Test a furnace pressure switch

Tools: Meter (VAC and ohms) · The hose still on the correct tap · The live IFC code — 31/32, E2, 3-flash — before you pull power

1. 1. Name which switch — or that there is no switch

   Count them. One hose = Comfort / single-stage. Two hoses, not teed = two-stage low vs high (59TP6, 59TN6, ML196 two-stage). Modulating Infinity 59MN7 is a pressure BAND / transducer — do not ohm a ‘low’ and a ‘high’ that are not there. Rooftop IGC has a draft switch in the heat section, not a split-system 96% collector.

   Expect: Only / low / high / band named before the meter comes out.

   If not: Load the plate (59SC5, 59TP6, 59MN7, GR9S96). Sister SKUs do not share switch counts.

2. 2. Expected state vs the live code

   Inducer OFF: switch must be OPEN. Closed with the inducer off is welded disc, water in the hose, or residual draft — Carrier 23, Goodman 2-flash / E1, Lennox 2-flash. Inducer ON: switch must CLOSE. Open with a spinning inducer is Carrier 31/32, Goodman 3-flash / E2, Lennox 3-flash. Write the live code before you unplug anything.

   Expect: Open at rest. Closed under draft. Code matches that state.

   If not: If the inducer is not actually spinning, this is an inducer / 120 V / wheel call, not a switch.

3. 3. 24 V in the circuit, then ohms out of it

   On a call: one side of the switch is IFC 24 V, the other side returns to the IFC when the disc closes. 24 V on one pin and 0 V on the other with the inducer on = switch open (or hose/draft). Unplug the harness and ohm the switch alone. Continuity with a known draft (inducer forced or a gentle vacuum on the port). Infinite ohms with draft = bad switch. Continuity with no draft = welded / wet hose.

   Expect: Harness voltage agrees with ohms. One measurement in circuit, one out.

   If not: Switch ohms good and IFC still faults = IFC input or a second switch you did not count. Do not throw the switch twice.

4. 4. Hose and the draft that moves the disc

   Hose on the correct tap, downhill, empty of water, not cracked, not teed across low and high. Then the draft: 96% trap/collector/PVC slope/attic ice; 80% chimney/liner/cap/common vent. A drowned collector makes a perfect switch look failed. Fix that before you open the bag.

   Expect: Dry hose, correct tap, vent family named.

   If not: Two-stage: 32 is the low-fire hose only. High-fire hose will not close on low inducer. See code 32 behavior.

• The switch closing is the last check, not the first part. Trap, vent, inducer, then the disc.
• A 5-minute open on many Carrier IFCs parks the inducer ~15 min. Waiting is not a bad switch.
• Rooftop heat-section PS hoses fill with water. Same ohm test, different drain.