How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask BillyHvac Your Own Question
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22633
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
Type Your HVAC Question Here...
BillyHvac is online now
A new question is answered every 9 seconds

Additional Information

This answer was rated:

Hello. I'm getting a "3 Flashes" ( (Pressure switch open...) diagnostic code with my PG9YAB furnace. To save time and efforts, I'd be glad to email a PDF of symptoms, what happened, and the steps I've taken so far...


Added - Here's the text version of my notes



==>  Payne PG9YAB (1 year old) Dual Pressure Switch
==> Honeywell HZ311 TrueZONE
==> Honeywell thermometers

SYMPTOM:  3 Flashes on Control Center board

WHAT HAPPENED (don’t know if this is the cause, though):
==>  Our house took a power surge from lightening (everything OK except the phone)
==>  HOWEVER:  The blower turned on, even with therm switch on Auto
==>  SO: I flipped the switches of both zones a few times and eventually the blower stopped (took a few hours, so I’m not sure if I stopped it or it stopped itself)
==>  Weather was warm so I didn’t try the furnace
==> The power surge happened about 3 weeks ago

==> We have 2 of these furnaces in the house and other one (1 zone) works fine…
==> The working furnace is in the basement and the problem child is in the attic
==> The power surge happened about 3 weeks ago…

TODAY (yesterday, now):
==>  It got cold so I set the therm to Heat and above room temp.  Waited and got nothing.
==> Same thing happens with the other zone…
==>  Set fan from “Auto” to “On” and fan works (both zones)

1. I turned the thermostats off
2. Checked the household circuit breakers – all OK (expected)
3.  I went to the attic and flipped the main electrical power switch off then back on.  Unfortunately, I didn’t think to look at the Control Center diag. light or the status lights of the HZ311 before I did this.
4. Tried the thermostats -- heat on WAY above room temperature, waited (10 mins.), nothing…
5. Switched fan from “Auto” to “On” and fan runs…
6. Followed the “Starting The Furnace” steps on page 4 of the PG9YAB User’s Manual –

Starting The Furnace
See Fig. 4 for an illustration of the gas control valve.
1. Turn the thermostat to its lowest temperature setting or to
OFF if equipped with a System Select Switch.
2. Turn OFF all electric power to the furnace at the disconnect
switch or circuit breaker.
3. Remove the burner compartment door by removing the
two(2) screws securing the door in front and lifting the door
up and outwards. Removing the door will expose the gas
control valve knob.
4. Turn the gas control valve to the OFF position. See Fig. 3.
5. Wait five (5) minutes to clear out any gas. If you then smell
gas, STOP! Follow the safety information on the cover of
this manual. If you do not smell gas, go to the next step.
6. Turn the gas control valve to the ON position. See Fig. 3.
7. Reinstall all doors.
8. Turn ON all electrical power to the furnace.
9. Set the thermostat to the desired temperature and the System
Select Switch to HEAT.
a. The furnace will activate an igniter which lights the main burners.

7. Afterwards, the Control Center diagnostic light was normal – Heartbeat / Pulse / Light – Dim
8. I waited for HZ311 Purge light to go off and all status lights seemed fine BUT this is where I get fuzzy…
9. EITHER the Control Center light was flashing 3 after RZ311 purge OR it started flashing after I tried the thermostats again.  I think it started flashing after Purge, but I’m really not sure…
10. Anyway, after all that I tried the thermostats again (same procedure) with the same results
11. And that’s where I am now

I hope this information helps!  Unfortunately, it’s close to 3:00AM my time so I won’t be crawling around in the attic anytime soon.  But, that’s OK because it’s not too cold right now (actually, it’s kind of pleasant) but I don’t want to wait until we start hitting single digit temperatures before getting the furnace to work… :)


My name is XXXXX XXXXX I apologize for the delay.

Do you still need assistance?


Customer: replied 3 years ago.

Hi there, Billy!


My apologies for not getting back sooner and, yep, I still need some help.


I'm doing a bunch of household chores today so I'll be bouncing in and out of the house. Fortunately, with the nice weather, the furnace isn't the highest priority (YET).


I'll be checking the computer whenever I get the chance. Did the information I posted help?


I read another post that walked the customer through testing the Control Center circuit board. I have a multi-meter and should be able to do that if you recommend it. I'm hoping that there's an easy fix, but I'll wait until I hear from the expert (that would be you)...








Ok, first we start with basics and work towards a fix.

Can you tell me if the exhaust motor attached to the vent pipe is turning on?


Customer: replied 3 years ago.

Would that be the Combustion Blower with the pressure switches attached? -->



That is correct.

Is that fan starting before 3 blinks appear?

Customer: replied 3 years ago.

Checking. This may take 15-20 mins...


No problem...standing by

Customer: replied 3 years ago.

OK... Before I touched anything I checked the light and it showed normal operation (bright-dim-bright-dim, etc.) and the thermostats were already off. The combustion fan wasn't doing anything (expected).

I turned one of the therms on "heat", above room temperature, and the house fan kicked in.

I went back to the furnace and the combustion fan was going but the light was 3 blinks. After a couple of minutes the house fan turned off, but the combustion blower was still running.

Then I turned the thermostat back off. The combustion motor stopped and the diagnostic light went back to flashing Normal Operation...


Thank you.

You have 2 pipes from the furnace (pvc)

One is exhaust and one is intake and one is exhaust.

The intake is the one from the top/center.

Can you remove that pipe...even if just an inch?

Customer: replied 3 years ago.

I think so. Need a few minutes. I'm in the basement and we have a 2-story house. The wife doesn't mind that I'm getting a workout, though!

Hmmm. Is there a way I can switch computers without losing this thread/connection?

Back in a few...

All you need is the link to this thread...or log into and sign in.

You can access from any computer or smartphone

Customer: replied 3 years ago.
Gotta love the iPad!!! I got the pipe off. Plenty of room to move it...

Now try the furnace again...tell me if 3 blink repeats

Customer: replied 3 years ago.
Ok. Combustion blower is going. Light was flashing normal for a few seconds (15 or so) then went to 3 flashes. Combustion fan is still going...
Thank you.

Now leave intake pipe off....remove the small rubber tubes to the pressure switch.
Check for cracks...trim if necessary.

Use a paper clip and rheem out the barbed fitting it attaches to on furnace side.

I want to eliminate obstructions before condeming a switch

Customer: replied 3 years ago.
Ok. I took the hoses off. No cracks and I was able to blow through them. I used the paper clip on the furnace side of both fittings.
Everything was clear on the combustion blower from the top switch.
BUT! The furnace-side fitting from the bottom switch (lower left corner) felt like there was padding behind. It cleared away pretty easy with the paper clip.
Kinda seems like an obstruction to me, but....?
The light is flashing Normal Operation...
Thank you....

have you tried unit again yet?...if not do so now.


Customer: replied 3 years ago.
Turned it back on. Light went back to 3 flashes. Combustion blower is going. Nothing else happening.
2 things: 1) there are some dead miller-moths on the base, below the blower and what felt like "padding" could have been one of those. 2) the intake pipe is still disconnected...
The combust blower just stopped after about 2 minutes... Light is still blinking 3...
I checked the furnace side fittings again with the paper clip and all clear.
Thank you,

last manual check.

Remove tube from lower switch from furnace side.
Start furnace
AFTER the exhaust fan starts draw air from the switch through the tube like a straw.

Do this long enough to see if the unit progresses to ignitor.

Then we will have answer.

Customer: replied 3 years ago.
Really hard to suck air... I can hear the switch click when I need a breath...
Fan shut off. No igniter...
Thank you...did it take a lot to activate the click?

Customer: replied 3 years ago.
The switch clicked easily but it took a lot to suck in. Kinda like sucking a really thick milkshake with a straw...
Also - I'm pretty sure I was wrong about the "padding/dead moth" thing. When I first tried the paper click it was tight against the hose next to the fitting. I noticed that when I checked it the 2nd time. Cockpit error...

if the switch clicked closed and did not progress to ignition we have a bad pressure switch.

If you watch/listen to the switch when combustion fan starts...would you say the switch closes?

Customer: replied 3 years ago.
First time I heard a click but its hard to tell where it came from. I left in on until combustion blower stopped. Turned it back on but the HZ311 was in Purge and clicked at the same time blower kicked in so , again I couldn't tell.
Third time... I hear a click but its too hard to tell if its one of the switches or something else. I think it is one of the switches because there's nothing else nearby except the gas control valve (does that click?)...

Is this natural gas or propane....I want to cover all the bases...

Customer: replied 3 years ago.
Natural gas
I am crossing off rare but possible...

Pull the main drain hose that drains water away from furnace...located on front wall under and to the left of combustion motor...does water come out?

Customer: replied 3 years ago.
No water.
Thank you.

your 2 pressure there an orange wire coming from the circuit board connector....and a yellow from opposite switch to board?

Customer: replied 3 years ago.
Yes. Circuit board Orange to "+" terminal on top switch. --> jumper from top switch "-" to bottom switch "+" then yellow from bottom switch to "-" to circuit board.
Ok John,

we want to take the orange wire from + on top switch off....and the yellow wire from bottom switch - off.

Start the furnace...AFTER the combustion fan runs.....we want to touch the 2 wires direct together and hold.

Since there is a plastic coating on the wire ends it is sometimes easier to simply have a small nail or paper clip in one end..and when motor starts slide the other one on.

The end result is we want metal to metal contact with the orange and yellow wire....after the combustion fan starts.

Leave these 2 wirres together until:

1 - the unit starts to proceed to ignition

2 - the 3 code appears again.

This is the final step.

Customer: replied 3 years ago.
Ok. I'm on it... Need a few minutes
standing by

Customer: replied 3 years ago.
Almost there! At first nothing happened but the light was flashing TWICE (switch closed when should be open).
While I was checking the light I accidentally dropped my bypass connection and when I reconnected the burner started.

This means the circuit board is ok
Ignitor is ok
gas valve ok.....

The issue is one of those 2 switches is not closing. have good air flow from the combustion motor
IF...there is no blockage in the other vent pipe

Then you have a bad pressure switch.

You would change the (2) switch assembly.

By bypassing the switches we told the board the combustion fan motor was running...and the vents were clear.

Do a final check on vent pipes....

You also can bypass one switch at atime (we did both) find which is bad...

But I always replace both at same time.

I thank you for your patience and assistance..


BillyHvac and other HVAC Specialists are ready to help you