FieldBench
How To
Electrical10 min · 4 steps

Test a furnace pressure switch

The switch is a witness, not the vent. Inducer off = open. Inducer on = closed. Ohm it out of the harness. Do not leave a jumper on a draft switch.

Still failing after this procedure?

How To stays open. Diagnose walks the next measurement.

Before you start

  • A jumped pressure switch is a CO / fire path if you walk away. Prove it, then take the jumper off.
  • Rollout and limit are not pressure switches. Do not jumper those to ‘see if it runs.’

On the truck

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

Steps

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

Field notes

  • 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.

Related

Sources

Practice distilled from public HVACR School and AC Service Tech material. OEM I/O and EPA 608 still win.