Code 3 - pressure switch stuck open. What do I do

sounds like u nee a new pressure switch. check 1\4 inch tubing going to pressure switch, remove it make sure no condesation in it., blower will only come on once flame is establised, ventor motor comes on closes pressure switch, brings on ignitor then gas the blower, make sure no blockage in vent pipe

You might try viewing this YouTube video which shows a way to unstick the pressure switch. https://www.youtube.com/watch?v=Wc_oFIL2aRY

A pressure switch code means there is an unsafe condition that should be looked at by a competent service tech. Sometimes there is a switch failure, but most of the time there is an unsafe condition.

TRI-COLOR (DSI LED) DIAGNOSTIC TABLE
No. of Red BLINKS
1 Flame sensed when no flame should be present
Verify the gas valve is operating and shutting down properly. Flame in burner assemble should extinguish promptly at the end of the cycle. Check orifices and gas pressure.

2 BLINKS Pressure switch stuck closed/ inducer error
Pressure switch stuck closed. Check switch function, verify inducer is turning off.

3 BLINKS 1st-stage pressure switch stuck open/inducer error
Check pressure switch function and tubing. Verify inducer is turning on the pulling sufficient vacuum to engage switch.

4 BLINKS Open limit switch Verify continuity through rollout switch circuit.

5 BLINKS Open rollout/open fuse detect Verify continuity through rollout switch circuit, check fuse.

6 BLINKS 1st-stage pressure switch cycle lockout If the first stage pressure switch cycles 5 times (open, closed) during one call for heat from the thermostat the control will lockout.Check pressure switch for fluttering, inconsistent closure or poor vacuum pressure.
7 BLINKS External lockout (retries exceeded) Failure to sense flame is often caused by carbon deposits on the
flame sensor, a disconnected or shorted flame sensor lead or a
poorly grounded furnace. Carbon deposits can be cleaned with
emery cloth. Verify sensor is not contacting the burner and is locatedin a good position to sense flame. Check sensor lead for shortingand verify furnace is grounded properly.

8 BLINKS External lockout (ignition recycles
exceeded where flame is established and then lost)Check items for exceeded retries listed above and verify valve is not dropping out allowing flame to be established and then lost.

9 BLINKS Grounding or Reversed polarity Verify the control and furnace are properly grounded. Check and reverse polarity (primary) if incorrect.

10 BLINKS Module gas valve contacts energized
with no call for heat Verify valve is not receiving voltage from a short. If a valve wiring is correct and condition persists, replace module.
11 BLINKS Limit switch open - possible blower
failure overheating limit Possible blower failure, restricted air flow through appliance or duct work. Verify continuity through limit switch circuit and correct overheating cause.

12 BLINKS Module Ignitor contact failure Fault code indicates the module ignitor contacts are not functioning
properly. Replace module.Solid Module - internal fault condition Module contacts for gas valve not operating or processor fault.
Reset control. if condition persists replace module.
3 double 2nd-stage Pressure Switch Stuck Open/Inducer Error
Check pressure switch function and tubing. Verify inducer.

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EACH OF THE FOLLOWING STATUS CODES IS A TWO DIGIT NUMBER WITH THE FIRST DIGIT DETERMINED BY THE
NUMBER OF SHORT FLASHES AND THE SECOND DIGIT BY THE NUMBER OF LONG FLASHES.

43 LOW-HEAT PRESSURE SWITCH OPEN WHILE MEDIUM-HEAT PRESSURE SWITCH IS CLOSED - Check for:

- Low inlet gas pressure (if LGPS used).

- Plugged condensate drain.

- Improper pressure switch wiring.

- Water in vent piping, possible sagging pipe.

- Stuck open low-heat pressure switch.
- Disconnected or obstructed pressure tubing.

I am sure you have gotten this fixed by now. A 1 code is a general lockout meaning it tried to light 3 times and did not. A 2 code is for a stuck closed pressure switch. That pressure switch is normally open. If unit is off, and you remove the wires and using a multimeter, ohm across it, it should be open. If not, it needs to be replaced.

i take it the flash code 3 blinks refers you to the pressure switch...see the schematic. If this is what you are saying than lets move on... the voltage across the pressure switch should read 0 if the furnace is running and calling for heat. you dont need to check each wire to ground but across the two quick connect tabs. you will either read 0 or 24 volts (or there about). if you read 0 the switch is closed and functioning as designed. if it reads 24 volts it is open and there is not enough negitive pressure to close it. this can be due to a leak in the rubber tubing or a clog in the pressure switch, tubing, or induced fan housing connection nipple. the flue pipe may also be clogged or an improper cap is used at the termination. last is the wiring could be faulty going to and from the pressure switch. Hardly ever have we found a broken or faulty switch but it could happen. trust the board in most cases.

Check the intake and exhaust plumbing for any obstructions. I had same issue, turned out to be ice in the air intake plumbing on outside of house. This may be the problem. The pressure switch can be tested by pulling hose from purge motor that goes to switch and applying a small amount of suction. The switch should make a clicking sound.