ML193

Flame with the valve off.

1. 1 Shut the gas cock first

   On Lennox ML193 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. Shut the cock. Leaking valve.

   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.

Pressure switch is made with the inducer off.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the Lennox ML193 active code. Write 2 flash. Is the inducer spinning on this call? Silent inducer is not a pressure switch. Unplug the switch. Water in the hose or a welded disc.

   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. 96% residual draft from a shared vent.

   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.

Draft not proven. On this 96% that is trap / PVC / collector — not a chimney.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the Lennox ML193 active code. Write 3 flash. Is the inducer spinning on this call? Silent inducer is not a pressure switch. Condensate trap, collector, PVC intake/exhaust, inducer.

   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 pull the ML180 metal-vent playbook. Same flash, opposite wet/dry checks.

   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.

Airflow / overtemp.

1. 1 Airflow before you touch the limit

   On Lennox ML193 a limit is airflow or overfire until proven otherwise. Do not replace the limit first. Filter, blower wheel, A-coil, and the heat-speed setting for THIS motor (tap, model plug, or communicating CFM — not a sister SKU chart). Tap or PSC speed, filter, coil, rise.

   Expect: Rise in the rating-plate window. Static vs this cabinet’s blower chart.

2. 2 Measure static and temperature rise

   Total external static vs the Lennox ML193 blower chart for this cabinet size. Supply minus return rise after 8–10 minutes. A new 5" media cabinet or closed supplies will trip this all winter.

   Expect: Rise typically 35–65 °F — rating plate wins.

   If fail: Fix the restriction or the heat-speed setting. The limit is doing its job.

   If pass: Airflow in range → step 3 (overfire / limit itself).

3. 3 Overfire, then the limit circuit

   Clock the meter / manifold on the stage that is tripping vs the rating plate. Then ohm the limit after it cools — a limit that never recloses is failed. Rollout in the same circuit is a different call (see rollout on this door card).

   Expect: Manifold on plate. Limit closed when cool.

Flame containment. Hazard.

1. 1 Treat this as flame outside the exchanger

   Do not reset the Lennox ML193 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. Exchanger, burners, flue. Reset only after the cause is gone.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on Lennox ML193 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 3-flash / 2-flash. Retries used up on draft.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the Lennox ML193 active code. Write 6 flash. Is the inducer spinning on this call? Silent inducer is not a pressure switch. Fix the 3-flash physics (trap/PVC) before you reset.

   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.

   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.

Failed to light / prove flame.

1. 1 Watch one full try — do not reset

   Write the Lennox ML193 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 cock, valve 24 VAC, rod.

   Expect: W → Lockouts

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.

Had flame, then lost it. Retries used up.

1. 1 Measure µA while it is running

   Do not clean first and guess. Series-µA the rod on Lennox ML193 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. Rod, ground, manifold after blower-on, vent that starves the flame.

   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.

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

3. 3 Prove it is not a switch opening

   Watch the live Lennox ML193 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.

L1/N reversed or ground missing.

1. 1 Hot and neutral at the IFC, not the disconnect

   On Lennox ML193 polarity/ground is measured at the board. A correctly landed disconnect with a swapped whip at the IFC still throws this. Hot and neutral at the IFC. Chassis ground. Flame sense will not work reversed.

   Expect: L1 hot, N neutral, ~120 VAC L1 to N, ~0 VAC N to ground.

2. 2 Burner-box and IFC ground

   Green ground to cabinet and IFC. Painted screw and missing star washer fail the ground check and kill flame sense.

   Expect: Near-zero ohms burner box to IFC ground screw.

3. 3 Then flame and HSI

   Polarity/ground faults take out HSI and flame sense together. After the wiring is right, run one heat cycle and confirm µA and igniter glow before you change those parts.

   Expect: Code cleared. One clean light. µA in range.