead lands.

3. 3 If it blows with the harness off

   Short is on the ICP N9MSE IFC or the gas-valve harness. Unplug the valve and try once more. Repeat blow with valve unplugged = IFC.

   Expect: Do not keep feeding fuses into a welded valve coil or a burned IFC trace.

Inducer running, the one PS did not close (or the draft path will not hold). On this 96% that is trap / PVC / inducer.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the ICP N9MSE active code. Write 31. Is the inducer spinning on this call? Silent inducer is not a pressure switch. One PS only — do not hunt a second high-fire switch.

   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. Dump the trap and open the collector. N9MSB 96% drowns overnight.

   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. PVC intake/exhaust, ice, wind, hose on the only collector tap.

   Expect: Pressure switch: One NO switch. Closed only with the inducer up. No second hose.

   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.

The winter callback. Airflow first; rollout is hazard.

1. 1 Treat this as flame outside the exchanger

   Do not reset the ICP N9MSE 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. Filter, heat-speed tap (this is not a VS 59TP6 menu), coil, return, TES. Rise vs plate. Overfire on a single-stage orifice.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on ICP N9MSE is a CO / fire call — cracked primary, misaligned burners, or a failed secondary. Confirm CO in the space before you leave. Rollout manual reset — inspect exchanger / burners / flue before you reset.

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

Trial without flame sense. Repeats → 14.

1. 1 Measure µA while it is running

   Do not clean first and guess. Series-µA the rod on ICP N9MSE 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. HSI glow and 120 VAC, gas cock, manifold, flame rod, IFC ground.

   Expect: Typically ≥ 1.0 µA DC on this family — confirm the door card. Rising while running.

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. If 31 is also stored, fix draft first or you will never get a fair 34.

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

3. 3 Prove it is not a switch opening

   Watch the live ICP N9MSE 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.